GPS time synchronization system based on digital seismograph

Abstract

The invention discloses a GPS time synchronization system based on a digital seismograph, which comprises a GPS receiver and an MCU, the GPS receiver outputs received time message information and second pulse information to the MCU, and the MCU provides the obtained whole second time information to an internal RTC clock for time calibration. A counter of the MCU carries out microsecond-level intra-second time subdivision in a counting mode, and when a data acquisition unit of the seismograph sends out a timestamp request, the MCU integrates and packages the whole second time and the microsecond time and then outputs a high-precision timestamp. The method has the advantages of providing an accurate timestamp for the collected seismic data, providing a time reference for a plurality of seismometers working at the same time, and the like.

Description

Technical Field

[0001] The invention relates to the technical field of digital seismographs, in particular to a GPS time synchronization system based on digital seismographs. Background Art

[0002] In the process of seismic exploration, in order to effectively analyze the seismic waveform data of a single seismograph, the precise time of data acquisition must be obtained. In addition, multiple seismographs are used in the process of seismic exploration. Often, multiple seismographs are distributed in different exploration areas and work simultaneously. Multiple seismographs must maintain time synchronization, otherwise it is impossible to invert massive data. At the same time, the time synchronization accuracy of digital seismographs must reach the microsecond level to meet the requirements of time resolution.

[0003] GPS is currently the most widely used and applied positioning system in the world. It includes satellite systems, ground monitoring systems and user receivers. In the actual timing process of the GPS timing system, the user receiver receives the information in the navigation message of the GPS system, calculates the deviation between the clock of the user receiver and the GPS dedicated time, and then converts it into the standard UTC time in the area where the user receiver is located through the relationship between the GPS dedicated time and the UTC time. It is necessary to ensure that the user clock time and the GPS dedicated time maintain high-precision synchronization so that users can obtain accurate time reference using GPS receivers.

[0004] Therefore, there is an urgent need to design a technical solution that can achieve high-precision time synchronization. Summary of the invention

[0005] The purpose of the present invention is to solve the above-mentioned deficiencies in the prior art and to provide a GPS time synchronization system based on a digital seismograph that provides an accurate time stamp for the collected seismic data and a time reference for multiple seismographs working simultaneously.

[0006] The technical solution adopted by the present invention to solve its technical problem is:

[0007] A GPS time synchronization system based on a digital seismograph, characterized in that: it comprises a GPS receiver and an MCU, wherein the GPS receiver outputs received time message information and second pulse information to the MCU, the MCU provides the acquired whole second time information to an internal RTC clock for time calibration, and the counter of the MCU performs microsecond-level subdivision of the second time by counting;

[0008] When the data acquisition unit of the seismograph sends a timestamp request, the MCU integrates and packages the whole second time and microsecond time and outputs a high-precision timestamp;

[0009] It provides accurate timestamps for seismic data collected by seismographs, ensures that the time synchronization accuracy of seismographs can reach the microsecond level, and provides a time benchmark for multiple seismographs working simultaneously.

[0010] The GPS receiver of the present invention adopts the GT-U13 dual-frequency multi-system module. The GPS receiver receives message information transmitted in the L1 frequency band and the L5 frequency band. The transmission protocol of the message information adopts the NMEA-0183 standard protocol. The hardware interfaces for obtaining the message information include UART, I2C and CAN.

[0011] The circuit of the GPS receiver of the present invention comprises a power supply part, an antenna part and a data receiving part, wherein the power supply part comprises a voltage VDD5, a PMOS tube, a low voltage difference linear regulator U25, a voltage VDD3.3, a resistor R211, a capacitor C211, a capacitor C212, a capacitor C213, a capacitor C214 and a capacitor C215, wherein the gate of the PMOS tube is electrically connected to the PD13 pin of the MCU, and the conduction and cutoff of the PMOS tube are controlled to turn on or off the power supply of the GPS receiver, the source of the PMOS tube is electrically connected to the voltage VDD5, the drain of the PMOS tube is electrically connected to the VCC pin of the low voltage difference linear regulator U25, when the PD13 pin outputs a low level, the source and drain of the PMOS tube are turned on, the drain of the PMOS tube is grounded via the capacitor C211 and the capacitor C212 arranged in parallel, and the voltage VDD5 is electrically connected to the PD13 pin of the MCU via the resistor R211;

[0012] The VCC pin of the low voltage drop linear regulator U25 is electrically connected to the EN pin, the GND pin is grounded, the BP pin is grounded via a capacitor C213, the VOUT pin is electrically connected to the voltage VDD3.3, and the VOUT pin is grounded via capacitors C214 and C215 arranged in parallel;

[0013] Capacitors C214 and C215 act as filters to stabilize the output voltage of the VOUT pin to a 3.3V power supply voltage, which serves as the power supply for the remaining two parts of the GPS receiver circuit. The setting of capacitor C213 reduces the noise and ringing phenomenon of the reference voltage of the control unit inside the low-dropout linear regulator U25, thereby reducing the output noise of the regulator.

[0014] The antenna part includes an antenna E211, an antenna E212, a signal amplifier U27, a capacitor C216, a capacitor C217 and an inductor L212, a VCC pin of the signal amplifier U27 is electrically connected to a voltage VDD3.3 pin and is grounded via a capacitor C217, a SHDN pin is electrically connected to a VCC pin, an RFOUT pin is electrically connected to an antenna E211 via a capacitor C216, an RFIN pin is electrically connected to an antenna E212 via an inductor L212, and a GND pin is grounded;

[0015] The data receiving part includes a GPS module U26, a voltage VDD3.3, an inductor L211, a diode D211, a resistor R212, a resistor R213 and a capacitor C218, a VCC-RF pin of the GPS module U26 is used as a power supply pin and is electrically connected to the antenna E211 via the inductor L211, a GND pin is grounded, an RF-IN pin is electrically connected to the antenna E211, a PPS pin is electrically connected to the PD15 pin of the MCU, a VDD-USB pin is electrically connected to the voltage VDD3.3, a UOUT0 pin is electrically connected to the PC6 pin of the MCU via the resistor R212, a UIN0 pin is electrically connected to the PC7 pin of the MCU via the resistor R213, a VCC pin is electrically connected to the VDD3.3 pin and is grounded via the capacitor C218, and a voltage VDD3.3 is electrically connected to the V-BCKP pin via the diode D211;

[0016] The PD15 pin of the MCU receives the second pulse signal, the connection between the UOUT0 pin and the PC6 pin of the MCU and the connection between the UIN0 pin and the PC7 pin of the MCU are used to send GPS message information to the MCU and receive configuration information from the MCU. When positioning and timing operations are required, the MCU controls the PD13 pin to output a low level, the PMOS tube is turned on, and the power of the GPS receiver is turned on. The GPS receiver receives the satellite message information and transmits it to the MCU through the PC6 pin for parsing. After the MCU parses the required information from the message, it controls the gate of the PMOS tube to a high level through the PD13 pin, the PMOS tube is turned off, and the power supply of the GPS receiver is turned off, thereby realizing the function of turning on the GPS receiver on demand and reducing power consumption.

