GPS clock generator technical solution

How to use GPS OEM for secondary development and generate high-precision clock generator is a hot issue. In systems such as power systems, CDMA2000, DVB, and DMB, high-precision GPS clock generators (GPS synchronous clocks) are critical to maintaining proper system operation.

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For example, in the DVB-T single frequency network (SFN), the synchronization accuracy reaches several tens of ns for the time synchronization requirement. How to carry out commercial-grade design for such a high-precision and high-stability system?

1 Introduction

In many areas of the power system, such as chronological recording, relay protection, fault location, energy billing, real-time information collection, etc., a uniform, high-precision time reference is required. One of the commonly used methods is to use GPS satellite signals for timing.

At present, there are many types of GPS-OEM boards on the market, which are moderately priced and have practical conditions. A GPS clock generator (GPS synchronous clock) that can accurately obtain GPS time information by using a GPS-OEM board for secondary development. This article uses the SUPERSTAR GPS OEM board produced by Canada Marconi as an example to introduce how to develop a GPS clock generator (GPS synchronous clock) for power systems. 

2 GPS timing module

The GPS clock generator (GPS synchronous clock) uses the SUPERSTAR GPS OEM board as the GPS receiver module, and the SUPERSTAR GPS OEM board is the parallel 12 tracking channel and the full-field GPS receiver module. The OEM board has a rechargeable lithium battery. The L1 frequency is 1575.42MHz, providing an output of pseudorange and carrier phase observations and a 1PPS (1 PULSE PER SECOND) pulse output. The OEM board provides two input and output serial ports, one 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 be read from the serial port. Another serial port is used for the output of differential data in RTCM format. This serial port can be used when there is no differential signal or only for GPS timing. The 1PPS pulse is a standard TTL logic output form. When the navigation output is active, the rising edge of the pulse corresponds to time. The 1PPS pulse is a positive pulse signal outputted per second with an amplitude of 5V. The rising edge of the 1PPS pulse is synchronized with the second pulse of UCT standard time, and the error is within plus or minus 1μs. The rising edge of this pulse signal can be used as the timing signal of UTC time. In addition, we can use the sync pulse circuit to expand the 1PPS signal to 1PPM (1 PULSE PER MINUTE), 1PPH (1 PULSE PER HOUR), etc. according to the actual situation. On time. The number of outputs per pulse can be extended according to the 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 from 300bps to 38400bps according to application requirements, with 8 data bits and 1 bit. Start bit, 1 stop bit, no parity. The GPS data information includes various information such as satellite status, longitude, latitude, time, altitude, speed, etc. For the development of the synchronous clock, we only need to read the time information. Therefore, the OEM board can be set through the main serial port to transmit only time information at a certain baud rate and some communication data format. For example, if we take the ASCII code of NMEA, 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. .

3 system components

The GPS clock generator (GPS synchronous clock) includes a GPS receiving module, a central processing unit, an RS-232/485 interface, a CAN bus interface, a sync pulse generating circuit, and a display circuit.

3.1 central processing unit

The GPS synchronous clock uses the DS80C320 as the CPU of the system. The DS80C320 is an 8-bit high-speed microcontroller from DALLAS, Inc., and is a single-chip microcomputer compatible with the MCS-51 series. Since the microprocessor core has been redesigned to eliminate redundant clock and memory cycles, if the clock operates at the same frequency and executes the same program code, the DS80C320 will execute at least 2.5 times faster than 8051.

DS80C320 and 80C32 have the same package. In addition to the I/O port, 2 timer/counter, serial port and other resources of the 80C32, there are some new resources, which are listed below.

a. serial port 1

The DS80C320 additionally provides the same hardware serial communication port as the 80C32. In the development of the GPS synchronous clock, we obtain the GPS time data from the serial communication port 0, and the serial communication port 1 is responsible for communicating with various grid automation devices. .

b. Double data pointer

The DS80C320 provides two data pointers. When the GPS clock receives the GPS OEM board information, the data pointer can be used to send data to different storage areas.

c. On-chip reset circuit

The DS80C320 has a complete set of power-up/power-down reset logic. Therefore, with the DS80C320, there is no need to add an external reset circuit. Simplified hardware and improved reliability.

d. Watchdog timer

The DS80C320 has a programmable watchdog timer that eliminates the need for an external watchdog circuit like the 80C32.
3.2 Synchronous pulse generation circuit

P3.2 and P3.3 are the strobe signal outputs of the 1PPM (1 PULSE PER MINUTE) and 1PPH (1 PULSE PER HOUR) pulses (to disable the transmission of the 1PPS pulse, both of which are normally low). Taking the 1PPM pulse as an example: When the UTC time information is read in by the serial port 0 and judged to be the 59 second time of a certain minute, the CPU sets P3.2 high, thereby transmitting a pulse at the time of the whole. When the time information is read again and it is judged to be a whole time, P3.2 is set low again to prohibit the pulse from being emitted. By cycling in sequence, an accurate 1PPM pulse signal is obtained. In the same way, a pulse signal of 1 PPH can also be generated.

Since the static empty node mode control terminal is well isolated from the signal channel and has high withstand voltage, the GPS clock generator (GPS synchronous clock) outputs the synchronous pulse signal as a switch quantity in a static empty node manner.

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 the year, month, day, hour, minute, and second once per second, or can be used as the communication lower-level machine to automate the grid in the upper computer when needed. The device provides accurate time information.

3.4CAN bus interface

The CAN (Controller Area Network) bus is one of the important fieldbuses and is currently used in some areas of the power system, such as substation integrated automation systems. Therefore, when designing the GPS clock generator (GPS synchronous clock), the CAN bus interface is configured. The GPS clock generator (GPS synchronous clock) uses the SJA1000 produced by PHILIPS as the CAN protocol controller, and the PCA82C250 serves as the interface between the SJA1000 and the physical bus. During the whole or the whole time, the DS80C320 can directly send the identifier and data to the SJA1000 send buffer through the address/data bus, and then set the send request bit TR in the command register CMR to start the CAN core module to read the send buffer. The data in the package is encapsulated into a complete CAN information frame according to the CAN protocol and sent to the bus through the transceiver. It can also respond to external interrupts when required by the grid automation device, and output the GPS data in the buffer sent by the MCU to the CAN protocol.

4 software design

The software of the GPS clock generator (GPS sync clock) consists of the main program and the receive/transmit interrupt subroutine. Initialize the system in the main program, including initialization of the SUPERSTAR GPS OEM board, two serial communication ports, built-in programmable watchdog, timers, etc. The program generates an interrupt of serial communication port 0 every second, reads UTC time data, converts it into Beijing time, and sends it through serial communication port 1 in BCD code format. In the interrupt subroutine, the time information is also judged, and the strobe signals of the 1PPM and 1PPH signals are generated at 59 seconds per minute and 59 minutes and 59 seconds per hour, and are prohibited at the whole time or at the entire time. A strobe signal is issued.

The GPS clock generator (GPS Synchronous Clock) can also send accurate time information at that time in response to the timing command sent by the grid automation device. To this end, it is only necessary to add an interrupt subroutine of serial communication port 1 in the program to provide real-time time information for the grid automation device according to a certain communication protocol.

5 Conclusion

The DS80C320-based GPS clock generator (GPS Synchronous Clock) introduced in this paper has been used in practical substation integrated automation systems. The results of commissioning and operation show that the clock device provides accurate time information for the entire system with high reliability, accuracy and practicality. In the aspect of power system accident analysis, fault location, phase measurement, etc., the synchronous clock device has a very broad application prospect.

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