On 25/11/2010 9:28 AM, ivp wrote:
>> We replaced the PIC in the first faulty unit and it is behaving
>> correctly now
> Do you mean you have two problems - a faulty chip and a power-
> up issue ?

Hi Joe,
I mean replacing the chip made the power-up issue go away for that=20
board. No other changes were made. Exact same PCB and firmware loaded.


>> The PIC has a 10K resistor from MCLR to Vdd, but no capacitor
>> to Vss. If I 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
> If you change nothing else, a cap would ensure that Vdd gets up
> before /mclr, holding the chip in reset. However, PWRTE is meant
> to perform that task

That's what I will be trying next I think. I left it off as I'm new to=20
ICSP, and just having the 10K pullup seems to satisfy the MCLR=20
requirements (low capacitance on the line) from what I have read.

>> It's almost as though the UART is not initialising correctly. I have
>> PWRTE enabled and the watchdog disabled
> If the receiver is powering-up before the PIC then the PIC will be
> seeing noise from the Data output, and that could be going into the
> UART buffer. I wonder if this noise is getting to the PIC's Vdd
> internally via the protection diodes before the PIC has powered
> up properly from the supply rail
>
> In projects of mine that use a receiver, the receiver's Vdd is sourced
> from the PIC, and is supplied only after the PIC has initialised. Done
> that way not with power-up in mind, but for power-saving during
> normal operation

The receiver is powered all the time, but its output goes to an NPN=20
transistor that has a pull-up to Vdd. There is code in the receive ISR=20
to dump bad packets and synchronise, but it seems like the receiver is=20
just not listening.  I may have to add some additional code to output=20
the state of the UART via a spare pin.

>> If I power it down and back up again however, it fails to respond
>> to commands via the UART
> Powering down may not allow enough energy to dissipate to cause
> a reset before powering up again, especially now that micros are such
> low power devices.. If you try one of those power downs and ground
> /mclr you may find it does reset properly

Powering up with 0V all round (left to sit for any more than a couple of=20
seconds) is when the issue occurs.
David...

--=20
___________________________________________
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
___________________________________________

--=20
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