GPS is in the present technology is maturest, and already practical one kind of positioning system, but in GPS positioning system, because it is an unidirectional guidance system, he is and so on passes to the ephemeris data the ground receiver, may when many concrete applications, for example in the vehicles dispatching system, needs the message data which obtains using GPS to pass to generally the control center, because its system stands by a datum with many subscriber switchboards is composed, the datum stands with subscriber switchboard between contacting, namely each kind of correct value which calculates based on the datum station transmits the subscriber switchboard, above all these need to use the data chain to complete the data the transmission. And, the data chain is composed of the modem and the broadcasting station. In the wireless data transmission system, a modem foot essential part, modem (Modem) is carries on the data the code and the modulation, then inputs to the broadcasting station launches, the subscriber switchboard receives it, after and data demodulation, sends in the GPS receiver to carry on the correction. This article introduced the monolithic integrated circuit controls Modem the principle and the method.
1 system composition
This system is mainly composed of the monolithic integrated circuit and the modem, the monolithic integrated circuit selects P89C58, its achievement control circuit’s main part, carries on the control to the K224 chip, and completes each chip the initialization work, meanwhile has to the data carries on interweaves the error correction code function. The monolithic integrated circuit and K224 connection’s connection as shown in Figure 1.
The overall system divides into two parts, a part shouts the structure primarily, another part for reply structure. Figure 1 gave the host to shout the part. When work, on the electricity replacement, carries on the initialization establishment first by P89C58 to K224, and carries on the initialization to its own serial port.
Overall system’s work process is: After from the GPS receiver receives the data, passes through the RS 232 interface circuits (i.e. by MAX232 constitution level switch electric circuit), becomes the TTL level, delivers to the P89C58 RXD end, P89C58 carries on after the data the block encoding, interlocks the code, delivers P89C58 the TXD end output, this output signal delivers K224 TXD, after undergoing the modulation, from K224 TXA end output modulation signal. When receiver from the RXA place receive after modulation signal, undergoes the K224 processing, after the demodulation, delivers by K224 through the RXD port to the P89C58 RXD receiving end, carries on by P89C58 to the data instead interweaves, decoding processing, hereafter, the data from monolithic integrated circuit’s TXD port place delivers, again after the ICL232 level switch, delivers by the RS232 level to the computer carries on the computation analysis.
2 monolithic integrated circuit’s initial establishments
In this system, because must use P89C58 to carry on the initialization establishment to each chip, and must use it to complete the data to interweave the error correction function, this needs to use monolithic integrated circuit’s special function register to carry on the establishment, simultaneously needs to be possible the position addressing area to complete using monolithic integrated circuit interior RAM interweaves the code function. Regarding the MCS-51 series’s monolithic integrated circuit, in its special function register carries on to the serial port controls has SCON and PCON. PCON is the power source control register, D7 position SMOD is the serial port baudrate coefficient control position, if SMOD=1, then baudrate double. The special function register SCON byte address is 98H, and may the position addressing. SCON uses for to establish the serial port the working, the receive transmission control as well as the establishment status flag.
In the serial port 4 working, mainly uses in expanding the parallel input outlet. Considered the above parameter, we establish the monolithic integrated circuit work in the serial mode 1, namely a data format is: 1 outset position, 8 data positions, 1 stop position, non-parity check. Because in the monolithic integrated circuit serial port 4 workings, and does not have 2 stop positions the data format, our necessity to not have collected 2 stop positions to select the method desirably 3, replaces the stop position with 9 bit data form’s. Under moreover, if saves these 1 stop position, because the GPS navascope outputs the data format is 2 stop positions, but monolithic integrated circuit work in way 1, when it receives to 1 stop position, thought that a data has received finishes, thus had 1 stop position time to carry on other processing. Regarding the baudrate was 2 400 b/s data, the element width was 1/2 400=0.42 ms, regarding the baudrate was 1 200 b/s data, the element width is 1/1 200=0.83 ms. In this system, what because uses is 11.059 2 MHz crystal oscillators, its machine cycle is approximate for 1μs. After the comparison, may see, the time which saves down has certain use value, thus has more time to carry on interweaves error correction code processing. Because time system work those who receive the data to use inquires the way, also after namely on electricity replacement, whether there is namely doesn’t stop the loop check data feeds, until examines has the data to lose the human. Thus, if has more time to carry on the data processing, then not will cause some data omission because of the processing data, thus lost the data.
