Pyroelectric infrared detector and integrated circuit

Abstract

The pyroelectric infrared detector (10) contains a pyroelectric photoelectric conversion element (21) and an integrated circuit (25) for processing a detection signal from the pyroelectric photoelectric conversion element in a package (11), wherein the integrated circuit comprises: a conversion unit (27) for performing analog-to-digital conversion of the detection signal; a signal processing unit (29) for performing signal processing based on a digital filter on the converted detection signal; and an input / output control unit (32) for outputting output data from the signal processing unit to an external device (15) via serial data communication. The cutoff frequency in the digital filter can be set and changed by applying a voltage to the integrated circuit or by a command input from an external device via serial data communication.

Description

Technical Field

[0001] The present invention relates to infrared (IR) detection, and in particular to a pyroelectric infrared detector and an integrated circuit (IC) having a built-in integrated circuit. Background Art

[0002] In recent years, with the development of wireless communication technology, wireless communication functions have been installed in various electronic devices, and the popularity of devices that have wireless communication functions and can be miniaturized and battery-driven has also been remarkable. In such electronic devices, for the purpose of saving power, most of them are equipped with pyroelectric infrared detectors so that the electronic device is driven only when a person or animal, etc., who is the detection object of the electronic device, is present. The pyroelectric infrared detector is configured with a pyroelectric photoelectric conversion element inside a package having a window that transmits infrared rays. The pyroelectric infrared detector installed in such an electronic device is mostly built-in with an integrated circuit as disclosed in patent documents 1 to 4. By building in an integrated circuit, the electronic device equipped with the pyroelectric infrared detector can be miniaturized, and a signal processing circuit can be configured inside the pyroelectric infrared detector composed of a metal package to reduce the influence of external electromagnetic noise. In addition to devices with wireless communication functions, pyroelectric infrared detectors with built-in integrated circuits are also used in a variety of devices and systems.

[0003] Among the integrated circuits built into the pyroelectric infrared detector, for example, there is a type that has the following elements: a conversion unit that performs analog-to-digital conversion on the detection signal obtained from the pyroelectric photoelectric conversion element; and a signal processing unit that performs noise removal and signal processing on the detection signal converted into a digital signal. Noise removal is performed, for example, by digital filtering using a fixed cutoff frequency. Another type of integrated circuit has: an amplifier that amplifies the detection signal obtained from the pyroelectric photoelectric conversion element with a fixed amplification gain; a conversion unit that performs analog-to-digital conversion on the amplified detection signal, performs noise removal processing and calculation on the detection signal converted into a digital signal according to a certain rule, and outputs the result to the outside.

[0004] As an output method of the integrated circuit, there are the following methods: converting into a digital signal inside the pyroelectric infrared detector, and then applying a fixed detection judgment threshold to the signal-processed detection signal to judge whether a detection object such as a human body or an animal is detected, and only outputting the information of detection or non-detection as a binary signal. Alternatively, sometimes the detection signal converted into a digital signal and further processed is output according to the proprietary interface standard of the manufacturer of the pyroelectric infrared detector. By adopting these output methods, the external device connected to the pyroelectric infrared detector can receive and use the detection signal in a digital form without setting up an analog circuit for processing the detection signal.

[0005] Prior Art Literature

[0006] Patent Literature

[0007] Patent Document 1: Japanese Patent Application Publication No. 2012-225763

[0008] Patent Document 2: Japanese Patent Application No. 2013-524178

[0009] Patent Document 3: Japanese Patent Application Publication No. 2016-70694

[0010] Patent Document 4: Japanese Patent Application Publication No. 2016-186478 Summary of the invention

[0011] Problems to be solved by the invention

[0012] In a pyroelectric infrared detector, it is necessary to filter the detection signal based on the output of the pyroelectric photoelectric conversion element to extract only the signal component of the detection object. The pyroelectric infrared detector detects various objects such as human bodies, animals, machines or flames with different moving speeds and infrared wavelengths of radiation as detection objects. Therefore, in order to extract only the signal component of the detection object, it is necessary to set an appropriate cutoff frequency in the filtering process according to the detection object and the purpose. However, the cutoff frequency of the conventional digital output type pyroelectric infrared detector is fixed, so it is sometimes impossible to use it depending on the purpose. In addition, in the case of connecting multiple pyroelectric infrared detectors to an external device including an external integrated circuit, the conventional digital output type pyroelectric infrared detector needs to set corresponding input and output (I / O) ports for each pyroelectric infrared detector separately.

