Fig. 2 shows a schematic circuit diagram of an automobile speedometer. This is a typical minimum application system of single chip microcomputer. Single chip microcomputer AT89C205 1 is famous for its low price, low power consumption, high reliability and easy programming. X25045 is an auxiliary chip of MCS-5 1 series single chip microcomputer circuits, which mainly plays the roles of reset, voltage detection, watchdog and EEPROM. The adoption of this chip greatly improves the reliability of the system, reduces the number of peripheral chips, and can realize the power-off storage of mileage accumulation. LCM 10 10 is an 8-segment LCD module with backlight. Using three-wire serial interface, it has the characteristics of low power consumption and convenient programming. The display screen * * * is displayed in two lines. The first line shows the accumulated mileage with 6 digits, and the second line shows the subtotal mileage with 4 digits (1 decimal). In the figure, K 1 is the subtotal mileage clearing key, and R4 is used to adjust the viewing angle contrast of LCD. Chip X25045 is a CMOS serial EEPROM with programmable μP monitor introduced by Xicor Company, with 4096 bits and organized in 5 12×8. It has a 4-byte page writing mode, 6.5438+ten thousand use cycles, and the data can be saved for 654.38+ 000 years. In order to ensure that the digits of the accumulated mileage unit or the decimal digits of the subtotal unit can be refreshed reliably, when these units are close to the limit service life, the data can be moved to a new unit by page change to continue counting. Figure 2 The pulse signal sent by Hall sensor can be divided into two paths after shaping. One path is sent to the INT 1 terminal of the single chip microcomputer for cumulative mileage counting, and the other path is sent to the speed signal input terminal of LM 18 19 driver (10 pin). Then the driving circuit outputs corresponding sine and cosine driving signals at pin 2 and 12 according to the frequency of the input signal, and passes through the coil. However, it should be noted that the size of capacitor C3 will change the smoothness of pointer deflection. The greater C3 is, the better smoothness will be, but at the same time, the time lag will also increase, and too small C3 will make hands shake. C4 can be used to adjust the linearity and hysteresis error of the circuit; The value of R4 can change the indicating scale point of the pointer. Conclusion In this design, AT89C205 1 is used to realize mileage accumulation, subtotal, clearing and storage, and LM 18 19 is used to drive the cross coil meter, thus realizing the indication of vehicle speed. The design scheme has the advantages of low cost, good pointer stability, fast response, strong earthquake resistance, high reliability and high cost performance. The practical application proves that the odometer can completely replace the traditional soft shaft drive speed odometer. Of course, this is only an implementation scheme, and the cross coil meter can also be driven by a single chip microcomputer through software, that is, the single chip microcomputer controls the sine coil and cosine coil of the meter respectively, omitting the LM 18 19 integrated circuit. I won't go into details here.