Remote Processing RPC-220 - Manual (Page 32)

WATCHDOG TIMER SECTION 9
Page 9-1
Figure 9-1 Watchdog Jum per Location
WATCHDOG TIMER S E C T IO N 9
A watch dog timer r esets the CPU if it enters erroneous
process or states or "loop fo rever " code . T he timer is
reloaded in periods that are shorter than the watchdog
interval. The timer is built into the CPU and is enabled
or disabled by jumper W6.
OPERATION
When the w atchdog timer is ena bled and overflow s, a
short reset pulse, internal to the CPU , is generated. A
short output pulse is also generated but does not have
any effect on the reset signal at J2 or J3.
Inform ation about the w atchdog star ts on page 3-48 0 in
the 8X0C552OV. PDF file.
The tim er consists o f an 8 bit timer and an 11 bit
prescaler. The 8 bit timer is loaded to prevent a reset
pulse. Watchdog timer interval is determined by the
following formula:
interval = 1.111 m s * timer_value
The w atchdog ser vice routin e is very sh ort:
watchdog:
ORL PCON,#10H ;set condition
MOV T3,timer_value ;load with
;interval
RET
This should be in assembly language even when using C
to prevent it from wrongly ma nipulating PCON. The
address of T3 is 0xff.
USE DURING DEVELOPMENT
The watchdog jumper W 6 is set to [1-2] to disable it. If
you want to enable it for debugging purposes, you can
set the jumper to enable (W6[2-3]) while your program
is running. If a watchdog crash happens, the program
will reset and the monitor program
When y our pr ogram is in flash, you can set W 6[2-3] all
of the time.
On power up or reset, the watchdog timer is reset to 0,
which is a timeout interval of 283 ms. This is enough
t im e fo r ba si c s ys te m in it ia li za ti on (m e m or y an d I/ O
states). If you are perform ing extensive initialization
(such as clearing 512K of RAM) then you should put the
w a tc h do g r ou ti ne a s p a r t o f th e pr o g ra m .