If you use external clock that runs at startup you can gett problem.
RC oscillators starts quick and a caned clocksource runs from the start i=
f
you have that powered on before the PIC.
Crystall oscillator take some time to start and after that you have 1024
clock cycle count in the PIC just to be sure the oscillator is stable.
My advise is to dissable the external oscillation by a pulldown on OSC in=
ut
during programming.
External crystall is usually not a problem.
Have a look in the programming specification for the PIC you use. There y=
ou
can find critical time for Vpp and Vdd etc.
Niklas

-----Ursprungligt meddelande-----
Fr=E5n: pic microcontroller discussion list [mailto:PICLIST@MITVMA.MIT.ED=
U]
F=F6r Peter Onion
Skickat:        den 21 januari 2002 12:15
Till:   PICLIST@MITVMA.MIT.EDU
=C4mne: [PIC]: Startup & ICSP problems with external clock source.

I've been using my own design of ICSP programmer for a few months now wit=
h
no
problems.

Until now I've only ever used crystals (From 1MHz up to 20MHz) for the
master
clock source.  Yesterday I tried using an external clock source (for bett=
er
accuracy) and ran into problems.

When I put my programmer into program mode the target pic doesn't recogni=
se
the
"Enter HV programming" sequence and instead starts running its program.

Somewhere I remember seeing a reference to "Not more than SoMany clock
pulses"
in a timing diagram to do with start up/reset/enter programming mode.  I'=
ve
just had a look in the 16F877 data sheets and I can't find again.

Is my problem cause by the external clock source starting up quicker than
the
internal oscillator+xtal would start ?

Peter.

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