[0017] The calculation formula of the internal frequency of the RTC clock of the present invention is:

[0018]

[0019] Where f CK_SPRE is the RTC internal frequency (Hz), f RTCCLK is the RTC clock input source frequency (Hz), prediv_S is the synchronous prescaler factor, and prediv_A is the asynchronous prescaler factor.

[0020] The RTC clock of the present invention uses a temperature compensated crystal oscillator TCXO as a stable external clock source. The temperature compensated crystal oscillator TCXO circuit includes a voltage VDD3, an active crystal oscillator Y3, an inductor L7, a capacitor C116, a capacitor C117, a capacitor C118, a resistor R117 and a resistor R116. The voltage VDD3 is electrically connected to the VDD pin of the active crystal oscillator Y3 via the inductor L7, the VDD pin is grounded via the capacitors C116 and C117 arranged in parallel, the VDD pin is electrically connected to the OUT pin via the resistor R117, the OUT pin is grounded via the capacitor C118, the OUT pin is electrically connected to the PC14 pin of the MCU via the resistor R116, and the GND pin is grounded;

[0021] The setting of inductor L7 prevents the high-frequency noise interference of the temperature compensation crystal oscillator TCXO from coupling with the VDD3 power supply voltage, thereby interfering with other devices powered by VDD3. Capacitors C116, C117 and C118 are used for power supply filtering. Resistor R116 is used to limit the output oscillation amplitude. Resistor R117 can be regarded as a feedback resistor, which adjusts the device inside the active crystal oscillator that originally works in the saturation region and has no gain effect to work in the linear region to meet the starting conditions.

[0022] The UTC time parsed by the GPS receiver of the present invention is used as the standard timeline, and the seismic data time information of the seismograph is synchronized with the standard timeline. After the seismic data is collected by the data acquisition unit of the seismograph, it is stored in the standard seismic data format of MiniSEED. The format is mainly divided into a frame header information part and a seismic data part. The frame header information part contains the precise acquisition timestamp of the first point data of the current data frame and the sampling rate information of the data frame. The acquisition time of other data points of a single data frame except the first data point is calculated according to the following formula:

[0023]

[0024] Where T N is the sampling time (s) of the Nth data point in the frame, T1 is the exact sampling time (s) of the first data point in the frame, N is the number of data points in the frame, and f Sample is the sampling rate (SPS) of the data points in the frame, where f Sample Configuration is performed through the analog-to-digital conversion chip in the data acquisition unit.

[0025] The sampling rate configuration method of the analog-to-digital conversion chip of the present invention is:

[0026] Step S1: The ADSBUS_IOInitial function configures the IO port of the three-channel bus multiplexer, and executes step S2;

[0027] Step S2: The ADS_IOInitial function configures the SYNC, DRDY and SPI bus related pins of the analog-to-digital conversion chip in the three-channel bus, and executes step S3;

[0028] Step S3: ADS_IOInitial function configures the SPI peripheral of MCU and enables it, and then executes step S4;

[0029] Step S4: dynamically apply for memory to store MiniSEED format seismic data, and execute step S5;

[0030] Step S5: ADS_IO_enable function turns on the external crystal oscillator source of the analog-to-digital conversion chip, and then executes step S6;

[0031] Step S6: The ADSBUS_ResetByPin function resets each pin of the analog-to-digital conversion chip, and then executes step S7;

[0032] Step S7: The ADSBUS_muxSwitch(i) function controls the multiplexer to select the i-th channel, and executes step S8;

[0033] Step S8: the ADS_cfgReg(Data_Rate) function writes the sampling rate Data_Rate into the cache unit adc_regdata according to the definition format of the DR[2:0] bit in the CONFIG0 register of the analog-to-digital conversion chip, and executes step S9;

[0034] Step S9: the ADS_cfgReg(Data_Rate) function writes other acquisition parameters into the corresponding positions of adc_regdata according to the bit definition format of the CONFIG0 register, and executes step S10;

[0035] Step S10: the Write_ADC_REG_Sequence function writes the one-byte content of adc_regdata into the register unit with the address 0x01 in the analog-to-digital conversion chip, and then executes step S11;

[0036] Step S11: Determine whether all three channels are configured, if yes, execute step S12, if no, execute step S7;

[0037] Step S12: The ADSBUS_SYNC function triggers the synchronization signal of the analog-to-digital conversion chip.

[0038] The method for the GPS receiver of the present invention to obtain and analyze message information is as follows:

[0039] Step S1: Find the GNGGA identifier in the message, point the parsing pointer to the GNGGA message, and execute step S2;

[0040] Step S2: Move the parsing pointer to the first comma in the message and return the offset value, and then execute step S3;

[0041] Step S3: Determine whether the offset value is non-0xFF, if yes, execute step S4, if no, execute step S15;

[0042] Step S4: convert the UTC time in string format into a digital format without decimal points and store it in the cache, and then execute step S5;

[0043] Step S5: Move the parsing pointer to the second and third commas in the message and return the offset value, and then execute step S6;

[0044] Step S6: Determine whether the offset value is non-0xFF, if yes, execute step S7, if no, execute step S16;

[0045] Step S7: convert the latitude information in the string format into a digital format and store it in a cache, and then execute step S8;

[0046] Step S8: Move the parsing pointer to the 4th and 5th commas in the message and return the offset value, and then execute step S9;

[0047] Step S9: Determine whether the offset value is non-0xFF, if yes, execute step S10, if no, execute step S17;

[0048] Step S10: convert the longitude information in the string format into a digital format and store it in a cache, and then execute step S11;

[0049] Step S11: determine whether the GNGGA parsing valid bit is 1, if yes, execute step S12, if no, jump to step S1;

[0050] Step S12: parse the GNRMC message and execute step S13;

[0051] Step S13: Determine whether the GNRMC parsing valid bit is 1, if yes, execute step S14, if no, jump to step S1;

[0052] Step S14: end;

[0053] Step S15: if the valid bit is 0, jump to step S5;

[0054] Step S16: if the valid bit is 0, jump to step S8;

[0055] Step S17: if the valid bit is 0, jump to step S11;

[0056] After the above steps, the second time information in the high-precision timestamp is successfully obtained.

