Hello Gentlemen and Gentlewomen,
I am using the SPI bus to load data to an external device that uses the MOSI and SCK pins, and I am using the 6 ISP pins for my programmer. App notes show placing 330 Ohm resistors on the MOSI and SCK pins so the external device does not interfere with the programming. I have built circuits both with and without 4.7 k resistors and they both seem to work. I have also read that in some cases I don't really need the resistors. Can someone clue me in to when I need these resistors and when I don't, and what size they should be.