My 6722 died yesterday !!

cstan_02 · Joined: 24 Jul 2005 Posts: 13

Hello,
Can you tell me what are reasons for the PIC to fail ??
And when it fails, it grounds the +5v supply. I was field testing my working code and it suddenly failed. I had tested my unit for 24 hour runs before and never had any problems. The power supply is stable(from a battery). Everything was fine ?? I am using a PIC18LF6722 with external 24Mhz oscillator. This happened before atleast once, which was more than six months back. That time i suspected that it had something to do with my hardware. This time I have separate power supplies for different classes of components, and all are stable. I just wanted to know how can I prevent this from happening again.

Thanks

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

It's possible that it's a CMOS latch-up problem. Describe the components
and circuits that are connected to the PIC pins.

cstan_02 · Joined: 24 Jul 2005 Posts: 13

Well to say, the circuit is simple with a spi flash memory which uses the 3.3v rail, one transciever connected to one of the uart and a gps on the other. The power for the latter two components are isolated from that of the main 6722 power. That is, there are two +5v rail and another 3.3v rail. Talking to the flash is through some voltage dividers. Other than that there is a MAX232, an external 24 Mhz oscillator and some few transistors.

Anyway, is there something like overclocking the PIC. Well I don't think so. But Mr. PCM_Programmer i don't understand by what you said it can be a CMOS latchup problem. Can you explain a bit more. That would be so nice !!

Thanks.

asmallri · Joined: 12 Aug 2004 Posts: 1634 Location: Perth, Australia

cstan_02 · Joined: 24 Jul 2005 Posts: 13

Thanks Andrew, I'll try to take care of the latchup problem next time. But is there any sure way of preventing this, like if i power everything in the circuit with a single supply, will it eleminate the problem ?? Or is it enough that I put capacitors to increase the VDD rise time of the components connected to the PIC I/O ??

Thanks.

asmallri · Joined: 12 Aug 2004 Posts: 1634 Location: Perth, Australia

Generally if you have a common power supply with a siingle power rail (VDD and VSS are common) then latchup will not occur. It is possible for latchup to occur if you have two seperate power rails. For example, assume you have a 9 to 12 volt power source that you use to derive 5volts for a PIC and 3.3 volts for some other device. Assume you have two regulators, a 7805 derives the 5 volts for the PIC from the power source and a 3.3 volt regulator derives its output from the +5 volt regulator. If you have a large capacitor on the output of the 3.3 volt regulator then the 5 volt rail will rise before the 3.3 volt rail. If the PIC asserts a high level out to the 3.3volt device during this period then latchup can occur.

This problem can be solved in two ways. First you mimimise the difference in rise time between the two power supplies and secondly you incorporate a delay in your PICs startup to enable the power supplies to stabalise before asserting any outputs.
_________________
Regards, Andrew

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cstan_02 · Joined: 24 Jul 2005 Posts: 13

Thanks Andrew, have a nice day !!

:-)