A real-time clock synchronization method and a corresponding electronic system

Abstract

A two-stage synchronization method is provided for synchronizing real-time clocks (RTC) of a plurality of electronic units included in an electronic system, each unit comprising its own RTC, and for simultaneously calibrating the RTCs according to an external reference pulse-per- second (PPS) signal. The method comprises a calibration process performed by a first unit (1) comprising its own RTC and configured to receive an external reference PPS signal (PPS0) from an external clock signal source, and a synchronization process performed by each second unit (2) receiving an internal reference PPS signal (PPS1) generated by the first unit (1). In the calibration process, the first unit (1) calibrates the internal reference PPS signal (PPS1) according to the external reference PPS signal (PPS0). In the synchronization process, each second unit (2) synchronizes its respective RTC with the internal reference PPS signal (PPS1) received from the first unit (1).

Description

[0001] DESCRIPTION

[0002] A REAL-TIME CLOCK SYNCHRONIZATION METHOD AND A CORRESPONDING ELECTRONIC SYSTEM

[0003] Relevant technical field

[0004] The present invention relates to an RTC synchronization method developed to ensure synchronization of real-time clocks (RTC) of units within an electronic system with each other, and to an electronic system in which said method is implemented.

[0005] State of the art

[0006] In most embedded systems, real-time clock (RTC) accuracy is achieved by means of 32.768 kHz crystal oscillators. These oscillators have limited accuracy due to reasons such as various manufacturing tolerances in the system, temperature deviations, or drifts caused by aging of the system. Therefore, in order to obtain an RTC with high precision, various calibration and compensation mechanisms need to be implemented. Temperature-compensated crystal oscillators (TCXO) have been developed for applications requiring more precise temperature stability and provide compensation for temperature-induced drifts. However, temperature-compensated crystal oscillators currently in use have an error margin of approximately ±1 ppm. This corresponds to a deviation of about 1 microsecond per second and therefore approximately 2.5 seconds per month. Particularly in electronic systems comprising multiple units that each perform different tasks and that operate in coordination with one another with precise timing, such as satellite systems, it is an important requirement that the RTCs of said units be synchronized with each other with high precision.

[0007] Various implementations for RTC calibration exist in the prior art. Patent document US2014247072A1 discloses an RTC correction circuit. Said circuit comprises an internal unit that collects temperature data of the chip. When it is detected that the chip is in a power-off state, a frequency division multiplier is calculated by using the temperature data of the chip, and a timer pulse signal for the RTC is generated using said multiplier.

[0008] Patent document US6630872B1 discloses a temperature-based RTC compensation method developed to obtain a corrected clock signal. In said method, a thermal model of the oscillator is used together with temperature data obtained by a temperature sensor.

[0009] Patent document US7797118B2 discloses an RTC calibration method and system. Said method comprises the steps of generating a fast clock signal and a slow clock signal from an uncompensated clock signal, detecting frequency errors of the clock signal, selectively and instantaneously replacing the uncompensated clock signal with the fast clock signal or the slowclock signal by means of a switching circuit, and thereby generating a compensated clock signal.

[0010] In such implementations, the use of additional clocks and / or additional circuits for adjusting the uncompensated clock is mentioned. This results in an increase in power consumption and space usage of the system. In such implementations, the digital circuit is generally synchronized using a higher-speed clock, which increases the required current. The implementations known in the prior art are directed to increasing the precision of existing RTCs and cannot provide a solution for synchronizing the RTCs of all components of an electronic system with each other.

[0011] Object of the Invention

[0012] An object of the present invention is to develop a method that enables synchronization of the real-time clocks (RTC) of units within an electronic system with each other, and an electronic system in which said method is implemented.

[0013] Another object of the present invention is to develop a two-stage method in which RTC calibration is performed based on an external reference clock signal and, in accordance with said calibration, the RTC signals of the units included in the system are synchronized; and an electronic system in which said method is implemented.

[0014] Definition of the figures

[0015] Exemplary applications of the RTC synchronization method and the electronic system developed by the present invention are shown in the accompanying drawings, and from these figures;

[0016] Figure 1 is a schematic representation of said electronic system.

