Brief Introduction of 51 Single Chip Microcomputer Watchdog Circuit

The watchdog circuit is composed of 89C51 single-chip microcomputer and X25045. The hardware connection diagram of X25045 is shown in Figure 2. The X25045 chip contains a watchdog timer, and the system monitoring time can be preset by software. If there is no bus activity within the preset time of the watchdog timer, X25045 will output a high-level signal from RESET, and output a positive pulse through the differential circuit C2 and R3 to reset the CPU. In the circuit of Figure 2, there are 3 reset signals for the CPU: power-on reset (C1, R2), manual reset (S, R1, R2) and watchdog reset (C2, R3), which are added to the RESET terminal after being synthesized by the OR gate. The time constants of C2 and R3 do not need to be too large, and hundreds of microseconds are sufficient, because the CPU oscillator is already working at this time.

Figure 2 X25045 watchdog circuit hardware connection diagram

The preset time of the watchdog timer is set by the corresponding bit of the status register of X25045. As shown in Table 2, there are 6 bits in the status register of X25045, of which WD1 and WD0 are related to the watchdog circuit, and the remaining bits are related to the working settings of EEPROM.

Table 2 X25045 status register

WD1＝0, WD0=0, preset time is 1.4s.
WD1＝0, WD0=1, preset time is 0.6s.
WD1＝1, WD0=0, preset time is 0.2s.
WD1＝1, WD0=1, disable watchdog.

The length of the watchdog circuit timing time can be determined by the cycle of the specific application program, and is usually slightly longer than the maximum cycle time when the system is working normally. When programming, you can add a dog feeding instruction in the appropriate place of the software to make the watchdog timing time never reach the preset time, and the system will not reset and work normally. When the system runs away and the program cannot be captured back by software traps or other methods, the watchdog timing time will quickly increase to the preset time, forcing the system to reset. [page]

The following is part of the watchdog program written in C language.

#include reg51.h
sbit cs=P1^2;/*Chip select signal is generated by P1.2*/
sbit sck=P1^3; /*Clock signal is generated by P1.3*/
sbit si=P1^0; /*SI is generated by P1.0*/
sbit so=P1^1; /*SO is generated by P1.1*/
sbit c=ACC^7; /*Define bit variables*/
bdata unsigned char com;
void tran() /*Send one byte data sub-function*/
{
unsigned char i;
for(i=0; i<8; i++)
{ ACC=com; /*Put data into a*/
si=c;
sck=0; /*sck generates an up jump*/
sck=1;
com=com<<1; /*Shift left one bit*/
}
return;
}
main()
{
com=0x06; /*Send write and read enable command*/
cs=0;
tran();
cs=1;
com=0x01; /*Send write status word command*/
cs=0;
tran();
com=0x00; /*Timing 1.4s*/
tran();
cs=1;
...;Program part for normal system operation
}

It should be noted that when the program is running normally, a dog feeding instruction should be added in an appropriate place so that the timing time of the system during normal operation does not reach the preset time. The system will not reset. The dog feeding instruction is as follows.

main()
{
...; The program part for normal system operation
{
cs=0; /*Generate cs pulse*/
cs=1;
}
}

The watchdog circuit of X25045 is very convenient to use. X25045 also integrates 512BEEPROM and voltage operation monitoring system. Only one chip, plus crystal oscillator and reset circuit can form a single-chip microcomputer application system, which is very suitable for the design of portable instruments and embedded systems.