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I am building a UART 3.3V to 5V Level translator between a Raspberry Pi and a ATMega328.

The 74HCT244 is my choice of translator but I can't find a good example of of hooking it up.

  1. Am I right that I should power it with 3.3V(VCC)?
  2. Output enabled(Grounded)
  3. 3.3v TX on Input 1.This should Output 1 at 3.3V to the ATMega328 that can work with 3.3V logic.
  4. 5v RX on Input 2 with Output Enabled. Connecting Output 2 at 3.3V to the RPI.

Number 4 to me is the bit I am not certain about. If I run the VCC at 3.3V will the input pins accept voltages above VCC.

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    \$\begingroup\$ Re $4 -- its clear from the table in the datasheet called "recommended operating conditions " that the maximum level of the input voltage Vi is VCC. \$\endgroup\$
    – tcrosley
    Commented May 6, 2015 at 8:13

2 Answers 2

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Instead of the 74HCT744, you could use a chip specially made for this, such as the TI TXB0104. SparkFun makes a convenient breakout board for this chip, priced at only $4, so you don't have to deal with as SMT part.

enter image description here

From the SparkFun description:

This 4-bit noninverting translator uses two separate configurable power-supply rails. The A port is designed to track VCCA. VCCA accepts any supply voltage from 1.2V to 3.6V. The B port is designed to track VCCB. VCCB accepts any supply voltage from 1.65V to 5.5V. This allows for universal low-voltage bidirectional translation between any of the 1.2-V, 1.5-V, 1.8-V, 2.5-V, 3.3-V, and 5-V voltage nodes. VCCA should not exceed VCCB. We have broken out each pin on this module for you to easily access both the A and B ports.

So you want to supply VCCA with 3.3v and VCCB with 5v.

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  • \$\begingroup\$ Thanks @tcrosley. I am looking at the TXS0102. It has only two lines and cost half the price but the package is tiny but usable. \$\endgroup\$ Commented May 8, 2015 at 8:38
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Run it on 5 volts.

schematic

simulate this circuit – Schematic created using CircuitLab

While using a resistive divider is not something to do with high-speed signals (due to the effects of parasitic capacitance) for UART signals you ought to be OK.

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