I want to use the "even" 3 pins of the 6-pin AVR ISP to power a Mega48 device and also to provide an analog control voltage on pin 4, which connects to MOSI. Unfortunately, for the M48, MOSI (PB3) is not an ADC input, so I would wire an ADC input pin in parallel with PB3.
The question is, what will happen when I drive that pin (and thus PB3) with an analog voltage, esp. one around Vcc/2? I know in general that's a Bad Thing, but how bad, exactly? Any thoughts?