The usage of 89S51 watchdog function is as follows

The usage of 89S51 watchdog function is as follows: 
In the initialization of the program, write 01EH first and then write 0E1H to the watchdog register (WDTRST address is 0A6H). The watchdog can 
be activated  .
Org 0000  
Ljmp begin 
Begin: 
Mov 0A6H,#01EH; send 1E first 
Mov 0A6H,#0E1H; send E1 later 

; Activate the watchdog in the program initialization. 
…… 
…… 
For: 
…… 
Mov 0A6H,#01EH ;Send 1E first 
Mov 0A6H,#0E1H ;Send E1 later 
;Feed the dog instruction 
…… 
Ljmp for 

In C language, a declaration statement must be added. 
Add a line in the AT89X51.h declaration file sfr WDTRST = 0xA6; 
Main() 
{ 
WDTRST=0x1E; 
WDTRST=0xE1;//Initialize the watchdog. 
While (1) 
{ 
WDTRST=0x1E; 
WDTRST=0xE1;//Feed the dog instruction 

} 
} 

Notes: 
1. The watchdog of 89S51 must be activated by the program before it starts working. Therefore, it is necessary to ensure that the CPU has a reliable power-on reset. 
Otherwise, the watchdog will not work. 
2. The watchdog uses the crystal oscillator of the CPU . The watchdog is also invalid when the crystal oscillator stops. 
3. 89S51 has only 14-bit counter. The watchdog must be fed at least once in 16383 machine cycles . And this time is 
fixed  and cannot be changed. When the crystal oscillator is 12M, the watchdog must be fed once every 16 milliseconds.
The timer can also be used to extend the watchdog feeding time from a few seconds to a few minutes. 
In general, a dedicated watchdog chip is used.