[0017] Figure 2 is an exemplary flow diagram illustrating the calibration stage of the method.

[0018] Figure 3 is an exemplary illustration of the synchronization stage of the method. The elements shown in the figures are represented symbolically, and the reference numerals corresponding to said elements are as follows:

[0019] First unit (1)

[0020] Second unit (2)

[0021] First processing unit (11)

[0022] Second processing unit (I2)

[0023] External reference PPS signal (PPSO)Internal reference PPS signal (PPS1)

[0024] Time difference (Terence)

[0025] Threshold time value (Threshold)

[0026] First RTC (RTC1)

[0027] Second RTC (RTC2)

[0028] Timer (T)

[0029] Second signal (S)

[0030] Description of the invention

[0031] By means of the present invention, a two-stage synchronization method is developed which enables synchronization of the RTCs of a plurality of electronic units included in an electronic system, each unit comprising its own real-time clock (RTC - Real Time Clock), with each other, and which simultaneously enables calibration of said RTCs according to an external reference “pulse-per-second” signal (PPS - Pulse Per Second). Said method comprises a calibration process performed by a first unit (1) comprising its own RTC and configured to receive an external reference PPS signal (PPSO) originating from an external clock signal source, preferably a GNSS signal, and a synchronization process performed by each second unit (2) receiving an internal reference PPS signal (PPS1) generated by the first unit (1), wherein said second unit (2) synchronizes its own RTC with the internal reference PPS signal (PPS1) received from the first unit. The aforementioned processes are illustrated in Figures 2 and 3. An exemplary embodiment of the system according to the invention is illustrated in Figure 1. As shown in Figure 1, the first unit (1) comprises at least one first processing unit (11) and at least one first RTC (RTC1). Each of the second units (2) comprises at least one second processing unit (I2), at least one second RTC (RTC2), and at least one timer (T).

[0032] Figure 2 illustrates the calibration stage of the method. The first processing unit (11) included in the first unit (1) is configured to continuously generate an internal reference PPS signal (PPS1) and to transmit said signal to a plurality of second units (2) included in the electronic system. It is intended that the second RTCs (RTC2) included in each of the second units (2) be synchronized with each other and with an external reference time signal. The first unit (1) is configured to receive a signal originating from a source of an external reference PPS signal (PPSO). The first unit (1) continuously generates the internal reference PPS signal (PPS1) and simultaneously periodically checks whether an external reference PPS signal (PPSO) has been received. When the first unit (1) receives an external reference PPS signal (PPSO), it calibrates its own RTC (RTC1) according to the received signal and compares the internal reference PPS signal (PPS1) generated by itself with the external reference PPS signal (PPSO), and calibratesthe internal reference PPS signal (PPS1) so as to ensure that the two signals coincide with each other, and transmits the generated internal reference PPS signal (PPS1) to the second units (2). As long as no external reference PPS signal (PPSO) is received, the first unit (1) continues generating the internal reference PPS signal (PPS1) and transmitting it to the second units (2). The aforementioned operations are carried out by the first processing unit (11) included in the first unit (1).

[0033] Each of the second units (2) comprises a second processing unit (I2) configured to synchronize its respective second RTC (RTC2) with the received internal reference PPS signal (PPS1), independently of and without awareness of the aforementioned calibration process. Figure 3 provides an exemplary illustration of the synchronization process performed by the second units (2). Accordingly, each of the second processing units (I2) compares, the received internal reference PPS signal (PPS1) with a second signal (S) generated by the second RTC (RTC2) by using the timer (T); determines the time difference (Tdifference) between the two signals, and compares the determined time difference (Tdifference) with a threshold time value (Tthreshoid). If the time difference (Tdifference) is below the threshold time value (Tthreshoid), no operation is performed. If the determined time difference (Tdifference) exceeds the threshold time value (Tthreshoid), the second signal (S) is shifted.

[0034] If the second processing unit (I2) determines during the comparison that the starting point of the internal reference PPS signal (PPS1) precedes the second signal (S), the second signal (S) is shifted backward; otherwise, it is shifted forward. In this manner, synchronization of the two signals is achieved. The aforementioned control and shifting operation is performed once per second. The shifting operation is achieved by increasing or decreasing the number of clock signals between two consecutive second signals (S).

