Datasheet

AT89C51IC2

4301D–8051–02/08

Figure 16. PCA PWM Mode

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 14 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 mod-

ules 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 appli-

cations the first solution is the best option.

This watchdog timer won’t generate a reset out on the reset pin.

CCAPnH

CCAPnL

ECOMn

CCAPMn, n= 0 to 4

0xDA to 0xDE

CAPNn MATn TOGn PWMn ECCFnCAPPn

8 bit comparator

CEXn

“0”

“1”

Enable

PCA counter/timer

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