[0013] The object of the present invention is to provide a pyroelectric infrared detector that can cope with various detection objects and uses and can prevent an increase in the number of input and output terminals in an external device that serves as a connection target even when multiple pyroelectric infrared detectors are used; and an integrated circuit that can be used in such a pyroelectric infrared detector.

[0014] Means for solving problems

[0015] The pyroelectric infrared detector of the present invention is characterized in that it comprises: a package; one or more thermoelectric photoelectric conversion elements, which are housed in the package; and an integrated circuit, which is housed in the package and processes the detection signal based on the output of the thermoelectric photoelectric conversion element, the integrated circuit comprising: a conversion unit, which performs analog-to-digital conversion of the detection signal; a signal processing unit, which performs signal processing based on a digital filter on the detection signal converted into a digital signal; and an input-output control unit, which outputs the output data from the signal processing unit to an external device via serial data communication, and the cutoff frequency in the digital filter can be set and changed by applying a voltage to the integrated circuit or by a command input from an external device via serial data communication.

[0016] The integrated circuit of the present invention performs input and signal processing of detection signals based on the output of a pyroelectric infrared detector, and is characterized in that the integrated circuit comprises: a conversion unit, which performs analog-to-digital conversion of the detection signal; a signal processing unit, which performs signal processing based on a digital filter on the detection signal converted into a digital signal; and an input-output control unit, which outputs the output data from the signal processing unit to an external device via serial data communication, and can set and change the cutoff frequency in the digital filter by applying a voltage to the integrated circuit or by a command input from an external device via serial data communication.

[0017] Effects of the Invention

[0018] In the present invention, since the cutoff frequency in the digital filter can be set and changed by applying a voltage to the integrated circuit or by a command input from an external device via serial data communication, one pyroelectric infrared detector can be adapted to various detection objects and uses. In addition, since the pyroelectric infrared detector can communicate with the external device via serial data communication, by using a technology that is commonly used in serial data communication and can connect multiple devices on a single bus line, even when multiple pyroelectric infrared detectors are used, the number of input and output terminals in the external device that is the connection target can be prevented from increasing. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a diagram showing the structure of the pyroelectric infrared detector according to the first embodiment.

[0020] Figure 2 This is a graph showing an example of frequency characteristics in digital filter processing.

[0021] Figure 3 It is a timing chart for explaining the operation of the maximum value holding unit.

[0022] Figure 4 It is a timing chart for explaining the operation of the detection and determination unit.

[0023] Figure 5 It is a diagram for explaining the connection of a plurality of pyroelectric infrared detectors in the second embodiment. DETAILED DESCRIPTION

[0024] [First embodiment]

[0025] Next, embodiments of the present invention will be described with reference to the drawings. Figure 1 The internal structure of the pyroelectric infrared detector according to the first embodiment of the present invention is shown. The pyroelectric infrared detector 10 is used to detect a detection object and output the detection result to an external device 15. The external device 15 is provided outside the pyroelectric infrared detector 10, and is, for example, an integrated circuit or various devices.

[0026] The pyroelectric infrared detector 10, for example, contains one or more thermoelectric photoelectric conversion elements 21 and an integrated circuit 25 for processing detection signals obtained from the thermoelectric photoelectric conversion elements 21 in an airtight manner inside a metal package 11. In the package 11, a window 12 for transmitting infrared rays is formed inside the package 11, and the thermoelectric photoelectric conversion element 21 is configured to receive infrared rays transmitted through the window 12. In the example shown in the figure, one thermoelectric photoelectric conversion element 21 is provided. A resistor 22 having a high resistance value is connected in parallel to the thermoelectric photoelectric conversion element 21. Furthermore, an impedance conversion element 23 is connected to one end of the thermoelectric photoelectric conversion element 21, and a filter circuit 24 as an analog low-pass filter is provided between the impedance conversion element 23 and the integrated circuit 25 in order to reduce high-frequency noise. In the example shown in the figure, a source follower circuit based on a field effect transistor is used as the impedance conversion element.