In this system receives the data the baudrate hypothesis is 1 200 b/s,2 400 b/s, uses timer T1 to take the baudrate generator, the T1 work makes the counting in autoloading way working 2, TL1 to use, when the automatic rewiring’s value places in TH1, the overflow rate may determine by the equation below:
Overflow rate = counting rate/[256-(TH1)]
Its baudrate determined by the equation below:
Baudrate = (fixed time/counter 1 overflow rate)/(32/2SMOD)
And SMOD is in the special function register PCON 8th special position.
3 interweave the error correction code design
The error correction code enhances the digital transmission reliable one kind of technology, is the correct transmission difference GPS correction signal important means. Data in transmission process, because the transmission channel internal noise and creates between the mark disturbs, as well as the transmission external environment’s disturbance, has the burst characteristics wrong occurrence frequently. In order to sharpen the antijamming ability, we will use interweave the error correction to code will be a succession of disperse wrongly to many symbols, will then use again can correct little the wrong symbol to carry on the code, like this will correct on the possibility has the long wrong symbol. For example, assigns a (n, k) linear grouping code, we use interweaves may long for bλ the single burst error to disperse to Lamda (n, k) symbol, thus causes in each (n, k) symbol only then the length is the b burst error, this sharpened the system antijamming ability, sharpened system’s error correction ability.
The MCS-51 monolithic integrated circuit internal data memory is 256 B generally, divides into 128 B the internal data memories and 128 B special function register. Regarding P89C58, its addressable data storage asked spatially may expand the supreme side 128 B. In the internal data memory, 20H~2FH (16 units) take the general working space, may also by CPU according to the byte addressing, may also according to the position direct addressing, carry on the position operation. We were precisely use 20H~27H to constitute interlock the code space.
Through 8×8 the moment data unit, circulates according to the line (row) lays aside the data, then circulates again according to the row (line) the readout, thus achieves the data to march forward lists, or the row carries on, like this completes interlocks the error correction code. When this process, needs to pay attention to the time the coordinate question, data processing asked that with the data transmission time wants the coordination, will otherwise cause the data not to be able to receive normally.
4 modem’s design
The K224 interior altogether has 8 8 b registers, is through carries on the initial establishment to the K224 operation to these 8 registers, determined that the active status completes.
2 piece of Modem establishes separately to work Yu Zhu to shout with the reply way, forbids INT (interrupt), after the scrambler, to forbid to defend the sound, the pass reply sound generator, to close DTMF, the simultaneous reception transmission uses the identical decision plane, opens the auto-adapted balancer.
Before Modem normal transmission data, between 2 Modem as well as Modem and DTE, to determine that bilateral questions and so on speed, need to carry on the essential liaison, this is the so-called handshake. In the handshake process, micro controller P89C58 must examine and confirm that the definite signal controls the handshake the process and the Modem condition. Micro controller P89C58 reads K224 an examination register’s bit position, but examines some bit position is merely condition in advance (1 or 0), immediately adopts with this condition related movement is not enough. In which reason has two: First is in the handshake process, examines the signal must continue period of time stably; Second is because has the noise disturbance, certain bits positions will possibly occur from 0 to 1 or from 1 to 0 sudden changes. Therefore the micro controller needs to confirm that examines register’s bit position condition, and continues to examine this signal whether to have continued the corresponding time.
Therefore, needs to design the examination handshake sequence the method. What this article uses is increases the counter the method, soon the signal should continue the time transformation is a pre-placed value, then establishes a counter, examines every time to a signal then adds 1, when increases to the pre-placed value, only then might think this signal detection.
The concrete computational method is as follows: Each handshake process must examine the signal has the time which must continue, but this signal arrives at the receiver to examine by the detector must have a detention, subtracts the delay time with this signal’s duration, then uses this signal the sampling frequency to be multiplied by this value again, the sampling frequency characteristic is bigger than or is equal to by the detection signal data rate. The above procedure establishment is through P89C58 after the system seven electricity replacements, carries on the initialization to K224. Figure 2 is the system work flow chart.
5 conclusions
This system grinds the Star51PH monolithic integrated circuit simulator simulation after the star, and after TOP2005 programmer programming, the result proof system is feasible, the modem receive in 1 200 b/s may the completely stable receive transmission, when works when the speed is big, the long-firing operation data meets cannot receive stably, needs to go a step further experimental verification. This system design structure is simple, the stable property is reliable.