[0057] The method for serving time to the RTC clock of the present invention is as follows:

[0058] Step S1: Initialize the microsecond counter and execute step S2;

[0059] Step S2: Initialize the GPS receiver and UART, and execute step S3;

[0060] Step S3: Determine whether the GPS message is valid, if valid, execute step S4, if invalid, execute step S3;

[0061] Step S4: parse the GPS message, longitude and latitude and time information, which takes time t0, and then execute step S5;

[0062] Step S5: Determine whether the millisecond bit of the parsed time information is 0, if yes, execute step S6, if no, execute step S3;

[0063] Step S6: Save the current time information T1, notify the PPS to interrupt and set the timing flag position to 1, and execute step S7;

[0064] Step S7: End.

[0065] The method of the PPS pulse-per-second trigger interruption of the present invention is:

[0066] Step S1: Determine whether the PPS second pulse can be used for timing flag is set to 1, if yes, execute step S2, if not, execute step S5;

[0067] Step S2: Set the RTC time to T2, T2=T1+1, the timing time is t1, and execute step S3;

[0068] Step S3: The microsecond counter sets the initial value after time compensation, starts counting upward, and executes step S4;

[0069] Step S4: clear the flag bit that can perform timing service to 0, and execute step S5;

[0070] Step S5: Return.

[0071] The beneficial effects of the present invention are: providing an accurate time stamp for the seismic data collected by the seismograph, ensuring that the time synchronization accuracy of the seismograph can reach the microsecond level, and providing a time reference for multiple seismographs working simultaneously. BRIEF DESCRIPTION OF THE DRAWINGS

[0072] Figure 1 It is a schematic diagram of the high-precision time synchronization system of the present invention.

[0073] Figure 2 It is the parameter index of GT-U13 of the present invention.

[0074] Figure 3 It is the circuit diagram of the GPS receiver of the present invention.

[0075] Figure 4 It is a schematic diagram of RTC clock frequency division of the present invention.

[0076] Figure 5 It is a sine wave oscillation circuit diagram of the present invention.

[0077] Figure 6 It is the RTC crystal oscillator circuit diagram of the present invention.

[0078] Figure 7 It is a flow chart of the sampling rate configuration of ADS1281 of the present invention.

[0079] Figure 8 This is the universal MiniSEED time format table of the present invention.

[0080] Fig. 9 This is the MiniSEED time format table after adjustment by the present invention.

[0081] Fig.10 It is the NEMA sentence format table of the present invention.

[0082] Fig.11 It is a common sentence of the NMEA protocol of the present invention.

[0083] Fig.12 It is a flow chart of GPS receiver message analysis of the present invention.

[0084] Fig.13 It is a schematic diagram of GPS timing of the present invention.

[0085] Fig.14 It is a firmware flow chart of the RTC clock timing part of the present invention.

[0086] Fig.15 It is a flow chart of the PPS second interrupt program of the present invention.

[0087] Fig.16 It is a schematic diagram of the time synchronization of seismic exploration according to the present invention.

[0088] Fig.17 It is the basic principle diagram of GPS positioning of the present invention.

[0089] Fig.18 It is a basic principle diagram of GPS time synchronization of the present invention. DETAILED DESCRIPTION

[0090] The present invention is described below in conjunction with the accompanying drawings and embodiments.

[0091] As attached Fig.16 As shown in the figure, it is a concept of seismic exploration data inversion. Seismometers P and Q are arranged in a single line. The artificial source O causes vibration. The vibration wave propagates through the medium and reaches the medium interface. After being reflected by the medium interface, it is received by seismometers P and Q. Because the two seismometers receive the reflected seismic waves of the same medium interface, assuming that the propagation speed of the seismic wave is constant, according to the relationship between time, wave speed and distance, we get:

[0092]

[0093] Where v is the velocity of seismic waves (m / s), t P is the time (s) for the seismic wave to reach the seismograph P, t Q is the time it takes for the seismic wave to reach the seismograph Q (s), l OP is the distance between the earthquake origin O and the seismograph P (m), l OQ is the distance between the earthquake origin O and the seismograph Q (m), H is the distance between the medium interface and the ground (m);

[0094] The above formula can be used to deduce the time difference Δt when the same seismic wave reaches two seismometers: PQ for:

[0095]

[0096] Taking the exploration of coal resources as an example, coal resources are mainly concentrated in the shallow surface, with a depth H of about 300m from the ground. The speed v of the vibration wave excited by the artificial source in the medium is 5km / s. Assuming that the distance l between the seismograph P and the artificial source O is OP is 100m, and the inter-channel distribution distance of the seismograph is about 20m, that is, l OQ It is about 120m. According to the above formula, the time difference between the same seismic wave reaching the two seismometers is 0.72ms. It can be seen that the time synchronization accuracy of the digital seismometer needs to reach the microsecond level to meet the time resolution requirements.

[0097] As attached Fig.17 As shown, GPS uses time information for positioning. Using the spatial position information and time information of four GPS satellites for approximate calculation, the following multivariate equations can be listed:

[0098]

[0099] Where (T i , X i , Y i , Z i) are the four-dimensional time and space coordinates of the four satellites, (T, X, Y, Z) are the four-dimensional time and space coordinates of the ground receiver, and c is the speed of the satellite signal, that is, the speed of light (m / s).

[0100] As attached Fig.18 As shown in the figure, it is the basic principle diagram of GPS time synchronization. It can be seen that the relationship between GPS dedicated time, satellite clock time and user clock time is expressed as:

[0101] t S_T =t GPS_T +Δt S

[0102] t U_R =t GPS_R +Δt U ;

[0103] Where, t S_T is the satellite-borne clock time corresponding to the transmission of the timing signal (s), t GPS_T is the GPS dedicated time corresponding to the time when the timing signal is transmitted (s), Δt S is the deviation between the satellite clock time and the GPS dedicated time (s), t U_R is the user clock time when receiving the timing signal (s), t GPS_R is the GPS dedicated time corresponding to the timing signal reception (s), Δt U is the deviation between the user clock time and GPS dedicated time (s);

[0104] t related to the satellite clock time S_T and Δt S It can be obtained from the navigation message, t U_R Provided by the user receiver, the actual geometric distance between the user receiver U and the satellite S is as follows:

[0105] R i =c×(t GPS_R -t GPS_T )-c×t D ;