[0027] The integrated circuit 25 includes an amplifier 26 for amplifying a detection signal input from the pyroelectric photoelectric conversion element 21 via the impedance conversion element 23 and the filter circuit 24; a conversion unit 27 for performing analog-to-digital conversion on the amplified detection signal; a signal processing unit 29 for performing digital filtering on the detection signal converted into a digital signal; and an input / output control unit 32 for outputting output data from the signal processing unit 29 to the external device 15 by serial data communication. In the example shown in the figure, two input / output terminals 33 and 34 called "Pin A" and "Pin B" are provided in the pyroelectric infrared detector 10 for serial data communication, and both the input / output terminals 33 and 34 are connected to the input / output control unit 32. When the detection signal output from the pyroelectric photoelectric conversion element 21 is large enough and a ΔΣ type converter is used for analog-to-digital conversion, the amplifier 26 is not necessarily required. Furthermore, the pyroelectric infrared detector 10 is provided with a power supply terminal 35 for supplying a power supply voltage VDD to the integrated circuit 25 , and a ground terminal 36 for connecting the integrated circuit 25 to a ground potential GND and electrically connecting the package 11 .

[0028] Here, the serial data communication for connection to the external device 15 is described. In the present invention, various methods such as synchronous serial data communication and asynchronous serial data communication can be used as the communication method of serial data communication, but it is preferable to use a communication method that can connect multiple slave devices to a bus line used for serial data communication. As an example of such a method, there is an I2C (Inter-Integrated circuit) method. Sometimes "I2C" is recorded as "I 2 C". When the I2C method is used, the external device 15 is used as a master device, the pyroelectric infrared detector 10 is used as a slave device, one of the input and output terminals 33 and 34 is used as a terminal for bidirectional serial data transmission, and the other is used as a terminal for receiving a serial clock (SCL) from the master device. As a device other than the I2C method, there is a device that uses one of the input and output terminals 33 and 34 for serial data communication and uses the other for transmission of a trigger signal or an interrupt signal from the pyroelectric infrared detector 10 to the external device 15.

[0029] The purpose of the digital filtering process in the signal processing unit 29 is to extract only the component of the detection object from the detection signal of the pyroelectric photoelectric conversion element 21. The sudden noise caused by external electromagnetic noise or rapid ambient temperature change is also removed by the digital filtering process. As the detection object of the pyroelectric infrared detector 10, various objects such as human body, animal, machine or flame are considered. Since their moving speed and wavelength of infrared radiation are different, if the cutoff frequency in the digital filtering process is not set or changed in accordance with the purpose of the pyroelectric infrared detector 10, the detection object may not be properly detected. Therefore, the pyroelectric infrared detector 10 of this embodiment is configured to be able to set and change the cutoff frequency in the digital filtering process by applying a voltage to the adjustment input pin (not shown) provided in the integrated circuit 25, or by a command input to the input / output control unit 32 by serial data communication from the external device 15. In the case of setting and changing the cutoff frequency by applying a voltage to the integrated circuit 25, it can be configured to be able to set and change the cutoff frequency in several stages according to the applied voltage. When the serial data communication is in the I2C mode, it is easy to send and receive commands for setting and changing the cutoff frequency, and it is possible to set any cutoff frequency characteristics based on the commands. Figure 1 In the example shown, a command transmitted by serial data communication of the I2C system is converted into a set value in the input / output control unit 32 and supplied to the signal processing unit 29 .

[0030] Figure 2 1 shows an example of the frequency characteristics of the digital filter implemented by the signal processing unit 29. As shown in the figure, it is preferable to be able to independently set and change the cut-off characteristic 41 on the low frequency side and the cut-off characteristic 42 on the high frequency side. By setting the cut-off characteristic 41 on the low frequency side and the cut-off characteristic 42 on the high frequency side according to the purpose of the pyroelectric infrared detector 10, the performance of detecting the detection object can be improved, and the false alarm caused by detecting an object that is not the detection object can be suppressed.