[0035] In an exemplary embodiment of the invention, one second consists of 32,768 clock signals, and the second RTC (RTC2) generates one second signal (S) for every 32,768 clock signals. The second processing unit (I2) compares the internal reference PPS signal (PPS1) with said second signal (S). If the internal reference PPS signal (PPS1) precedes the second signal (S) and the time difference between them exceeds the threshold time value (Tthreshoid), it is determined that the RTC signal is slow, and in order to accelerate it, the number of clock signals is reduced by one to 32,767; that is, the next second signal (S) is generated at the 32,767th clock signal. If it is determined that the internal reference PPS signal (PPS1) begins after the second signal (S) and that the time difference between them exceeds the threshold time value (Tthreshoid), it is determined that the RTC signal is fast, and in order to slow it down, the number of clock signals is increased by one to 32,769; that is, the next second signal (S) is generated at the 32,769th clock signal. The comparison is repeated once per second. If,during the subsequent comparison, the difference still exceeds the threshold value, the same operation is repeated, thereby maintaining the difference between the two signals, within the threshold time value (Tthreshoid) window and thereby maintaining synchronization between the internal reference PPS signal (PPS1) and the second signal (S).

[0036] The electronic system developed according to the present invention comprises at least one first unit (1) comprising at least one first RTC (RTC1) and at least one first processing unit (11) associated with said first RTC (RTC1), said first processing unit (11) being configured to: continuously generate an internal reference PPS signal (PPS1);

[0037] check whether an external reference PPS signal (PPSO) is received from a source of an external reference PPS signal (PPSO);

[0038] if an external reference PPS signal (PPSO) is received, calibrate the internal reference PPS signal (PPS1) according to the external reference PPS signal (PPSO);

[0039] and a plurality of second units (2), each comprising at least one second RTC (RTC2), at least one timer (T), and at least one second processing unit (I2) associated with said second RTC (RTC2) and timer (T), said second processing unit (I2) being configured to:

[0040] continuously receive the internal reference PPS signal (PPS1);

[0041] determine, once per second, the time difference (Tdifference) between the internal reference PPS signal (PPS1) and the second signal (S) generated by the second RTC (RTC2) by using said timer (T), and compare the determined time difference (Tdifference) with a threshold time value (Tthreshoid) ;

[0042] if the time difference (Tdifference) exceeds the threshold time value (Tthreshoid) :

[0043] o decrease the number of clock signals between two second signals (S) by one if the internal reference PPS signal (PPS1) precedes the second signal (S); o increase the number of clock signals between two second signals (S) by one if the internal reference PPS signal (PPS1) follows the second signal (S);

[0044] if the time difference (Tdifference) is below the threshold time value (Tthreshoid) , return to the step of determining the time difference (Tdifference) between the internal reference PPS signal (PPS1) and the next second signal (S).

[0045] In a preferred embodiment of the invention, the electronic system in question is a satellite electronic system and enables the RTCs of the satellite components included in the system to be synchronized with each other and simultaneously calibrated according to a GNSS signal.The electronic system comprises a plurality of second units (2) configured to perform various functions executed on the satellite. The first unit (1) is preferably an Electrical Power System (EPS - Electrical Power System). The second units (2) may include, but are not limited to, communication units, antenna feed units, imaging units, control units, and similar units. The external reference PPS signal (PPSO) is obtained from a GNSS and cannot be received continuously. Therefore, even in situations where an external reference time signal cannot be received, an internal reference PPS signal is continuously generated by the first unit (1), said signal is transmitted to all second units (2), and synchronization of all second units (2) with said signal is ensured. When an external reference time signal is received from the GNSS, the internal reference PPS signal (PPS1) generated by the first unit (1) is calibrated using the external reference PPS signal (PPSO). Thus, synchronization among the satellite units is maintained even during periods when GNSS signals are unavailable.

