On 24/11/2010 22:45, David Duffy (AVD) wrote:
> I have a project that uses a PIC16F1934-I/PT running on 5V via a 78L05
> regulator which is fed from a 12V battery. The 5V rail has one 47uF
> electro as well as one 1uF and two 100nF decoupling caps.
>
> A 433MHz receiver module feeds the UART (via a transistor to provide
> inversion) and it has an output to drive a logic level MOSFET.
>
> Of the dozen or so of these built so far, a couple have failed in a
> curious way.  After programming the chip (via ICSP), the unit behaves
> correctly. If I power it down and back up again however, it fails to
> respond to commands via the UART.
>
> The oscillator is a 20MHz 3 pin resonator and I have HS mode selected.
> Interestingly, the status (heat beat) LED flashes at the same rate in
> both good and fault conditions.
>
> The PIC has a 10K resistor from MCLR to Vdd, but no capacitor to Vss. If
> a reset it by shorting to Vss, the unit works fine again. It's only when
> the board is first powered up that the fault occurs.
>
> It's almost as though the UART is not initialising correctly. I have
> PWRTE enabled and the watchdog disabled.
>
> We replaced the PIC in the first faulty unit and it is behaving
> correctly now. I have done nothing to the second faulty unit and can
> replicate the fault easily.
>
> I'm loath to replace the PIC on the second board just yet, as I'd like
> to know why this is happening first.  Any ideas?  About the only thing I
> can think of is Vdd rising too slowly?
> David...
>

All the things that have been mentioned sound plausible - also, I'd=20
maybe try pulling out the 47uF cap to see if it makes a difference.

--=20
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.