89C51 microcontroller + watchdog circuit composed X25045

   Using 89C51 microcontroller and X25045 composed watchdog circuit , X25045 hardware connection diagram shown in Figure 1. The X25045 chip contains a watchdog timer can be preset by the system software to monitor the time. In the watchdog timer preset time if there is no bus activity, the X25045 will RESET outputs a high level signal, through differential circuit C2, R3 outputs a positive pulse, the CPU is reset.The circuit of Figure 1, the reset signal to the CPU of a total 3: Power-on reset (C1, R2), manual reset (S, R1, R2) and Watchdog Reset (C2, R3), added after the adoption of a comprehensive RESET terminal or gate. C2, R3 do not have a time constant is too large, there can be hundreds of microseconds, because then the CPU oscillator already working.