[0106] Where Ri is the actual geometric distance between the user receiver and the satellite (m), c is the speed of light (m / s), and t D is the additional delay introduced by the atmosphere (s);

[0107] Among them, t D The pseudo-range between the user receiver U and the satellite S can be obtained from the navigation message: As shown in the following formula, there are no unknown parameters in the following formula, that is, the pseudorange is known:

[0108]

[0109] Further deduction leads to the following expression:

[0110]

[0111] It can be seen from the above formula that the pseudo-range expression between the user receiver and the satellite contains the actual geometric distance R between the two. i The distance error c×t caused by the noise delay when the signal propagates in the atmosphere D and the distance error c×(Δt U -Δt S ), where only Δt U is an unknown parameter. Fig.17 The GPS positioning principle shown in the figure is to calculate the deviation Δt between the user clock time and the GPS dedicated time by performing multiple operations and averaging the multivariate equations listed. U The user receiver corrects the user clock based on this deviation, so that the user clock and GPS dedicated time maintain high-precision synchronization, enabling the user to obtain an accurate time reference using a GPS receiver.

[0112] As attached Figure 1 As shown, a GPS time synchronization system based on a digital seismograph includes a GPS receiver and an MCU. The GPS receiver outputs the received time message information and second pulse information to the MCU. The MCU provides the acquired whole second time information to an internal RTC clock (Real Time Clock) for time calibration. The counter of the MCU performs microsecond-level time subdivision in a second by counting.

[0113] When the data acquisition unit of the seismograph sends a timestamp request, the MCU integrates and packages the whole second time and microsecond time and outputs a high-precision timestamp;

[0114] It provides accurate timestamps for seismic data collected by seismographs, ensures that the time synchronization accuracy of seismographs can reach the microsecond level, and provides a time benchmark for multiple seismographs working simultaneously.

[0115] The GPS receiver adopts the GT-U13 dual-frequency multi-system module, which has the technical advantages of small package and low power consumption. The parameter indicators are as follows Figure 2As shown, the GPS receiver can receive message information transmitted in the L1 frequency band (1575.42MHz) and the L5 frequency band (1176.45MHz). The transmission protocol of the message information adopts the NMEA-0183 standard protocol. The hardware interfaces for obtaining the message information include UART (Universal Asynchronous Receiver / Transmitter), I2C (Inter-Integrated Circuit) and CAN (Controller Area Network), etc.

[0116] As attached Figure 3 As shown, the circuit of the GPS receiver includes a power supply part, an antenna part and a data receiving part. The power supply part includes a voltage VDD5, a PMOS tube, a low voltage difference linear regulator U25, a voltage VDD3.3, a resistor R211, a capacitor C211, a capacitor C212, a capacitor C213, a capacitor C214 and a capacitor C215. The gate of the PMOS tube is electrically connected to the PD13 pin of the MCU to control the conduction and cutoff of the PMOS tube to turn on or off the power supply of the GPS receiver. The source of the PMOS tube is electrically connected to the voltage VDD5. In this embodiment, VDD5 is a 5V voltage domain of the seismograph system. The drain of the PMOS tube is electrically connected to the VCC pin of the low voltage difference linear regulator U25. When the PD13 pin outputs a low level, the source and drain of the PMOS tube are turned on, and the drain of the PMOS tube is grounded via the capacitor C211 and the capacitor C212 arranged in parallel. The voltage VDD5 is electrically connected to the PD13 pin of the MCU via the resistor R211.

[0117] The VCC pin of the low voltage drop linear regulator U25 is electrically connected to the EN pin, the GND pin is grounded, the BP pin is grounded via a capacitor C213, the VOUT pin is electrically connected to the voltage VDD3.3, and the VOUT pin is grounded via capacitors C214 and C215 arranged in parallel;

[0118] Capacitors C214 and C215 play a filtering role, so that the output voltage of the VOUT pin is stabilized to a power supply voltage of 3.3V, which serves as the power supply for the other two parts of the GPS receiver circuit. The setting of capacitor C213 reduces the noise and ringing phenomenon of the reference voltage of the internal control unit of the low-voltage dropout linear regulator U25, thereby reducing the output noise of the regulator; in this embodiment, the low-voltage dropout linear regulator U25 adopts RT9193, and the nominal value of capacitor C213 is 22nF;

[0119] The antenna part adopts the design form of an active antenna, including an antenna E211, an antenna E212, a signal amplifier U27, a capacitor C216, a capacitor C217 and an inductor L212, a VCC pin of the signal amplifier U27 is electrically connected to a voltage VDD3.3 pin, and is grounded via a capacitor C217, a SHDN pin is electrically connected to a VCC pin, an RFOUT pin is electrically connected to an antenna E211 via a capacitor C216, an RFIN pin is electrically connected to an antenna E212 via an inductor L212, and a GND pin is grounded;

[0120] In this embodiment, the signal amplifier U27 of the radio frequency RF signal adopts the MAX2659, and the VCC-RF pin of the GT-U13 is used as the power supply pin of the active antenna;

[0121] The data receiving part includes a GPS module U26, a voltage VDD3.3, an inductor L211, a diode D211, a resistor R212, a resistor R213 and a capacitor C218, a VCC-RF pin of the GPS module U26 is used as a power supply pin and is electrically connected to the antenna E211 via the inductor L211, a GND pin is grounded, an RF-IN pin is electrically connected to the antenna E211, a PPS pin is electrically connected to the PD15 pin of the MCU, a VDD-USB pin is electrically connected to the voltage VDD3.3, a UOUT0 pin is electrically connected to the PC6 pin of the MCU via the resistor R212, a UIN0 pin is electrically connected to the PC7 pin of the MCU via the resistor R213, a VCC pin is electrically connected to the VDD3.3 pin and is grounded via the capacitor C218, and a voltage VDD3.3 is electrically connected to the V-BCKP pin via the diode D211;

[0122] In this embodiment, the GPS module U26 model adopts GT-U13. GT-U13 provides multiple interfaces for communicating with MCU. The UART interface is selected to connect with PC6 and PC7 of MCU. GPS message information is sent to MCU and configuration information is received from MCU at a baud rate of 115200. The PD15 pin of MCU is connected with the PPS pin of GT-U13 to receive a second pulse signal with a frequency of 1Hz.

