2.4 MOSI

When programming the AVR in Serial mode, the In-System Programmer supplies data to the target on the MOSI pin. This pin is always driven by the programmer, and the target system should never attempt to drive this wire when target reset is active.

The target AVR microcontroller will always set up its MOSI pin to be an input with no pull-up whenever Reset is active. See also the description of the Reset wire.