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Interfacing a PIC16F 5V to PIC18F 3.3V using two MAX232IDR connecting to UART Pins

Antonio Arnone
Prodigy 30 points
Other Parts Discussed in Thread: MAX232, PCA9306, TRS3232E

Hello 

I am connecting two pics Microchip by UART using the MAX232IDR. The first PIC is a 16F 5V the second one is a 18F 3.3V. The card that use the 18F has also a 5V power Line so i have connected the MAX232IDR to 5V. The PIC is connected at 3.3V which I believe the UART PINs will transmit around 3V. 

Now I am wondering is i can have any problem in transmitting and receiving. Should I connect the MAX232 at 3.3V too ?

Please someone can help?

Regards

Antonio 

 

  • 0
    TI__Guru 62920 points
    Hi Antonio,

    With VCC = 5 V, the output high level fro the MAX232 receiver to the MCU will be about 5 V, which may be too high if the MCU is powered via 3.3 V. (You should check the MCU datasheet to confirm.) Depending on the input voltage range of the MCU, you may be able to use a resistive voltage divider to reduce the voltage. Or, a pull-up resistance can be placed at the MCU input and a diode could be used to block the higher-voltage signals from the MAX232 from reaching the MCU. Or, you could use a level translator (PCA9306 is once example). Or, you could operate with VCC = 3.3 V. If that were the case, though, you would need to switch to a different transceiver that was designed for operation at this lower voltage. TRS3232E would be a p2p equivalent to MAX232 that supports VCC = 3.3 V. Let us know if you have any questions.

    Regards,
    Max