In-system programming circuit

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Hi all,

I want to do In-System programming for an ATmega48 with and AVRISP MKII.
During programming I need to isolate the target board VCC (3.3V) from the programming voltage because the programmer cannot provide power for the complete board. The target board is unmounted, so I cannot use the target board power itself.
A series diode for isolating board VCC and programming VCC consumes about 0.4V, which is to much.

Does anybody have an implementation idea using a FET or similar instead of the series diode?

I will implement series resistors for MISO, MOSI and SCK. I did not find any recommendation for resistor values. Any recommendation from the forum?

Thanks,
HJ

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I use 1000 ohm series resistors. I guess you could use larger resistances.

I can think of a few ideas that would let you use the AVRISP, but I don't know if they are practical. It seems to me you would have to break the VCC connection in the ISP cable. Then you would have to power each board separately. I guess you would have to insure the signals didn't exceed the VCC of either board by much more than a couple of tenths of a volt. If you could keep each VCC close to the other, that might do the job. Otherwise you could use 2.7 volt (or whatever) zeners and resistors in the signal paths.

Personally I would put the AVRISP back in it's box and use a Butterfly running ButtLoad. The Butterfly could be powered by whatever power supply you are using for your target. You would need a serial port on the PC or a USB to RS232 adapter.