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I need to interface a 2.4V IC and a 3.3V IC via UART i/f. The VIH spec of the 3V3 IC is low enough to allow UART_TX_2V4 to be directly connected to UART_RX_3V3. My main concern is the UART_RX_2V4 pin which would see 3.3V levels and see current sinking in to its protection diodes. TO deal with this, I can have a current limiting resistor - shown in circuit below.

  1. Is this an acceptable way of interfacing or is there something I tend to lose by not going for a level shifter?
  2. Also - what spec would be I need to check on the 2V4 IC side to make sure that the UART_RX_2V4 pad is not damaged?

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Can you post the datasheet for the 2.4V IC? There's a good chance this is not specified and not technically allowed. \$\endgroup\$
    – Justin
    Commented Jul 29, 2016 at 18:23
  • \$\begingroup\$ You could add a 2.4V zener diode to the right side of the resistor. \$\endgroup\$
    – Rev
    Commented Jul 29, 2016 at 18:30
  • \$\begingroup\$ @NK2020 - From your earlier question here, we know the 3.3V UART is an FT232R. The comments you received in that question explained that you can run its I/O pins at 2.4 V, to match your 2.4 V MCU. (a) Can you tell us what MCU you are using at 2.4 V, or is it a secret? (b) Why don't you run the I/O pins of the FT232R at 2.4 V, as suggested in comments to that earlier question? What is stopping you from doing that, and thereby avoiding level-shifting, like you are asking about here? \$\endgroup\$
    – SamGibson ♦
    Commented Jul 29, 2016 at 20:49
  • \$\begingroup\$ Hi @SamGibson: the 3.3V IC I refer to in this question is not the FT232R. I have generalised the circuit and the schematic in this question for this to be helpful to other people as well. \$\endgroup\$
    – NK2020
    Commented Jul 30, 2016 at 2:50

2 Answers 2

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First, check the current sinking capability of gpio of IC 2.4v and current sourcing capability of gpio of IC 3.3v.

If sourcing current of the IC is lesser then sinking, then there shouldn't be any problem. But if it is higher then, u need to calculate the resistor value which will limit the sourcing current towards sinking. A simple ohms law will do for calculation of the resistor.

Also, check whether the IC 2.4v pins are tolerant to 3.3v. If it is, it will simplify the solution.

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The simpler proven method would be a resistor voltage divider from 3.3V to 2.4V. This avoids any issue with current through the protection diodes, which are not meant for general level shifting purposes.

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