[0123] When positioning and timing operations are required, the MCU controls the PD13 pin to output a low level, the PMOS tube is turned on, and the power of the GPS receiver is turned on. The GPS receiver receives satellite message information and transmits it to the MCU through the PC6 pin for analysis. After the MCU parses the required information from the message, it controls the gate of the PMOS tube to a high level through the PD13 pin, the PMOS tube is turned off, and the power supply of the GPS receiver is turned off, thereby realizing the function of turning on the GPS receiver on demand and reducing power consumption.

[0124] The GPS receiver gives the accurate UTC time to the real-time clock system RTC inside the MCU. The clock generation diagram of the RTC internal frequency is 1Hz as shown in the figure. Figure 4 As shown, the OSC32_OUT pin remains in a high-impedance state, and an external crystal oscillator with a frequency of 32.768Hz is input to the MCU through OSC32_IN. The clock selector is used to select the clock source of the RTC. There are three clock input sources, namely LSI, LSE and HSE_RTC. The lower 2 bits of the RTCSEL (RTC Clock Source Selection) register inside the MCU are controlled to be 01, and the LSE oscillator with a frequency of 32.768kHz is set as the clock input source of the RTC. According to the following formula, the synchronous pre-division coefficient is set to 0xFF, and the asynchronous pre-division coefficient is set to 0x7F, so that the 32.768kHz can be divided to obtain a clock frequency of 1Hz.

[0125]

[0126] Where f CK_SPRE is the RTC internal frequency (Hz), f RTCCLK is the RTC clock input source frequency (Hz), prediv_S is the synchronous prescaler factor, and prediv_A is the asynchronous prescaler factor.

[0127] The external crystal oscillator will inevitably produce clock drift due to the influence of external factors such as temperature. A temperature compensated crystal oscillator (TCXO) is used as a stable external clock source.

[0128] The crystal oscillator generates oscillations through excitation. Generally speaking, a typical sinusoidal oscillation circuit can be decomposed into the following: Figure 5 The structure diagram shown, Figure 5 In the figure, the upper box is the gain network, the lower box is the feedback network, and the polarity of the feedback is positive. Figure 5 In (b), when the circuit has no input, the feedback component is the total input quantity. Due to external excitation or electrical disturbance, the circuit will generate an output signal with a small amplitude but rich frequency. If the frequency-selective network designed in the oscillation circuit only produces a positive feedback process for the sine wave with a frequency of f0, the amplitude of the output signal of this frequency will become larger and larger.

[0129] Transistors have nonlinear characteristics. When the amplitude of the output signal increases to a certain level, the gain of the feedback circuit will decrease, and the circuit will eventually reach dynamic balance. According to the dynamic balance relationship of the circuit, there is an expression as shown below.

[0130]

[0131] In the formula is the input signal, is the amplifier circuit gain, is the feedback signal output by the feedback network, is the gain of the feedback circuit;

[0132] The process represented by the formula is that the input signal maintains the output signal through the forward amplifier circuit, and the output signal continuously stimulates the input signal through the positive feedback network. From the formula, it can be seen that the condition for the dynamic balance of the oscillation circuit is:

[0133]

[0134] (n is an integer);

[0135] The premise for the oscillation circuit to reach dynamic balance through the positive feedback process is that it needs to be excited by a small signal with complex frequency. Therefore, the starting condition of the sinusoidal oscillation circuit is:

[0136]

[0137] Based on the starting conditions of the sine wave oscillation circuit, the design is as follows Figure 6 The circuit of the active crystal oscillator TCXO shown in the figure comprises a voltage VDD3, an active crystal oscillator Y3, an inductor L7, a capacitor C116, a capacitor C117, a capacitor C118, a resistor R117 and a resistor R116. The voltage VDD3 is electrically connected to the VDD pin of the active crystal oscillator Y3 via the inductor L7. The VDD pin is grounded via the capacitors C116 and C117 arranged in parallel. The VDD pin is electrically connected to the OUT pin via the resistor R117. The OUT pin is grounded via the capacitor C118. The OUT pin is electrically connected to the PC14 pin of the MCU via the resistor R116. The GND pin is grounded. In this embodiment, the OUT pin outputs a sine wave with a frequency of 32.768kHz, which is output to the PC14 pin of the MCU via the small resistor R116 of 47Ω, so as to limit the output oscillation amplitude.

[0138] The setting of inductor L7 prevents the high-frequency noise interference of the temperature compensation crystal oscillator TCXO from coupling with the VDD3 power supply voltage, thereby interfering with other devices powered by VDD3. Capacitors C116, C117 and C118 are used for power supply filtering. Resistor R116 is used to limit the output oscillation amplitude. Resistor R117 can be regarded as a feedback resistor, which adjusts the device inside the active crystal oscillator that originally works in the saturation region and has no gain effect to work in the linear region to meet the starting conditions.

[0139] In order to ensure that the data collected by multiple seismographs in different spaces can be accurately aligned in time, the scheme of aligning multiple seismographs with the same standard timeline is adopted. This can weaken the coupling relationship between multiple seismographs and prevent them from affecting each other. The time error depends entirely on the difference between a single seismograph and the standard timeline. The UTC time parsed from the GPS receiver is selected as the standard timeline. It is only necessary to ensure the synchronization between the seismic data time information of a single seismograph and the standard time.

[0140] After the seismic data is collected by the data acquisition unit of the seismograph, it is stored in the standard seismic data format of MiniSEED. The format is mainly divided into a frame header information part and a seismic data part. The frame header information part contains the precise acquisition timestamp of the first point data of the current data frame and the sampling rate information of the data frame. The acquisition time of other data points in a single data frame except the first data point is calculated according to the following formula:

[0141]

[0142] Where T N is the sampling time (s) of the Nth data point in the frame, T1 is the exact sampling time (s) of the first data point in the frame, N is the number of data points in the frame, and f Sample is the sampling rate (SPS) of the data points in the frame, where f Sample The configuration is performed through the analog-to-digital conversion chip in the data acquisition unit. In this embodiment, the analog-to-digital conversion chip model is ADS1281. The specific configuration method is:

[0143] Step S1: The ADSBUS_IOInitial function configures the IO port of the three-channel bus multiplexer, and executes step S2;

[0144] Step S2: The ADS_IOInitial function configures the SYNC, DRDY and SPI bus related pins of the analog-to-digital conversion chip in the three-channel bus, and executes step S3;

[0145] Step S3: ADS_IOInitial function configures the SPI peripheral of MCU and enables it, and then executes step S4;

[0146] Step S4: dynamically apply for memory to store MiniSEED format seismic data, and execute step S5;

[0147] Step S5: ADS_IO_enable function turns on the external crystal oscillator source of the analog-to-digital conversion chip, and then executes step S6;

