I have a project that uses a PIC16F1934-I/PT running on 5V via a 78L05=20
regulator which is fed from a 12V battery. The 5V rail has one 47uF=20
electro as well as one 1uF and two 100nF decoupling caps.

A 433MHz receiver module feeds the UART (via a transistor to provide=20
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=20
curious way.  After programming the chip (via ICSP), the unit behaves=20
correctly. If I power it down and back up again however, it fails to=20
respond to commands via the UART.

The oscillator is a 20MHz 3 pin resonator and I have HS mode selected. =20
Interestingly, the status (heat beat) LED flashes at the same rate in=20
both good and fault conditions.

The PIC has a 10K resistor from MCLR to Vdd, but no capacitor to Vss. If=20
a reset it by shorting to Vss, the unit works fine again. It's only when=20
the board is first powered up that the fault occurs.

It's almost as though the UART is not initialising correctly. I have=20
PWRTE enabled and the watchdog disabled.

We replaced the PIC in the first faulty unit and it is behaving=20
correctly now. I have done nothing to the second faulty unit and can=20
replicate the fault easily.

I'm loath to replace the PIC on the second board just yet, as I'd like=20
to know why this is happening first.  Any ideas?  About the only thing I=20
can think of is Vdd rising too slowly?
David...

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___________________________________________
David Duffy        Audio Visual Devices P/L
Unit 8, 10 Hook St, Capalaba 4157 Australia
Ph: +61 7 38235717      Fax: +61 7 38234717
Our Web Site: www.audiovisualdevices.com.au
___________________________________________

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