[0031] In the pyroelectric infrared detector 10, the level of the detection signal generated by the pyroelectric photoelectric conversion element 21 when receiving infrared light varies greatly, depending on the shape of the light-receiving electrode provided in the pyroelectric photoelectric conversion element 21 and the energy of the received infrared light. Therefore, when the dynamic range of the analog-to-digital conversion in the conversion unit 27 is too wide compared to the level of the detection signal, the resolution in the analog-to-digital conversion is effectively reduced. This leads to a reduction in detection performance when it is desired to detect small movements with little change in infrared energy or when it is desired to detect the detection object at an ambient temperature equal to the surface temperature of the detection object. In order to improve the detection performance, it is necessary that the dynamic range in the analog-to-digital conversion is suitable for the level of the detection signal and its change amount. Therefore, in the pyroelectric infrared detector 10 of the present embodiment, it is preferable to be able to change at least one of the amplification gain of the detection signal in the amplifier 26 and the dynamic range of the analog-to-digital conversion in the conversion unit 27 by applying a voltage to an adjustment input pin (not shown) provided in the integrated circuit 25, or by a command from the external device 15 input to the input / output control unit 32 by means of serial data communication. Figure 1 In the illustrated configuration, commands sent via I2C serial data communication are converted into set values ​​in the input / output control unit 32. The integrated circuit 25 is provided with a setting unit 28 that actually sets and changes the amplification gain of the amplifier 26 and the dynamic range of the converter 27 based on the set values.

[0032] By making it possible to variably adjust at least one of the amplification gain of the amplifier 26 and the dynamic range of the conversion unit 27, even for the detection signal when the energy of the infrared light incident on the pyroelectric photoelectric conversion element 21 is small or the movement of the detection object is small, the resolution in the analog-to-digital conversion will not be insufficient, and the detection signal can be converted into a digital signal regardless of the size of the detection signal without damaging the detection resolution. As a result, the detection performance can be maintained or improved, and high micro-motion detection performance relative to the detection object can be obtained.

[0033] When the I2C method is used as the communication method for serial data communication, the external device 15 becomes the master device and the pyroelectric infrared detector 10 becomes the slave device. The external device 15 makes a data request to the pyroelectric infrared detector 10, and the integrated circuit 25 of the pyroelectric infrared detector 10 sends the detection data to the external device 15 in response to the data request. Since the data request interval is determined by the software executed by the external device 15, when the data request interval is long, sometimes the detection signal of the detection object fails to be sent to the external device 15 side. When the pyroelectric infrared detector 10 is mounted on a battery-driven device, it is effective to reduce the frequency of data requests from the external integrated circuit as the external device 15 to the pyroelectric infrared detector 10 in order to achieve power saving of the device. However, by reducing the frequency of data requests, there is a possibility that the detection signal that is originally required is missed, that is, underreporting occurs. Power saving and underreporting are in a trade-off relationship. Therefore, it is preferable to provide a maximum value holding unit 30 in the integrated circuit 25 for holding the maximum amplitude of the detection signal processed by the signal processing unit 29 during a period when there is no data request and the direction of vibration of the detection signal at that time.

[0034] Figure 3 The operation of the maximum value holding unit 30 is described. Here, the time variation of the detection signal 43 processed by the signal processing unit is shown in a graph, but in fact, the detection signal 43 is in the form of digital data that varies with time. The maximum value 44 in the detection signal 43 is held in the maximum value holding unit 30, and when the data request 45 from the external device 15 is input, the maximum value data 46 is sent to the external device 15 at the timing of the data request 45. By setting the maximum value holding unit 30 that holds the maximum value of the detection signal in this way, even if the interval between the data requests from the external device 15 is long, it is possible to confirm the maximum value of the detection signal caused by the detection of the detection object, and it is possible to achieve power saving of the external device 15 and prevent missed reports. Here, the maximum value 44 when the detection signal 43 changes in the positive direction, that is, when it vibrates, is held in the maximum value holding unit 30, but when the detection object is detected, the detection signal 43 may also change in the negative direction, so the maximum amplitude in the positive direction or the maximum amplitude in the negative direction of the detection signal is set as an absolute value, and its absolute value and the direction of vibration of the detection signal at this time are held in the maximum value holding unit 30.

[0035] Regarding power saving of the device equipped with the pyroelectric infrared detector 10, it is effective to set the device to a dormant state in normal times and operate the device only when the pyroelectric infrared detector 10 determines that the detection object is detected. In order to realize such an operation, it is preferable that the integrated circuit 25 is provided with a detection determination unit 31 that compares the detection signal processed by the signal processing unit 29 with a threshold value to perform detection determination of the detection object, and when the detection determination unit 31 determines that the detection object is detected, a trigger signal or an interrupt signal is output to the external device 15 via the input / output control unit 32.

