I have been unsuccessfully investigating the mystery of start-up failure on
our PIC16F870 board.

The circuitry that is implemented is as follows:

+5V power supply with risetime <300 usec
NOT_MCLR driven by reset generator which holds it low for approximately 250
msec from power-up
Crystal oscillator in HS mode running at 8 MHz which appears to start up <20
msec from power-up
Watchdog timer and brownout detector disabled

With program loaded in flash there appears to be no execution following
power-up.

The program can be started by pulsing NOT_MCLR low several seconds (or
minutes) after power-up.

The program can also be started by enabling the watchdog timer.

This behaviour smacks of the processor entering sleep mode from power-up and
needing to be woken before anything happens. This is not consistent with my
interpretation of the data sheets and application notes.

Does anyone understand what is going on here (or knows someone who does)?

Rod
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