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Hi, I use AT89S52 in a project. I send and receive data through TXD, RXD pins to a GSM module for sending SMS to another mobile equipment. I use watchdog facility and the watchdog counter is reset at every 256 counts. My crystal frequency is 11.0592 MHz. It happens once in a while that the uC doesn't respond and remains as if it hangs. Then it has to be RESET again. I doubt whether it happens while serial data reception and sending happens simultaneously. I wrote the program in C using KEIL IDE. My questions are 1) What are the special precautions to be taken in this case?(sending and receiving simultaneously) 2)why this happens even when the watchdog is active? It will be highly appreciated if anybody can give some guidance.
Thank you very much for the nice explanation. As mentioned earlier I faced a problem of program stops (may be due to voltage spikes). I added some filtering and made some changes in component values in RESET circuit . This gave good results. Still I wanted to take extra precaution. It is not that I am using external watchdog instead of trying to improve my design An external watchdog is powered separately, Is it the case always? I use the same power source for both and the external supervisor circuit is expected to RESET my processor if it stops.Is it OK?
Component values in reset circuit? Are you not using a tripod - a special reset chip just taking VCC, GND as inputs and having reset as only output? RC resets are seldom good.
Powered separately as in having a separate lead for power. An internal watchdog is powered from the same VCC leads as the processor core, which means that the watchdog can't just cut the power to the processor. With an external watchdog, it's possible to have the watchdog cut the power for a second to the processor by letting the watchdog drive a FET.
I tried supervisor IC DS1232. Working well.It has got a watchdog also which is monitoring the ALE pin of my uC toggling an external watchdog with ALE is no good, ALE can, and most likely will keep running with runaway code.
Anyhow the '1232 is a supervisor AND watchdog, so why do you still talk about the RC reset.
I, personally, am of the opinion that an internal watchdog (ONLY the kinds that require two consequent writes to be fed) is a better choice, but here is a case where opinions differ. Anyhow there is no discussion about two points a) any WD is better than none b) if the WD fires you have a problem that you should fix
Erik
Why do I consider an internal watchdog a better choice? I think the likelyhood of a port pin toggling in runaway code is greater than two specific consequent writes
Earlier I was using RC circuit. Now started using DS1232
"I think the likelyhood of a port pin toggling in runaway code is greater than two specific consequent writes" There exists intelligent external watchdogs that are using some form of protcol. Possibly an I2C message, or at least a pulsed sequence with limitations for max/min pulse length and pulse distance.
An external watchdog that only needs a low-to-high (or high-to-low) transition with a maximum delay is definitely dangerous. The code may have a lot of OP-codes that may affect the processor port.
Using an NXP LPC23xx processor, the situation could be improved by using the mask registers, so the relevant pin is write-protected between the kicks, making sure that just a wild port write with a random value will not affect the watchdog.
An external watchdog that only needs a low-to-high (or high-to-low) transition with a maximum delay such as the ever popular '1232 is definitely dangerous
as stated