[0148] Step S6: The ADSBUS_ResetByPin function resets each pin of the analog-to-digital conversion chip, and then executes step S7;

[0149] Step S7: The ADSBUS_muxSwitch(i) function controls the multiplexer to select the i-th channel, and executes step S8;

[0150] Step S8: the ADS_cfgReg(Data_Rate) function writes the sampling rate Data_Rate into the cache unit adc_regdata according to the definition format of the DR[2:0] bit in the CONFIG0 register of the analog-to-digital conversion chip, and executes step S9;

[0151] Step S9: the ADS_cfgReg(Data_Rate) function writes other acquisition parameters into the corresponding positions of adc_regdata according to the bit definition format of the CONFIG0 register, and executes step S10;

[0152] Step S10: the Write_ADC_REG_Sequence function writes the one-byte content of adc_regdata into the register unit with the address 0x01 in the analog-to-digital conversion chip, and then executes step S11;

[0153] Step S11: Determine whether all three channels are configured, if yes, execute step S12, if no, execute step S7;

[0154] Step S12: The ADSBUS_SYNC function triggers the synchronization signal of the analog-to-digital conversion chip.

[0155] In order to obtain the accurate sampling time of each seismic data point, it is only necessary to ensure that the time of the first data point in the MiniSEED frame format is aligned with the standard timeline; the frame header of the general MiniSEED format uses a total of 10 bytes to represent the time information, such as Figure 8 As shown, the highest accuracy of the time format is millisecond level (Msec). When the error with the standard time line is not considered, the reliable time accuracy of the seismic data timestamp can only reach the millisecond level at most. The quantization error of the frame format's own timestamp is also not conducive to the subsequent processing of seismic data.

[0156] The number of decimal numbers that can be represented by a binary number with a certain number of bits is shown below:

[0157]

[0158] Where Ndec is the maximum number of decimal digits that can be represented by a binary number, and Nbin is the number of bits in the binary number.

[0159] The range of microsecond values ​​in time information is 0 to 999999 microseconds. According to the above formula, the number of bits in a binary number is at least 20, and the number of bits in a byte is 8. Therefore, at least 3 bytes of space are required to store the microsecond information in the time information. Figure 8 Based on the MiniSEED frame header format, the time information format is adjusted. The 2 bytes of milliseconds (Msec) and the 1 byte of unused (Unused) are combined, and 3 bytes are used to represent high-precision microsecond time information. The memory space occupied by the remaining time information bits is not modified. The adjusted time format is as follows: Fig. 9 shown.

[0160] The message information output from the GPS receiver to the MCU from the serial port follows the NEMA protocol. The NEMA protocol contains multiple different types of message statements. The format of a single statement is as follows: Fig.10 As shown, its content mainly includes an identifier for identifying the start and end of a frame, a data frame for containing specific data information, and a check field for transmission reliability. Fig.10 In the text, the RMC statement is taken as an example to explain in detail a message information of type GPRMC sent by a GPS satellite.

[0161] Depend on Fig.10 It can be seen that the data content in the protocol message is related to the data type. The information that the seismograph needs to obtain when working includes UTC time information and longitude and latitude information. Therefore, the GGA and RMC statement types contained in the NEMA protocol are selected for parsing. The information contained is as follows: Fig.11 shown.

[0162] Because the number of satellites and signal quality in different geographical locations are different, the quantity and type of message information that can be obtained are uncertain. Sometimes, too many message sentences are transmitted, which causes valid information to be submerged. Therefore, it is necessary to limit the message type of the GPS receiver. The MCU uses the serial port to send setting instructions to the GPS receiver to limit the message.

[0163] In this embodiment, the process of the GPS receiver acquiring and analyzing message information is as follows: Fig.12 As shown, the method for the GPS receiver to obtain and parse the message information is as follows:

[0164] Step S1: Find the GNGGA identifier in the message, point the parsing pointer to the GNGGA message, and execute step S2;

[0165] Step S2: Move the parsing pointer to the first comma in the message and return the offset value, and then execute step S3;

[0166] Step S3: Determine whether the offset value is non-0xFF, if yes, execute step S4, if no, execute step S15;

[0167] Step S4: convert the UTC time in string format into a digital format without decimal points and store it in the cache, and then execute step S5;

[0168] Step S5: Move the parsing pointer to the second and third commas in the message and return the offset value, and then execute step S6;

[0169] Step S6: Determine whether the offset value is non-0xFF, if yes, execute step S7, if no, execute step S16;

[0170] Step S7: convert the latitude information in the string format into a digital format and store it in a cache, and then execute step S8;

[0171] Step S8: Move the parsing pointer to the 4th and 5th commas in the message and return the offset value, and then execute step S9;

[0172] Step S9: Determine whether the offset value is non-0xFF, if yes, execute step S10, if no, execute step S17;

[0173] Step S10: convert the longitude information in the string format into a digital format and store it in a cache, and then execute step S11;

[0174] Step S11: determine whether the GNGGA parsing valid bit is 1, if yes, execute step S12, if no, jump to step S1;

[0175] Step S12: parse the GNRMC message and execute step S13;

[0176] Step S13: Determine whether the GNRMC parsing valid bit is 1, if yes, execute step S14, if no, jump to step S1;

[0177] Step S14: end;

[0178] Step S15: if the valid bit is 0, jump to step S5;

[0179] Step S16: if the valid bit is 0, jump to step S8;

[0180] Step S17: if the valid bit is 0, jump to step S11;

[0181] After all GNGGA messages are parsed, determine whether the parsed valid bit is 1. If it is 1, it means that all GNGGA message information has been parsed successfully. Then parse the GNRMC message information in the same way. If it is 0, it means that a parsing of the GNGGA message has failed, and then return to the message receiving program to continue receiving. If all GNGGA and GNRMC messages are parsed successfully, the process of GPS receiver obtaining message information and parsing is completed. After the above steps, the second time information in the high-precision timestamp is successfully obtained. The remaining microsecond time information is obtained through the PPS pulse of the GPS receiver and the 32-bit counter inside the MCU. The PPS second pulse is a rising edge signal with ns-level accuracy, and has the following relationship with the time information output from the GPS receiver serial port:

[0182] (1) The PPS pulse-per-second and NEMA message data are sent at the same time, with an error of nanoseconds. The time information contained in the NEMA message data is the time of the rising edge of the PPS pulse-per-second accurate to milliseconds.

