| ??? 08/12/04 16:54 Read: times Msg Score: +1 +1 Good Answer/Helpful |
#75851 - Level translation Responding to: ???'s previous message |
The Atmel recommended 74VHC1GT buffer is deceptively named; it has TTL input levels, whereas 'normal' 74(V)HC devices have CMOS level inputs.
In general, a 3V output can be connected to the input of a 5V 74HCT device, but not a 74HC device at 5V. The reason is that the Vih for 74HC is 0.7*Vcc, which at 5V Vcc equals 3.5V. On the other hand, if you need to supply your logic with 3V, the situation is reversed, because 74HCT needs 5V ±10% and so you must use 74HC. That is unless you also need the inputs of your 3V logic to be 5V tolerant, in which case you should use something like 74LCX. Confusing isn't it? ;o) Rob. |
