AT89C5131A-RDTUM datasheet(68/188 Pages) ATMEL

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4337K–USB–04/08

AT89C5130A/31A-M

Figure 14-6. PCA PWM Mode

14.5

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 14-4 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 low.

In order to hold off the reset, the user has three options:

Periodically change the compare value so it will never match the PCA timer

Periodically change the PCA timer value so it will never match the compare values, or

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.

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 ECCFn

CAPPn

8-bit Comparator

CEXn

“0”

“1”

≥

Enable

PCA Counter/Timer

Overflow

AT89C5131A-RDTUM Datasheet(PDF) 68 Page - ATMEL Corporation