Does anyone know if it is OK for me to drive MISO, MOSI and SCK from an AVR processor even when the Pololu USB programmer (pololu.com/catalog/product/1300) is connected and powered? That is, when the programmer isn’t actively programming the chip, does it float those lines? I don’t want to fry it
Regarding MISO: The programmer is the SPI master, so it never drives the MISO line.
MOSI: The programmer only drives this line while the reset line is being driven low.
SCK: Because of how the AVR ISP interface was designed, our programmer actually has to drive SCK during the brief period (250 microseconds) where the AVR is running, right after initial reset period of the AVR, but before the reset period where programming occurs. This does have the potential to cause a short, but only for 250 us. To be safe, you can delay for a few milliseconds at the beginning of your AVR code before driving SCK.
In short: yes you can drive those lines but it is a good idea to delay a few milliseconds before you first start doing it.