receiver. Only after acknowledge from the receiver is received successfully, substation can confirm whether the data block is effective.
B. Communication software design
Communication software design should take the following three factors into account.
1) Wireless receiver receives radio signal from code emitter, and analyzes it into digital signal. Micro Control Unit (MCU) in the Receiver has 4 wire SPI interface, LPC2114 CPU has 2 wire SPI interfaces, so data is transmitted to substation through SPI bus in the form of external interrupt.
2) Sending digital signal to display circuit. LPC2114 device supports I2C bus transmission, so data can be transmitted to display drive circuit through I2C bus.
3) Packing data. If monitoring center sends out instruction of periodic detection, it is transmitted to substations. Real time coordinate is required by the whole system, so data is required to be transmitted in real time. Considering code emitter transmits data one time every 3 seconds, data is packaged every 8 seconds according to agreement during timer interrupt, so that the received data can be transmitted to substation
CPU in time. Based on analysis above, there are 4 interrupts in sensor node software. 1) External interrupt for data transmission. 2) I2C interrupt for data display. 3) Timer interrupt for package.
4) UART interrupt for receiving periodic detection instruction.
Data block received by substation CPU need to have time data, so the time in substation and receiver should be the same. Time data comes from monitoring center passed by LPC2114 CPU to Mega128 MCU.
C. Data packaging protocol
Data packaging protocols [8] between substation and receiver are designed in two aspects.
1) Substation sends out instructions as follows.
a) Tuning time instruction. Frame structure: frame head (5 FF) + receiver No.(1 byte) + instruction No.(23) + year(1 byte) + month(1 byte) + day(1 byte) + time(1 byte) + minute(1 byte) + second(1 byte) + check(2 byte). b) Periodic detection instruction. Frame structure:
frame head (5 FF) + receiver No.(1 byte) + instruction No.(20) + check(2 byte).
c) Information confirm instruction. Frame structure : frame head (5 FF) +
receiver No.(1 byte) + instruction No.(21) + check(2 byte). 2) Receiver sends back instructions as follows.
a) Recognition of tuning time instruction. Frame structure: frame head (5 FF) + receiver No.(1 byte) + instruction No.(23) + check(2 byte).
b) Recognition of periodic detection instruction. frame structure : frame head(5 FF) + receiver No.(1byte) + instruction No.(20) + day(1 byte) + time(1 byte) + minute(1 byte) + second(1 byte) + number of people(1 byte) + identify code number 1(2 byte)+ ……+ identify code number n(2 byte) + check(2byte).
c) Recognition of information confirm instruction. frame structure: frame head (5 FF) + receiver No.(1byte) + instruction No.(21) + check(2 byte).
D. Network drills
There are 11 substations in this personnel positioning system, one substation can load 6 receivers at most. So the networks can monitor 11_6_n miners at most. With the communication between substation and wireless receivers going on, receivers probably dropped at random. Once it drops, it would no longer be detected at interrupt interface. This phenomenon is found out after a long time of communication, and it is difficult to determine the reasons by the use of simulation tools. By looking up datasheet and analyzing, reasons for the fault are found out. Sequence in time for fault is shown in Figure 3.
Fig.3 Sequence in time for fault
Because the program running time is not the same in different receivers, time taken on analyzing communication protocol instruction is also different. Someone finishes a little earlier, whereas others a little later. For example, substations send out inquiring instruction at t1, receiver with 0X01 address finishes analyzing the instruction at t2. If its address meets the conditions of inquiring, it begins to response at time t3, while receiver with 0X04 finishes analyzing the instruction at t4. Receiver with 0X01 finishes response at t5. During the time of t3 and t4, Data sent by 0X01 can not be received by 0X04. When ending symbol from 0X01 comes, it is considered a new framereceived by 0X04, then it begins to analyze again. Sometimes it can analyze the fault frame “successfully” for the parity check-up limit, in this way program could eventually lead to confusion and drop.
Fig.4 State machine flow chart
Through analysis in-depth, it is found that there is some flaw in the structure of communication procedure: send buffer and receive buffer share the same one of LPC2114 CPU. Though this method can save a lot of RAM units, in the circumstance of multi-receivers, problems will appear. To improve it, queue is used for serial buffer because there is enough RAM units in LPC2114. Send buffer and receive buffer are separated. State machine method is used to send and receive data, in this way it can solve two problems: frequent on-off interrupt handling and error decoding. Flow chart adding error control measure is shown in Figure 4.
This state machine method has well solved the problem of receiver jam. After it was tested for a week, all receivers have worked normally, no jam happened.
