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Intefacing a 5v UART to 3.3V once

Discussion in 'Electronic Design' started by GNU, Jun 4, 2004.

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  1. GNU

    GNU Guest

    I want to comminicate two UARTs : 3.3V (UART1 Vinmax = 3.7V) with a TTL
    one (UART2 5V) from 9600Baud to 250Kb/s like this :

    UART1 (TTL 5V) UART2 (3.3V)
    TX1 ------------> RX2
    RX1 <----------- TX2

    From TX2->RX1, i think there's no problème. My question is how to
    interface Tx1&RX2, The first sends 5v to Rx2 wich accepts only 3.3
    signals. Need your help please.
  2. Paul Burke

    Paul Burke Guest

    Use something like a 74LVC14 (2 gates in series to invert twice) running
    off the 3.3V supply. The inputs are 5V tolerant, the output should be
    high enough to driver the 5V UART.

    I assume by 9600 to 250kb you aren't hoping the uarts will do the
    conversion for you....

    Paul Burke
  3. Tam/WB2TT

    Tam/WB2TT Guest

    People have used resistive dividers. For example, 1.8K series resistor and
    3.3K shunt. Or use a 1.8K a resistor pack; this will give you 1.8K series
    and 3.6K parallel.

  4. Tim Wescott

    Tim Wescott Guest

    First verify that your 3.3V UART isn't 5V tolerant (some are). If it is
    5V tolerant then you can just hook up to it directly. If not then the
    resistive divider should work. Check the input capacitance of the 3.3V
    UART to make sure that it won't slow your edges down too much at high
    baud rates, and the logic level high voltage on the 5V UART to make sure
    that 3.3V is high enough to be interpreted as a '1'.
  5. First verify that your 3.3V UART isn't 5V tolerant (some are). If it is
    I was using a when I got this problem PIC, which has a gotcha: the
    UART input sets itself to schmitt-trigger mode, which leaves very
    little room at the high side.

    Wouter van Ooijen

    -- ------------------------------------
    PICmicro chips, programmers, consulting
  6. Tim Wescott

    Tim Wescott Guest

    That's a bummer. You could use a single non-inverting Tiny-logic buffer
    to boost it up.
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