I have a 3.3V UART (two pins) plus a direction selector pin (also 3.3V.) I need to talk to and listen on a single duplex serial bus at 5V with 20 mA drive strength. The signaling rate is 3 Mbit/s, although switching directions is allowed to take 4 microseconds.
An automatic bidirectional translator like TXB0102 is not right because it ignores the direction signal.
A 74hct125 (one element going each way) doesn't do level translation.
A simple MOSFET for translation isn't good enough because it's not push/pull and thus doesn't get the drive strength both ways (plus strong pull-ups are bad for this bus.)
Finally, I prefer something in DIP-8 but can go up to DIP-14 if no external components are needed, and I'll break down and use SMDs if there is no other choice.
So, what existing component is there that fulfills these requirements? I've browsed parametric search at Digi-Key but not found anything that his all the requirements above…
Best Answer
The 74HCT125 is fine. Power it with 5V and it will read your 3V3 Tx and enable signal just fine because \$V_{IH}\$ is only 1.6V (2V worst case). For the 3V3 Rx use a resistive voltage divider for 'level translation' from the 74HCT125.