I'm designing my first 3V3 circuit with ATmega8. While the uC itself supports both 5V and 3V3, some of components do not. I'm going to place ISP socket on board and I'm wondering how to ensure that a 5V programmer won't damage lower voltage components. My first idea is:
- route Vcc from ISP through 3V3 low-drop regulator (already available on board) and add a diode to protect the programmer from being powered backwards with 12V which appears on board "before" the regulator,
- add voltage dividers on MOSI and SCK lines which, if I understand well, are always programmer's outputs and circuit's inputs.
Does it make sense?
In case someone connects a programmer already working with 3V3, the dividers will drop it to approx. 2.18V. Won't that be too low to maintain correct logic levels?