----- Original Message -----
From: "Dan Michaels" <oricom@USWEST.NET>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Friday, September 29, 2000 3:04 AM
Subject: Re: [PIC]: ICSP


> Andy Howard wrote:
> .........
> >I'm thinking of adding an ICSP socket to a future product for field
upgrades
> >so I'd be interested to hear if you find any gotchas when you come to
play
> >with it.
> >
>
> I talked with Jeff of melabs a few weeks ago about using the
> EPIC programmer for ICSP, and the major gotcha he mentioned was
> regarding the exact cktry tied to /MCLR pin on the PIC. You have
> to make sure you can pull the pin high fast enough [goes to current
> drive out of the programmer], and also that the programmer doesn't
> affect Vcc by backfeeding thru the /MCLR ckt.
>
> Apparently, on some marginal P/S designs, Vpp can pull up the Vcc
> bus. Maybe low current regulators, whatever. I did try this and
> found Vpp actually kicked up the output of a 7805 [1A v.reg] a
> couple of tenths of a volt. Have to watch the size of the pullup
> resistor on /MCLR - ie, not too low - if that is all you have
> on the pin. If you put a cap there, then you may not be able to
> pull the pin up fast enough.


Thanks for that Dan, the Vpp/Vcc interaction looks like a classic gotcha.
I'll certainly look out for that one.
In the Mchip documents they suggest fitting a Schottky diode between the R-C
junction and the MCLR/Vpp pin to isolate the capacitor and resistor.

















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