Abstract: Elaborates the GPS precise timing system’s basic constitution and the system function, introduced that realizes the method take the MCS-51 monolithic organization as the core high accuracy time display system’s concrete hardware, elaborated in detail the system each kind of software module’s design mentality and carries out the flow.
Key word: Global positioning system; MCS-51; DS12C887; Design
1 introduction
GPS is the Global Positioning System abbreviation, is carries on using the navigation satellite measured when and range finder’s global positioning system. It has the precision high, all-weather and whole world coating ability. Applies GPS in the clock countdown system can realize the high accuracy time demonstration function. Based on the GPS high accuracy, the countdown sign are the satellite measured when technical, the computer technology and the communication three organic synthesis. Looked from the functional module that the overall system divides into GPS measured when the receiving system and the clock display system. It mainly completes the following function:
· fixed time receives the GPS satellite transmission the data and carries on the recognition and the buffer;
· carries on the format conversion to the GPS chronometric data, causes the coding form to be suitable for to receive;
· in assigns in the time the refurbishing DS12C887 clock’s time;
· reads the DS12C887 time, carries on the countdown to convert and to demonstrate.
2 hardware designs
Is simpler based on the GPS high accuracy countdown sign’s hardware architecture, including the control module and the display module, as shown in Figure 1.
2.1 control modules
the 89C51 monolithic integrated circuit is the entire control system’s core, completes to the serial port controller’s initialization and the data read-write, but must carry on the recognition, the dump and the demonstration to receive each kind of data.
Because Japanese Electro-optic Company GSV-15 OEM the GPS receive board transmission’s data outputs one 10, the baudrate take the serial mode as the 4800bit, MCS-51 monolithic integrated circuit serial port I/O format match, therefore using monolithic integrated circuit serial port directly from OEM receive data.
DS12C887 has provides the high precision year, the month, Japan, divides, a second function, after its calibration, in power failure in the situation, within 10 years still could accurate carry on the time, and can with the monolithic integrated circuit direct connected.
2.2 display modules
This system realizes the serial mode control nixietube using 74HC595 to demonstrate that takes the mouth line to be few, the hardware architecture is simple. the 74HC595 string/and transforms the shift register to have the lock to save with 3 condition output functions. 14 feet (SER) is the serial input end; 12 feet (RCLR) to shift the clock end, rises along moves in 14 foot data the register; 11 feet (SRCLR) is the lock saves the clock end, rises saves the data along the lock; 8 feet (E) is 3 condition output control end, meets when the low level output data.
Monolithic integrated circuit output data and shifting clock, when the clock rises along moves in 74HC595 the data. P1.0 output data, when shift pulse rise along arrival data emigration. P1.1 output shift pulse. The P1.2 output lock saves the pulse. The data appears on the parallel out-port demonstrated after the 74HC07 actuation nixietube, because the data has been locked saves, when transmission next group of data, the preceding group of data’s content is invariable, carries on the data by this way the transmission and the demonstration. Because has received the OEM board content in the control module with monolithic integrated circuit’s serial port, therefore in the display module software must P1.0, P1.1, P1.2 define as the simulation serial port. Thus, P1.0, P1.2, P1.3 may take the serial port use.
3 software designs
3.1 master routines
Including to the monolithic integrated circuit own mechanism’s establishment serial port’s initialization and so on, as shown in Figure 2.
3.2 GPS measured when receives
GPS measured when receives the data by the serial port receive, carries on operations and so on read-, recognition, transformation and memory by the monolithic integrated circuit to it, when the procedure starts, first distinguishes the received signal, looked whether it is the signal which must receive, the OEM board receive’s time signal’s prefix is GPZDA. Because receives the time signal is ASCⅡCode, must therefore transform the receive data as the binary number. Because receives the time is the Greenwich Mean Time, must therefore transform as the Beijing standard time, after will then transform the time reads in DS12C887. DS12C887 is one kind of quite accurate timing circuit, does not need to receive the GPS signal frequently. The author stipulated that 1 hour receives 1 GPS signal, its flow as shown in Figure 3.
3.3 clock countdown demonstration
Simulates the serial port in this procedure, reads DS12C887 the data, carries on the demonstration using 74HC595 to its data, flow as shown in Figure 4.
3.4 countdown subroutines
Take calculates from the present to January 1, 2006 as the example, the monolithic integrated circuit reads DS12C887 the time cell, and deposits it in take 69H as in the outset unit. Reads the month first, use table look-up method computation its under month to the scheduled time number of days. Then judges its month using the table look-up way is again 31 days, 30 days, 28 days or 29 days, then the number of days which obtains the table look-up subtract the read date, like this obtain two number of days adding together the actual number of days. Carries on when, divides, a second computation, first January 1, 2006 0.0:0 0 second transformation is on December 31, 2006 23.59:00 60 seconds, when such direct use, divides, a second cancellation obtains difference, divides, the second. Countdown flow as shown in Figure 5.
4 concluding remark
This article states based on GPS high accuracy, non-erroneous countdown sign after debugging movement and parameter installation, movement stable reliable, the long-term movement accumulates continuously the error is zero, time curacy error <15ms then. This system’s control module structure is simple, is advantageous for the actual development application.
