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LSF0102 - Uart 3.3V <-> 5V

ee06091
Intellectual 340 points

Hi,

I need to translator the uart signals (tx and rx) between two mcus that run at 3.3v and 5v. The mcus are atmega328p that can source/sink up to 40mA and an atxmega8e5 that can source/sink 25mA.

After read the datasheet I'm a little confused so I need your help to confirm the schamtic I desing:



Is this ok? My confusion comes because I didn't understand if I need the Rpu only for bidirectional signals or also for unidirectional signals like my case.


Thank you

  • 0
    TI__Mastermind 46480 points

    Your set up is correct. I dont know which way you are translating. If you are translating down then no pullups are nessessary. If you are translating both directions then only the pullups on the high side are needed.

    However it is goood to have pads where the resistors can be used if you have ringing on th bus.

    once the Vref_A voltage is reached the output is basically high impedance and can be easily influenced by outside noise so to have the option to add resistor is always good.

  • 0 in reply to Chris Cockrill
    Intellectual 340 points

    3.3V                                               5V

    TX - A1                                     B1 - RX

    RX - A2                                     B2 - TX

    I'm translating down on B2 to A2 and up on A1 to B1. So don't I need any pullups?


    I don't understand what do you mean here "once the Vref_A voltage is reached the output is basically high impedance".

  • 0 in reply to ee06091
    TI__Mastermind 46480 points

    the I/Os are all clamped to Vref_A  level. the output can be pulled highr than Vref A with a pull up resistor.  So a pull resistor is not required if you are down translating to the Vref A level.  If you are up translating thene a pull resistor is needed to pull it on up past VrefA.

    I have attached a appnote on voltage clamps that show how they work. 

    voltage clamps made easy.pdf