[0183] (2) When the GPS receiver receives satellite information, the time information is received earlier than the longitude and latitude information, and when only the time information is received, the PPS second pulse is not output;

[0184] (3) The millisecond bit of the UTC time received initially is not 0. After several PPS second pulses, the GPS receiver will automatically adjust the PPS trigger time to the whole second to send message information to the MCU. That is, the PPS second pulse will be triggered at the whole second.

[0185] The schematic diagram of using the interrupt information triggered by the PPS second pulse and the message information obtained from the serial port to synchronize the RTC clock inside the seismograph is as follows: Fig.13 As shown, the upper and lower time lines represent the time line triggered by the PPS second pulse and the line of the RTC clock respectively. Time T1 represents the moment when the MCU receives the millisecond bit of the serial port information as 0, that is, the first full second moment. Time period t0 represents the time required for the MCU to parse the message information. The entire parsing time is less than the moment T2 when the next second pulse arrives. The whole second moment of time T1 parsed is saved in time period t0. When the next second pulse arrives at time T2, the RTC clock is timed, that is, 1 second is added to time T1 and timed to the RTC clock. Because the timing function also requires a certain amount of time, as shown in time period t1 in the figure, the RTC clock is granted the time of time T2 at time Ta. The time interval between the two time lines is time period t1. The length of time period t1 is calculated by experimental methods, and compensated by moving the count value of the microsecond counter in the second interrupt of the RTC.

[0186] The specific timing flow chart is as follows Fig.14 and Fig.15As shown, the method for timing the RTC clock is:

[0187] Step S1: Initialize the microsecond counter and execute step S2;

[0188] Step S2: Initialize the GPS receiver and UART, and execute step S3;

[0189] Step S3: Determine whether the GPS message is valid, if valid, execute step S4, if invalid, execute step S3;

[0190] Step S4: parse the GPS message, longitude and latitude and time information, which takes time t0, and then execute step S5;

[0191] Step S5: Determine whether the millisecond bit of the parsed time information is 0, if yes, execute step S6, if no, execute step S3;

[0192] Step S6: Save the current time information T1, notify the PPS to interrupt and set the timing flag position to 1, and execute step S7;

[0193] Step S7: End.

[0194] The method of PPS second pulse trigger interrupt is:

[0195] Step S1: Determine whether the PPS second pulse can be used for timing flag is set to 1, if yes, execute step S2, if not, execute step S5;

[0196] Step S2: Set the RTC time to T2, T2=T1+1, the timing time is t1, and execute step S3;

[0197] Step S3: The microsecond counter sets the initial value after time compensation, starts counting upward, and executes step S4;

[0198] Step S4: clear the flag bit that can perform timing service to 0, and execute step S5;

[0199] Step S5: Return.

[0200] After one time synchronization is completed, the time synchronization process is repeated through the interval time calibration method to improve the time accuracy of the RTC clock.

Claims

1. A GPS time synchronization system based on a digital seismograph, characterized in that: It includes a GPS receiver and an MCU. The GPS receiver outputs the received time message information and second pulse information to the MCU. The MCU provides the acquired whole second time information to the internal RTC clock for time calibration. The counter of the MCU performs microsecond-level subdivision of the second time by counting. When the data acquisition unit of the seismograph sends a timestamp request, the MCU integrates and packages the whole second time and microsecond time and outputs a high-precision timestamp.

2. A GPS time synchronization system based on a digital seismograph according to claim 1, characterized in that: The GPS receiver adopts the GT-U13 dual-frequency multi-system module. The GPS receiver receives message information transmitted in the L1 frequency band and the L5 frequency band. The transmission protocol of the message information adopts the NMEA-0183 standard protocol. The hardware interfaces for obtaining the message information include UART, I2C and CAN.

3. A GPS time synchronization system based on a digital seismograph according to claim 1 or 2, characterized in that: The circuit of the GPS receiver includes a power supply part, an antenna part and a data receiving part. The power supply part includes a voltage VDD5, a PMOS tube, a low-voltage difference linear regulator U25, a voltage VDD3.3, a resistor R211, a capacitor C211, a capacitor C212, a capacitor C213, a capacitor C214 and a capacitor C215. The gate of the PMOS tube is electrically connected to the PD13 pin of the MCU to control the conduction and cutoff of the PMOS tube to turn on or off the power supply of the GPS receiver. The source of the PMOS tube is electrically connected to the voltage VDD5, and the drain of the PMOS tube is electrically connected to the VCC pin of the low-voltage difference linear regulator U25. When the PD13 pin outputs a low level, the source and drain of the PMOS tube are turned on, and the drain of the PMOS tube is grounded via the capacitor C211 and the capacitor C212 arranged in parallel, and the voltage VDD5 is electrically connected to the PD13 pin of the MCU via the resistor R211; The VCC pin of the low voltage drop linear regulator U25 is electrically connected to the EN pin, the GND pin is grounded, the BP pin is grounded via a capacitor C213, the VOUT pin is electrically connected to the voltage VDD3.3, and the VOUT pin is grounded via capacitors C214 and C215 arranged in parallel; The antenna part includes an antenna E211, an antenna E212, a signal amplifier U27, a capacitor C216, a capacitor C217 and an inductor L212, a VCC pin of the signal amplifier U27 is electrically connected to a voltage VDD3.3 pin and is grounded via a capacitor C217, a SHDN pin is electrically connected to a VCC pin, an RFOUT pin is electrically connected to an antenna E211 via a capacitor C216, an RFIN pin is electrically connected to an antenna E212 via an inductor L212, and a GND pin is grounded; The data receiving part includes a GPS module U26, a voltage VDD3.3, an inductor L211, a diode D211, a resistor R212, a resistor R213 and a capacitor C218. The VCC-RF pin of the GPS module U26 is used as a power supply pin and is electrically connected to the antenna E211 via the inductor L211, the GND pin is grounded, the RF-IN pin is electrically connected to the antenna E211, the PPS pin is electrically connected to the PD15 pin of the MCU, the VDD-USB pin is electrically connected to the voltage VDD3.3, the UOUT0 pin is electrically connected to the PC6 pin of the MCU via the resistor R212, the UIN0 pin is electrically connected to the PC7 pin of the MCU via the resistor R213, the VCC pin is electrically connected to the VDD3.3 pin and is grounded via the capacitor C218, and the voltage VDD3.3 is electrically connected to the V-BCKP pin via the diode D211.

4. A GPS time synchronization system based on a digital seismograph according to claim 1 or 2, characterized in that: The calculation formula of the internal frequency of the RTC clock is: Where f CK_SPRE is the RTC internal frequency (Hz), f RTCCLK is the RTC clock input source frequency (Hz), prediv_S is the synchronous prescaler factor, and prediv_A is the asynchronous prescaler factor.

