AT89C51ED2-RDTIM datasheet(47/137 Pages) ATMEL

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AT89C51RD2/ED2

4235E–8051–04/04

Figure 21. 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 19 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:

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.

Disable the watchdog by clearing the WDTE bit before a match occurs and then

re-enable it.

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

AT89C51ED2-RDTIM 数据表(PDF) 47 Page - ATMEL Corporation