Rainbow Electronics T89C51RD2 - Manual (Page 31)

31 Rev. F - 15 February, 2001
T89C51RD2
SFR the output will be low, when it is equal to or greater than the output will be high. When CL overflows from
FF to 00, CCAPLn is reloaded with the value in CCAPHn. This allows updating the PWM without glitches. The
PWM and ECOM bits in the module's CCAPMn register must be set to enable the PWM mode.
Figure 12. PCA PWM Mode
6.5.5. 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 10 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:
•
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. 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.
CL
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
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