Discussion on GPS Clock Generator (GPS Synchronous Clock)

In power systems, CDMA2000, DVB, DMB and other systems, high-precision GPS clock generators (GPS synchronized clocks) are of vital importance to maintaining the normal operation of the systems.

How to use GPS OEM to carry out secondary development and produce high-precision clock generators is a hot research issue.

For example, in DVB-T single frequency network (SFN), the time synchronization requirement is tens of nanoseconds. How to design a commercial-grade system with such high precision and high stability?

1 Introduction

In many areas of the power system, such as time sequence recording, relay protection, fault location, energy billing, real-time information collection, etc., a unified, high-precision time reference is required. Using GPS satellite signals for time synchronization is one of the commonly used methods.

At present, there are many types of GPS-OEM boards on the market, which are moderately priced and have practical conditions. By using the GPS-OEM board for secondary development, a GPS clock generator (GPS synchronous clock) with accurate GPS time information can be obtained. This article takes the SUPERSTAR GPS OEM board produced by Marconi Company of Canada as an example to introduce how to develop a GPS clock generator (GPS synchronous clock) for power systems. ?

2GPS timing module?

The GPS clock generator (GPS synchronous clock) uses the SUPERSTAR GPS OEM board as the GPS receiving module. The SUPERSTAR GPS OEM board is a parallel 12-tracking channel, full-view GPS receiving module. The OEM board has a rechargeable lithium battery. The L1 frequency is 1575.42MHz, providing the output of pseudorange and carrier phase observation values ​​and 1PPS (1 PULSE PER SECOND) pulse output. The OEM board provides two input and output serial ports, one of which is used as the main communication port. The OEM board can be set through this serial port, and the international standard time, date, location and other information can also be read from this serial port. The other serial port is used to output differential data in RTCM format. When there is no differential signal or it is only used for GPS timing, this serial port can be used. The 1PPS pulse is a standard TTL logic output form. When the navigation output is valid, the rising edge of the pulse corresponds to the time. The 1PPS pulse is a positive pulse signal output per second, with an amplitude of 5V. The rising edge of the 1PPS pulse is synchronized with the second pulse of the UCT standard time, and the error is within plus or minus 1μs. The rising edge of this pulse signal can be used as the UTC time synchronization signal. In addition, we can use the synchronous pulse circuit to expand the 1PPS signal to 1PPM (1 PULSE PER MINUTE), 1PPH (1 PULSE PER HOUR), etc. for synchronization according to actual conditions. The number of output ports for each pulse can be expanded according to application requirements.

The communication data format of the main serial port of SUPERSTAR GPS OEM board can adopt CMC BINARY binary or NMEA ASCII code supported by CMC. The baud rate can be adjusted between 300bps and 38400bps according to the application requirements. It has 8 data bits, 1 start bit, 1 stop bit, and no parity bit. GPS data information contains various information such as satellite status, longitude, latitude, time, altitude, speed, etc. For the development of synchronous clock, we only need to read its time information. Therefore, the OEM board can be set through the main serial port to send only time information at a certain baud rate and a certain communication data format. For example, if we use NMEA ASCII code, we only need to read the time data with "$GPZDA" as the command header, and then we can easily separate the year, month, day, hour, minute, and second of UTC time from it.

3 System composition

The GPS clock generator (GPS synchronous clock) includes several parts such as GPS receiving module, central processing unit, RS-232/485 interface, CAN bus interface, synchronous pulse generating circuit, display circuit, etc.

3.1 Central Processing Unit

The GPS synchronization clock uses DS80C320 as the system CPU. DS80C320 is an 8-bit high-speed single-chip microcomputer launched by DALLAS Corporation of the United States. It is a single-chip microcomputer compatible with the MCS-51 series. Due to the redesign of the microprocessor core, redundant clock and storage cycles are omitted. If the clock operating frequency is the same and the same program code is executed, the execution speed of DS80C320 is at least 2.5 times that of 8051.

