Switching regulator modules typically have an enable input with a high resistance internal pullup to Vin.
Vin is much higher than the Attiny Vcc--in my case, up to 25 volts while my ATTiny84 is on a 3.3V linreg.
The electrical spec for VIH is VCC+.5 max. However, there is probably an input protect diode that will clamp the input voltage when such high value pullup resistors are used. Current would be something like 25 uA maximum.
We used to worry about CMOS latchup, if inputs were higher voltage than VCC. But, I suspect that enable inputs on DC/DC modules are routinely driven from 3.3V processors without issue.
Is this ok, or should I add a few parts to prevent current from flowing into an I/O when it is high? (schottky diode clamp to vcc, or a transistor stage).