89C51RB2 Datasheet, PDF (28/52 Pages) NXP Semiconductors – 80C51 8-bit Flash microcontroller family 16KB/32KB/64KB ISP/IAP Flash with 512B/512B/1KB RAM

Philips Semiconductors
80C51 8-bit Flash microcontroller family
16KB/32KB/64KB ISP/IAP Flash with 512B/512B/1KB RAM
Preliminary specification
89C51RB2/89C51RC2/
89C51RD2
WRITE TO
CCAP4L
RESET
WRITE TO
CCAP4H
1
0
ENABLE
CIDL
WDTE
––
––
CCAP4H
CCAP4L
MODULE 4
16–BIT COMPARATOR
MATCH
––
CPS1
CPS0
ECF
CMOD
(C1H)
RESET
CH
CL
PCA TIMER/COUNTER
––
ECOMn CAPPn CAPNn MATn
TOGn
0
0
1
X
Figure 25. PCA Watchdog Timer m(Module 4 only)
PWMn
0
ECCFn
X
CCAPM4
(C6H)
SU01105
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 25 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
PCA timer,
2. periodically change the PCA timer value so it will never match
the compare values, or
3. disable the watchdog by clearing the WDTE bit before a match
occurs and then re-enable it.
The first two options are more reliable because the watchdog
timer is never disabled as in option #3. If the program counter ever
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 26 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 26.
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
2¹⁶ count of the PCA timer.
1999 Sep 23
28