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5V to 3.3V UART conversion

Discussion in 'Electronic Design' started by [email protected], Feb 22, 2006.

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  1. Guest


    I am trying to link the 5V serial port on my ADuC831 (8051) to the 3.3V
    serial port on my Analog Devices DSP board. Transmission is one way
    only (5V -> 3.3V). I have no idea what to do to drop the voltage.

    I read somewhere that it should simply be a case of adding a series
    resistor, the value of which could be calculated by finding out the max
    current from the data sheets of both boards. However I haven't been
    able to find any current values on the data sheets.

    Should I use a resistor, transistor, a diode maybe??

    Greatful for all suggestions!


  2. Paul Burke

    Paul Burke Guest

    Assuming the DSP inputs aren't 5V tolerant:

    Two resistors in a voltage divider.
    One resistor and a zener.
    A level translating IC - a 3.3V IC with 5V tolerant inputs.

    Comms the other way should be no problem.

    Paul Burke
  3. Guest

    how would i determine the resistor value if i choose the zener aproach?
    will any resistor do?
  4. Paul Burke

    Paul Burke Guest

    Depends how fast the edges need to be. The capacitance of the input plus
    the zener will combine with the resistor to give you a time constant
    (CR). The resistor needs to be small enough to make the rise time within
    whatever the data sheet recommends. 1k or thereabouts should be OK, at a

    Paul Burke
  5. linnix

    linnix Guest

    It's much easier with a transistor, using 3V on the collector and bias
    resistor on the base. This gives you a better drive current,
    especially for long links.
  6. Hi

    I've used the resistor divider approach successfully, simple and v

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