UART: interfacing b/w 3.3V and 2.4V - current limiting resistor

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?

UART: interfacing b/w 3.3V and 2.4V - current limiting resistor

UART: interfacing b/w 3.3V and 2.4V - current limiting resistor

– Schematic created using CircuitLab


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.

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.

Category: uart Time: 2016-07-29 Views: 0

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