T89C51AC2-RLSI-M datasheet(81/117 Pages) ATMEL

81 / 117 page

Draft.A - March 30, 2001

Preview - Confidential

T89C51AC2

15.7. PCA Watchdog Timer

An on-board watchdog timer is available with the PCA to improve system reliability without increasing chip

count. Watchdog timers are useful for systems that are sensitive 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. 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.

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. If other PCA

modules are being used the second option not recommended either. 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.

T89C51AC2-RLSI-M 数据表(PDF) 81 Page - ATMEL Corporation