The DS80C320 has exactly the same package as the 80C32. In addition to the I/O ports, 2 timers/counters, serial ports and other resources that the 80C32 has, it also has some additional resources, which are listed below.

a.Serial port 1

DS80C320 provides an additional hardware serial communication port that is the same as 80C32. In the development of GPS synchronized clock, we obtain GPS time data through serial communication port 0, while serial communication port 1 is responsible for communicating with various power grid automation devices.

b.Double data pointer?

DS80C320 provides two data pointers. When the GPS clock receives the GPS OEM board information, it can use these two data pointers to send data to different storage areas.

c. On-chip reset circuit

DS80C320 has a complete set of power-on/power-off reset logic. Therefore, when using DS80C320, there is no need to add an external reset circuit. This simplifies the hardware and improves reliability.

d. Watchdog timer

DS80C320 has a programmable watchdog timer, so there is no need for an external watchdog circuit like 80C32.
3.2 Synchronous pulse generation circuit?

P3.2 and P3.3 are the strobe signal output terminals of 1PPM (1 PULSE PER MINUTE) and 1PPH (1 PULSE PER HOUR) pulses (to prohibit the sending of 1PPS pulses, these two control terminals are usually set to low). Take the generation of 1PPM pulses as an example: when the UTC time information is read from serial port 0 and it is judged to be 59 seconds of a minute, the CPU sets P3.2 high, thereby sending a pulse at the full minute. When the time information is read again and it is judged to be the full minute, P3.2 is set low again to prohibit the sending of pulses. By repeating the cycle in sequence, an accurate 1PPM pulse signal can be obtained. The same method can also be used to generate a 1PPH pulse signal.

Since the static empty node mode has better isolation between the control end and the signal channel and higher voltage resistance, the GPS clock generator (GPS synchronous clock) outputs the synchronization pulse signal as a switch quantity in a static empty node mode.

3.3RS-232/485 interface?

The GPS clock generator (GPS synchronous clock) has two communication interfaces, RS232 and RS485, to meet the requirements of different communication systems. Through these two interfaces, the synchronous clock can output complete UTC time information including year, month, day, hour, minute and second once per second, and can also be used as a communication slave to provide accurate time information for the power grid automation device in the upper computer when needed.

3.4CAN bus interface?

CAN (Controller Area Network) bus is one of the important field buses. At present, it is also used in some fields of power system (such as substation integrated automation system). Therefore, when designing GPS clock generator (GPS synchronous clock), CAN bus interface is configured. GPS clock generator (GPS synchronous clock) uses SJA1000 produced by PHILIPS as CAN protocol controller, and PCA82C250 as the interface between SJA1000 and physical bus. At the time of full minute or full time, DS80C320 can directly send identifier and data to the send buffer of SJA1000 through address/data bus, and then set the send request bit TR in command register CMR, start CAN core module to read data in send buffer, encapsulate into a complete CAN information frame according to CAN protocol and send to bus through transceiver. It can also respond to external interrupt when required by power grid automation device, and output GPS data in the send buffer of microcontroller to the outside according to CAN protocol.

4. Software Design

The software of GPS clock generator (GPS synchronous clock) consists of main program and receiving/transmitting interrupt subroutine. The system is initialized in the main program, including the initialization of SUPERSTAR GPS OEM board, two serial communication ports, built-in programmable watchdog, timer, etc. The program generates an interrupt of serial communication port 0 once a second, reads UTC time data, converts it into Beijing time, and sends it out through serial communication port 1 in BCD code format. In the interrupt subroutine, the time information will also be judged, and the strobe signal of 1PPM and 1PPH signals will be generated at 59 seconds of each minute and 59 minutes and 59 seconds of each hour. At the full hour or full minute, the strobe signal is prohibited.

The GPS clock generator (GPS synchronized clock) can also respond to the time calibration command sent by the power grid automation device and send out the accurate time information at that time. To do this, you only need to add an interrupt subroutine of serial communication port 1 in the program to provide real-time time information for the power grid automation device according to a certain communication protocol.

5. Conclusion?

The GPS clock generator (GPS synchronous clock) based on DS80C320 introduced in this paper has been used in the actual substation integrated automation system. The results of debugging and operation show that the clock device provides accurate time information for the entire system and has high reliability, accuracy and practicality. In power system accident analysis, fault location, phase measurement and other aspects, this synchronous clock device has a very broad application prospect.