AT89C51ED2-RDTUM datasheet(46/137 Pages) ATMEL

46 / 137 page

4235K–8051–05/08

AT89C51RD2/ED2

Figure 13-6. PCA PWM Mode

13.5

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 13-4 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 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-RDTUM 数据表(PDF) 46 Page - ATMEL Corporation