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
