Help

I have PIC16F84 -04I/SO controllers changing their configuration bits on the
fly.  Some have been installed for a few weeks, others have been in the
field for two years. The circuits are potted and therefore I can rule out
any tampering.

Environment : 14 to 21V DC, temperature range -8 deg Celsius to +80 deg
Celsius, outdoors
Oscillator : Murata CSTCS 4.00MHz (integrated 18pF caps)
Original Configuration : OSC=XT, WDT=ON, PUT=ON, CP=ON
Supply : 78DS05F 5V regulator designed for automotive industry
Voltage supervisor : Motorola MC33064
Source Power-Up : Very slow rise & fall from 0V DC to 19V DC (sometimes in
the region of 20 minutes).

The reset circuitry has been designed such that the circuit draws more
current in reset than when operational, this stops the supervisory chip
hunting in and out of reset.

Failed units' configuration bits are generally as follows; OSC=RC, WDT=ON,
PUT=OFF, CP=OFF

I have also found that we can get some failed units to run normally by
powering them up quickly (normal 5V DC supply).

Applying a 4MHz oscillator to pin 16 (OSC1) shows the device is still
operational, just that the on board oscillator will not function with the
OSC configuration bit set to RC.

Has anyone come across something similar, or can anyone suggest reasons for
this anomaly?

Regards

Mr Tom Phillips
Systems Engineer (Solar)
Merlin Gerin SA (Pty) Ltd t/a CONLOG
Tel       :   +27-31-268-1111 (S/Board)
              +27-31-268-1476 (Direct)
Fax       :   +27-31-208-7790
e-mail    :  phillipst@conlog.co.za
Internet  : www.conlog.co.za

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