On Wed, Nov 30, 2022 at 08:10:27PM -0500, Paul Koning via cctalk wrote:
5V tolerant does not mean 5V compatible. I have right
now some 5V devices
I want to control, and it's not exactly clear whether a 3.3V device will
drive outputs high enough to reliably make 5V devices see them as high.
Arduinos can be had in actual 5V models (5V power, standard 5V logic
levels in and out). Not the fast ARM ones but for many purposes good
There's no single "standard 5V logic levels". The usual comparison is
between TTL and CMOS of course, but there are also the subfamilies which are
mostly compatible, right up until they aren't.
Anyway, you can usually drive a TTL(-compatible) input from a 3.3V output
because TTL treats anything above 2V as a logic 1. The "usually" caveat is
because there are pathological devices out there, so check those datasheets
to see if they're compatible. In particular, some microcontrollers can only
source limited current and some rare old TTL devices are quite thirsty. (At
least they won't get rarer if you try this, as you'll burn out the