P89C668 Philips Semiconductors, P89C668 Datasheet - Page 28

• Manufacturer Part Number: P89C668
• Description: 80C51 8-bit Flash microcontroller family 64KB ISP FLASH with 8KB RAM
• Manufacturer: Philips Semiconductors

Philips Semiconductors
PCA Watchdog Timer
An on-board watchdog timer is available with the PCA to improve the
reliability of the system without increasing chip count. Watchdog
timers are useful for systems that are susceptible to noise, power
glitches, or electrostatic discharge. Module 4 is the only PCA module
that can be programmed as a watchdog. However, this module can
still be used for other modes if the watchdog is not needed.
Figure 26 shows a diagram of how the watchdog works. The user
pre-loads a 16-bit value in the compare registers. Just like the other
compare modes, this 16-bit value is compared to the PCA timer
value. If a match is allowed to occur, an internal reset will be
generated. This will not cause the RST pin to be driven high.
In order to hold off the reset, the user has three options:
1. periodically change the compare value so it will never match the
2. periodically change the PCA timer value so it will never match
3. disable the watchdog by clearing the WDTE bit before a match
2001 Jul 27
80C51 8-bit Flash microcontroller family
64KB ISP Flash with 8KB RAM
PCA timer,
the compare values, or
occurs and then re-enable it.
WRITE TO
CCAP4H
1
WRITE TO
CCAP4L
0
RESET
ENABLE
Figure 26. PCA Watchdog Timer m(Module 4 only)
CCAP4H
16–BIT COMPARATOR
CH
PCA TIMER/COUNTER
CIDL
––
ECOMn
WDTE
CCAP4L
CL
CAPPn
28
––
0
MODULE 4
timer is never disabled as in option #3. If the program counter ever
MATCH
The first two options are more reliable because the watchdog
goes astray, a match will eventually occur and cause an internal
reset. The second option is also not recommended if other PCA
modules are being used. Remember, the PCA timer is the time
base for all modules; changing the time base for other modules
would not be a good idea. Thus, in most applications the first
solution is the best option.
Figure 27 shows the code for initializing the watchdog timer.
Module 4 can be configured in either compare mode, and the WDTE
bit in CMOD must also be set. The user’s software then must
periodically change (CCAP4H,CCAP4L) to keep a match from
occurring with the PCA timer (CH,CL). This code is given in the
WATCHDOG routine in Figure 27.
This routine should not be part of an interrupt service routine,
because if the program counter goes astray and gets stuck in an
infinite loop, interrupts will still be serviced and the watchdog will
keep getting reset. Thus, the purpose of the watchdog would be
defeated. Instead, call this subroutine from the main program within
2
16
CAPNn
count of the PCA timer.
––
0
MATn
––
1
TOGn
CPS1
X
CPS0
PWMn
0
ECCFn
ECF
X
P89C668
CMOD
(C1H)
RESET
CCAPM4
(C6H)
SU01105
Preliminary data