[0036] Figure 4 : is a diagram for explaining the operation of the detection and determination unit 31. The detection signal from the thermoelectric photoelectric conversion element 21 sometimes changes not only in the positive direction but also in the negative direction according to the direction of the movement of the detection object. Therefore, in the detection and determination unit 31, a threshold value 51 on the upper limit side and a threshold value 52 on the lower limit side are set for the detection signal 43 processed by the signal processing unit 29. The detection and determination unit 31 compares the detection signal 43 with the threshold values ​​51 and 52, and outputs a trigger signal 53 or an interrupt signal when the detection signal 43 exceeds the threshold value 51 on the upper limit side or is lower than the threshold value 52 on the lower limit side. The threshold values ​​51 and 52 are preferably changeable by applying a voltage to an adjustment input pin (not shown) provided in the integrated circuit 25, or by a command from the external device 15 input to the input / output control unit 32 by serial data communication. If the external device 15 connected to the pyroelectric infrared detector 10 is an integrated circuit or the like that serves as a connection interface with the pyroelectric infrared detector 10 in the device equipped with the pyroelectric infrared detector 10, the device can be restored from the dormant state and operated only when there is a trigger signal or an interrupt signal from the detection determination unit 31, thereby achieving power saving of the device equipped with the pyroelectric infrared detector 10. In the case where the maximum value holding unit 30 is provided in the integrated circuit 25, the external device 15 restored from the dormant state by the trigger signal or the interrupt signal can send a data request to the pyroelectric infrared detector 10, and obtain the absolute value of the maximum amplitude stored in the maximum value holding unit 30 and the direction of vibration of the detection signal.

[0037] [Second Embodiment]

[0038] The pyroelectric infrared detector 10 of the first embodiment can be connected to the external device 15 via a bus line of serial data communication, and therefore a plurality of pyroelectric infrared detectors 10 can be connected to one external device 15 . Figure 5An example of connecting a plurality of pyroelectric infrared detectors 10 based on the first embodiment to an external device 15 is shown. Here, the I2C method is used as a communication method for serial data communication. In the example shown in the figure, one external device 15 as a master device is connected to the bus line 54 of the I2C method, and three pyroelectric infrared detectors 10 as slave devices are connected. The bus line 54 is composed of a signal line of serial data (SDA) and a signal line of serial clock (SCL). Based on the I2C standard, a unique access address is set for each pyroelectric infrared detector 10. In order to facilitate individual identification when setting the pyroelectric infrared detector 10, the package 11 of the pyroelectric infrared detector 10 is printed or engraved so that the access address of the pyroelectric infrared detector 10 can be visually recognized. The number of pyroelectric infrared detectors 10 that can be connected to the bus line 54 at the same time is not limited to 3, and more pyroelectric infrared detectors 10 can be connected to the bus line 54. The external device 15 can individually identify the pyroelectric infrared detectors 10 connected to the bus line 54 and access a desired pyroelectric infrared detector 10 by using the access addresses of the pyroelectric infrared detectors 10 stored in advance.

[0039] Even when the I2C method is used as the communication method for serial data communication, when a detection object is detected in any pyroelectric infrared detector 10 connected to the bus line 54, the external device 15 can receive a trigger signal or an interrupt signal via the bus line 54, and can make a data request 45 to each pyroelectric infrared detector 10 by scanning the access address, and obtain the data held by the maximum value holding unit 30 of each pyroelectric infrared detector 10.

[0040] According to the second embodiment, in each pyroelectric infrared detector 10, it is possible to remove external electromagnetic noise and the like without damaging the quality of the detection signal from the pyroelectric photoelectric conversion element 21, and process the detection signal according to the purpose. Moreover, a plurality of such pyroelectric infrared detectors 10 can be connected to the same bus line 54. Therefore, as the external device 15, a cheap component with a small number of input and output terminals, i.e., a small number of pins, can be used.

