Using a 3.3V MCU (Arm) to control reset pin of a 5V MCU (AVR) without a level shifter
Do you see any issues with this?


two issues for you to consider:

1) the 5v pin has likely a logic high level of 4v (5*80%, for ST gpio). so your 3.3v pin may not be able to "drive" it high;

2) if the 5v INPUT pin is every turned into an OUTPUT pin, you may see the 3.3v device being powered by the 5v pin, and the two power sources are shorted to each other, through the 3.3v pin's clamping diodes. 

The Right solution is to use a translator, which can be as simple as a transistor;

The Simple solution is 

1) to run the 3.3v pin as OD output, and the 5v pin as input with pull-up enabled. Clear the data register on that pin so even if it is turned to be an output pin, it can do no damage; and

2) to use a resistor in between the two pins to limit current when bad things happen - and bad things will happen.

The implicit assumption here is that the 3.3v pin is 5v tolerant. Or all bets are off.