1 Dec
2022
1 Dec
'22
4:51 p.m.
On 12/1/22 05:35, Peter Corlett via cctalk wrote>
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
microcontroller first.)
A more significant issue is the substitution of 74HC for 74HCT logic in
applications driven by TTL in bus applications, where the loading can be
significant. Vih min tends to be around 3 volts for HC devices.
Bottom line is that you may be able to get away with using an HC instead
of an HCT device as a TTL bus receiver, but don't count on it always
working.
--Chuck