5. A GPS time synchronization system based on a digital seismograph according to claim 1 or 2, characterized in that: The RTC clock uses a temperature compensated crystal oscillator TCXO as a stable external clock source. The temperature compensated crystal oscillator TCXO circuit includes a voltage VDD3, an active crystal oscillator Y3, an inductor L7, a capacitor C116, a capacitor C117, a capacitor C118, a resistor R117 and a resistor R116. The voltage VDD3 is electrically connected to the VDD pin of the active crystal oscillator Y3 via the inductor L7, the VDD pin is grounded via the capacitors C116 and C117 arranged in parallel, the VDD pin is electrically connected to the OUT pin via the resistor R117, the OUT pin is grounded via the capacitor C118, the OUT pin is electrically connected to the PC14 pin of the MCU via the resistor R116, and the GND pin is grounded.

6. A GPS time synchronization system based on a digital seismograph according to claim 1 or 2, characterized in that: The UTC time parsed by the GPS receiver is used as the standard timeline, and the seismic data time information of the seismograph is synchronized with the standard timeline. After the seismic data is collected by the data acquisition unit of the seismograph, it is stored in the standard seismic data format of MiniSEED. The format is mainly divided into a frame header information part and a seismic data part. The frame header information part contains the precise acquisition timestamp of the first point data of the current data frame and the sampling rate information of the data frame. The acquisition time of other data points of a single data frame except the first data point is calculated according to the following formula: Where T N is the sampling time (s) of the Nth data point in the frame, T1 is the exact sampling time (s) of the first data point in the frame, N is the number of data points in the frame, and f Sample is the sampling rate (SPS) of the data points in the frame, where f Sample Configuration is performed through the analog-to-digital conversion chip in the data acquisition unit.

7. A GPS time synchronization system based on a digital seismograph according to claim 6, characterized in that: The sampling rate configuration method of the analog-to-digital conversion chip is: Step S1: The ADSBUS_IOInitial function configures the IO port of the three-channel bus multiplexer, and executes step S2; Step S2: The ADS_IOInitial function configures the SYNC, DRDY and SPI bus related pins of the analog-to-digital conversion chip in the three-channel bus, and executes step S3; Step S3: ADS_IOInitial function configures the SPI peripheral of MCU and enables it, and then executes step S4; Step S4: dynamically apply for memory to store MiniSEED format seismic data, and execute step S5; Step S5: ADS_IO_enable function turns on the external crystal oscillator source of the analog-to-digital conversion chip, and then executes step S6; Step S6: The ADSBUS_ResetByPin function resets each pin of the analog-to-digital conversion chip, and then executes step S7; Step S7: The ADSBUS_muxSwitch(i) function controls the multiplexer to select the i-th channel, and executes step S8; Step S8: the ADS_cfgReg(Data_Rate) function writes the sampling rate Data_Rate into the cache unit adc_regdata according to the definition format of the DR[2:0] bit in the CONFIG0 register of the analog-to-digital conversion chip, and executes step S9; Step S9: the ADS_cfgReg(Data_Rate) function writes other acquisition parameters into the corresponding positions of adc_regdata according to the bit definition format of the CONFIG0 register, and executes step S10; Step S10: the Write_ADC_REG_Sequence function writes the one-byte content of adc_regdata into the register unit with the address 0x01 in the analog-to-digital conversion chip, and then executes step S11; Step S11: Determine whether all three channels are configured, if yes, execute step S12, if no, execute step S7; Step S12: The ADSBUS_SYNC function triggers the synchronization signal of the analog-to-digital conversion chip.

8. A GPS time synchronization system based on a digital seismograph according to claim 1, 2 or 7, characterized in that: The method for the GPS receiver to obtain and analyze message information is as follows: Step S1: Find the GNGGA identifier in the message, point the parsing pointer to the GNGGA message, and execute step S2; Step S2: Move the parsing pointer to the first comma in the message and return the offset value, and then execute step S3; Step S3: Determine whether the offset value is non-0xFF, if yes, execute step S4, if no, execute step S15; Step S4: convert the UTC time in string format into a digital format without decimal points and store it in the cache, and then execute step S5; Step S5: Move the parsing pointer to the second and third commas in the message and return the offset value, and then execute step S6; Step S6: Determine whether the offset value is non-0xFF, if yes, execute step S7, if no, execute step S16; Step S7: convert the latitude information in the string format into a digital format and store it in a cache, and then execute step S8; Step S8: Move the parsing pointer to the 4th and 5th commas in the message and return the offset value, and then execute step S9; Step S9: Determine whether the offset value is non-0xFF, if yes, execute step S10, if no, execute step S17; Step S10: convert the longitude information in the string format into a digital format and store it in a cache, and then execute step S11; Step S11: determine whether the GNGGA parsing valid bit is 1, if yes, execute step S12, if no, jump to step S1; Step S12: parse the GNRMC message and execute step S13; Step S13: Determine whether the GNRMC parsing valid bit is 1, if yes, execute step S14, if no, jump to step S1; Step S14: end; Step S15: if the valid bit is 0, jump to step S5; Step S16: if the valid bit is 0, jump to step S8; Step S17: parse the valid bit to be 0, and jump to step S11.

9. A GPS time synchronization system based on a digital seismograph according to claim 8, characterized in that: The method for timing the RTC clock is: Step S1: Initialize the microsecond counter and execute step S2; Step S2: Initialize the GPS receiver and UART, and execute step S3; Step S3: Determine whether the GPS message is valid, if valid, execute step S4, if invalid, execute step S3; Step S4: parse the GPS message, longitude and latitude and time information, which takes time t0, and then execute step S5; Step S5: Determine whether the millisecond bit of the parsed time information is 0, if yes, execute step S6, if no, execute step S3; Step S6: Save the current time information T1, notify the PPS to interrupt and set the timing flag position to 1, and execute step S7; Step S7: End.

10. A GPS time synchronization system based on a digital seismograph according to claim 9, characterized in that: The method of PPS second pulse trigger interrupt is: Step S1: Determine whether the PPS second pulse can be used for timing flag is set to 1, if yes, execute step S2, if not, execute step S5; Step S2: Set the RTC time to T2, T2=T1+1, the timing time is t1, and execute step S3; Step S3: The microsecond counter sets the initial value after time compensation, starts counting upward, and executes step S4; Step S4: clear the flag bit that can perform timing service to 0, and execute step S5; Step S5: Return.

Images