[0041] -Description of Reference Numerals-

[0042] 10 Pyroelectric infrared detector

[0043] 11 Package

[0044] 12 Window

[0045] 15 External devices

[0046] 21 Thermoelectric photoelectric conversion element

[0047] 23 Impedance transformation element

[0048] 25 Integrated Circuits

[0049] 26 Amplifier

[0050] 27 Transformation

[0051] 28 Setting section

[0052] 29 Signal Processing Unit

[0053] 30 Maximum value holding unit

[0054] 31 Detection and judgment unit

[0055] 32 Input and output control unit

Claims

1. A pyroelectric infrared detector, characterized in that: have: Packages; One or more thermoelectric elements housed in the package; and an integrated circuit housed in the package and processing a detection signal based on an output of the thermoelectric element, The integrated circuit comprises: A conversion unit, performing analog-to-digital conversion of the detection signal; a signal processing unit that performs signal processing using a digital filter on the detection signal converted into a digital signal; and an input / output control unit that outputs the output data from the signal processing unit to an external device via serial data communication, The cutoff frequency in the digital filter can be set and changed by applying a voltage to the integrated circuit or by a command input from the external device via the serial data communication.

2. The pyroelectric infrared detector according to claim 1, wherein: The cutoff frequency on the high frequency side and the cutoff frequency on the low frequency side in the digital filter can be set separately.

3. The pyroelectric infrared detector according to claim 1 or 2, wherein: The dynamic range of the analog-to-digital conversion in the conversion section can be changed by applying a voltage to the integrated circuit or by a command input from the external device via the serial data communication.

4. The pyroelectric infrared detector according to claim 1 or 2, wherein: The integrated circuit includes an amplifier for amplifying the detection signal at a stage preceding the conversion unit. The amplification gain of the amplifier section can be changed by applying a voltage to the integrated circuit or by a command input from the external device via the serial data communication.

5. The pyroelectric infrared detector according to claim 3, wherein: The integrated circuit includes an amplifier for amplifying the detection signal at a stage preceding the conversion unit. The amplification gain of the amplifier section can be changed by applying a voltage to the integrated circuit or by a command input from the external device via the serial data communication.

6. The pyroelectric infrared detector according to claim 1 or 2, wherein: The integrated circuit comprises: a maximum value holding unit for holding an absolute value of a maximum amplitude of the detection signal processed by the signal processing unit and a direction of vibration of the detection signal corresponding to the absolute value; The integrated circuit transmits the current value of the detection signal and the maximum value held in the maximum value holding section to the external device in response to a data request from the external device.

7. The pyroelectric infrared detector according to claim 3, wherein: The integrated circuit comprises: a maximum value holding unit for holding an absolute value of a maximum amplitude of the detection signal processed by the signal processing unit and a direction of vibration of the detection signal corresponding to the absolute value; The integrated circuit transmits the current value of the detection signal and the maximum value held in the maximum value holding section to the external device in response to a data request from the external device.

8. The pyroelectric infrared detector according to claim 1 or 2, wherein: The integrated circuit comprises: a detection determination unit, which compares the detection signal processed by the signal processing unit with a threshold value to perform detection determination of a detection object, and outputs a trigger signal or an interrupt signal to the external device according to the result of the detection determination, The threshold value can be changed by applying a voltage to the integrated circuit or by a command input from the external device via the serial data communication.

9. The pyroelectric infrared detector according to claim 1 or 2, wherein: An access address for specifying the pyroelectric infrared detector is set in the serial data communication, and the access address is printed or engraved on the pyroelectric infrared detector so that the access address can be visually recognized.

10. The pyroelectric infrared detector according to claim 9, wherein: The serial data communication is an I2C serial data communication. When a plurality of the pyroelectric infrared detectors are connected to one I2C bus line, individual identification of the pyroelectric infrared detectors can be performed using the access address.

11. The pyroelectric infrared detector according to claim 8, wherein: The serial data communication is an I2C serial data communication, and the trigger signal or interrupt signal output by the detection and determination unit is output to the external device using one I2C bus line.

12. An integrated circuit for inputting and processing a detection signal based on an output of a pyroelectric infrared detector, characterized in that: The integrated circuit comprises: A conversion unit, performing analog-to-digital conversion of the detection signal; a signal processing unit that performs signal processing using a digital filter on the detection signal converted into a digital signal; and an input / output control unit that outputs the output data from the signal processing unit to an external device via serial data communication, The cutoff frequency in the digital filter can be set and changed by applying a voltage to the integrated circuit or by a command input from the external device via the serial data communication.

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