AT89C51AC3-RDTIM datasheet(99/140 Pages) ATMEL

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AT89C51AC3

4383D–8051–02/08

Figure 58. PCA PWM Mode

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:

•

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.

The first two options are more reliable because the WatchDog timer is never disabled as

in the third option. 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 appli-

cations the first solution is the best option.

CL rolls over from FFh TO 00h loads

CCAPnH contents into CCAPnL

CCAPnL

CCAPnH

8-Bit

Comparator

CL (8 bits)

“0”

“1”

CL < CCAPnL

CL > = CCAPnL

CEX

PWMn

CCAPMn.1

ECOMn

CCAPMn.6

AT89C51AC3-RDTIM 数据表(PDF) 99 Page - ATMEL Corporation

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