> It turned out that 5 usec delay between Vpp and Vdd rise wasn't enough
> and I need 30 usec to program the device reliable.

That might be a spec problem, but are you sure your Vdd rises that fast?

IIRC I use a few ms delay.

Wouter van Ooijen

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Van Ooijen Technische Informatica: www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: www.voti.nl/hvu
 

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