I'm using a Attiny45 microcontroller and USBtinyISP as my programer, and this is the first time I need to hook stuff up (in this case just LEDs, but eventually I'll want to conect a raspberry pi) to the programing pins (SCK, MISO, MOSI). As I want the microcontroller to be re-programmable when it's all soldered up. I first breadboarded the circuit to make sure it works. This is what I found:
If I have a LED and resistor in series between SCK or MOSI, avrdude throws this error at me:
avrdude: initialization failed, rc=-1
unplugging the LED gets rid of the error, but isn't ISP ment to allow this?(Arduino's have a LED connected to pin 13 SCK)
However on MISO this never happens and I'm free to upload the program with the LED attached. Why is this? Is there a way to obtain this behaviour on the SCK and MOSI? How is this issue going to be affected if I replace the LEDs with raspberry pi GPIO thru a logic level converter?