I am populating a custom PC board a section at a time and testing each section before doing the next. I added a couple ICs that connect to SCK and MOSI. After that when I try to upload a test program (with an AVRISP mkII), I get an error saying something about a short circuit. The existing test program runs fine and there isn't any unreasonable current draw.
So there is something about the addition of the ICs that the SPI doesn't like while programming.
So as a test, I removed the ICs... AVRISP mkII is happy now.
Is there some circuitry I should include to help separate the ISP connector from the rest of the circuit?
(I'm working on getting a schematic cleaned up - there is a lot of extra stuff that can be removed for clarity)