MC81F4x16
16. WATCH DOG TIMER
BCK[2:0]
Basic interval timer INT enable
fxx/1024
fxx/512
fxx/256
fxx/128
fxx/64
fxx/32
fxx/16
fxx/8
Start the CPU
M
U
X
clear
BTCL
clear
Watchdog
Counter (7-bit)
clear
overflow
8-Bit Up Counter
BITR
overflow
BTIR
BTINT
BTIE
fxx
Prescaler
Basic interval timer INT request
WDTSR
7-bit Comparator
WDTIE
7-bit Compare data
WDTON
WDTIR
WDTCL
WDTR
To RESET CPU
Watchdog timer INT enable
WDTINT
Watchdog timer INT request
Figure 16-1 Block diagram of Basic Interval Timer/Watchdog Timer
The watchdog timer rapidly detects the CPU malfunction such as endless looping caused by noise or
the like, and resumes the CPU to the normal state. The watchdog timer signal for detecting
malfunction can be selected either a reset CPU or a interrupt request.
When the watchdog timer is not being used for malfunction detection, it can be used as a timer to
generate an interrupt at fixed intervals.
The watchdog timer uses the Basic Interval Timer as a clock source.
The watchdog timer consists of 7-bit binary counter and the watchdog timer data register. When the
value of 7-bit binary counter is equal to the lower 7 bits of WDTR, the interrupt request flag is
generated. This can be used as Watchdog timer interrupt or reset the CPU in accordance with the bit
WDTON.
Watchdog reset feature is disabled when the watchdog timer status register(WDTSR) value is „0A5h‟.
Note that, WDTSR‟s reset value is „00h‟. And reset value of WDTON is „1‟. So watchdog timer reset is
enabled at reset time.
112
October 19, 2009 Ver.1.35
FINECHIPS [ FINECHIPS ]