[0046] During the synchronization process performed by the second units, the time difference between the clock signal of the second RTC (RTC2) and the internal reference PPS signal (PPS1) is determined by the timer (T). The timer (T) is preferably integrated into the second processing unit (I2) or at least operatively connected to the second processing unit (I2). The timer (T) preferably has a high resolution, thereby enabling detection of small differences between the two signals. Said timer preferably has a higher resolution than the RTC. However, it is not required that its precision be as high as that of the RTC. By means of the method according to the invention, RTCs having high precision (preferably about 1 ppm) can be calibrated by using timers (T) having relatively lower precision but higher resolution. Thus, high-precision synchronization can be achieved without requiring high-precision timers in each unit.

Claims

CLAIMS1. An electronic system comprising:at least one first unit (1) comprising at least one first RTC (RTC1) and at least one first processing unit (11) associated with said first RTC (RTC1), said first processing unit (11) being configured to:generate an internal reference PPS signal (PPS1);check whether an external reference PPS signal (PPSO) is received from a source of an external reference PPS signal (PPSO); andif an external reference PPS signal (PPSO) is received, calibrate the internal reference PPS signal (PPS1) according to the external reference PPS signal (PPSO);anda plurality of second units (2), each comprising at least one second RTC (RTC2), at least one timer (T), and at least one second processing unit (I2) associated with said second RTC (RTC2) and said timer (T), said second processing unit (I2) being configured to:receive said internal reference PPS signal (PPS1);determine, once per second, a time difference (Tdifference) between the internal reference PPS signal (PPS1) and a second signal (S) generated by the second RTC (RTC2) by using said timer (T), and compare the determined time difference (Tdifference) with a threshold time value (Tthreshoid);if the time difference (Tdifference) exceeds the threshold time value (Tthreshoid):o decrease the number of clock signals between two consecutive second signals (S) by one if the internal reference PPS signal (PPS1) precedes the second signal (S); oro increase the number of clock signals between two consecutive second signals (S) by one if the internal reference PPS signal (PPS1) follows the second signal (S);if the time difference (Tdifference) is below the threshold time value (Tthreshoid), return to the step of determining the time difference (Tdifference) between the internal reference PPS signal (PPS1) and a next second signal (S).

2. The electronic system according to claim 1, wherein the timer (T) is integrated into the second processing unit (I2).

3. The satellite electronic system according to claim 1 or 2, wherein the first unit (1) is an electrical power system (EPS).

4. The satellite electronic system according to claim 3, wherein the second unit (2) comprises at least one of communication units, antenna feed units, imaging units, and control units of the satellite.

5. An RTC synchronization method performed in an electronic system according to any one of claims 1 to 4, comprising:the following steps carried out by a first processing unit (11) included in a first unit (1): continuously generating an internal reference PPS signal (PPS1) and simultaneously, periodically checking whether an external reference PPS signal (PPSO) is received; if an external reference PPS signal (PPSO) is received, calibrating the first RTC (RTC1) according to the received signal and calibrating the internal reference PPS signal (PPS1) such that the internal reference PPS signal (PPS1) coincides with the external reference PPS signal (PPSO);as long as no external reference PPS signal (PPSO) is received, generating the internal reference PPS signal (PPS1) and transmitting the generated internal reference PPS signal (PPS1) to second units (2);and the following steps carried out by a second processing unit (I2) included in a second unit (2):comparing the received internal reference PPS signal (PPS1) with a second signal (S) generated by the second RTC (RTC2);determining a time difference (Tdifference) between the two signals;comparing the determined time difference (Tdifference) with a threshold time value (Tfhreshold),performing no operation if the time difference (Tdifference) is below the threshold time value (Tthreshoid); andshifting the second signal (S) if the time difference (Tdifference) exceeds the threshold time Value (Tfhreshold).

6. The method according to claim 5, wherein the external reference PPS signal (PPSO) is a GNSS signal.

7. The method according to claim 5 or 6, wherein the second processing unit (I2), if it determines that a starting point of the internal reference PPS signal (PPS1) precedes the second signal (S), shifts the second signal (S) backward, and otherwise shifts the second signal (S) forward.

8. The method according to claim 7, wherein the shifting operation is achieved by increasing or decreasing the number of clock signals between two consecutive second signals (S).

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