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SN74LV4066A: 5V input to pin2A derails operation of SN74LV4066A

Part Number: SN74LV4066A
Other Parts Discussed in Thread: TMUX1511


I am using the 4066 as below to switch RX/TX to different ports:

At initial time while all the control lines are 0, 5V to ERXD, (5V from the TX of the connected serial port, from another system) causes mis-operation.

Mis-operation is that this is a custom board on a raspberryPI hat, so basically RXD is connected to RPI4s RXD port. When the rpi boots, all the control lines are zero.

I have purposefully placed rx/tx combinations thinking it was single direction switch, so directionality is preserved.

Taking the below diagram into account, how can 5V at input A, while C being low, cause mis-operation.

  • The recommended opertating conditions forbid I/O voltages above VCC. (The I/O pins have clamping diodes to VCC.)

    The TMUX1511 supports input voltages beyond the supply.

  • Hello,

    In addition to Clemens comments the TMUX1511  could be a good replacement  for the 4066 device because it they have a feature called power off protection  which prevents power sequencing like your are seeing now.

    1. The TMUX1511 which is a 1:1 4channel device but only has power off protection up to 3.6v on a signal pin when VDD=0v so in your current system that could be a problem.

    Here are some additional questions that can help us narrow down a new device.

    1. Does the control logic for the new device have to be the same Low= switch off  and High= switch on?

    2. What package of the 4066 device are you using? does the package size matter?

    Please let me know!



  • Hello,

    1. yes the control lines are the same

    2. I am using SN74LV4066ADR

    The datasheet says:

    these switches are designed to handle both
    analog and digital signals. Each switch permits
    signals with amplitudes up to 5.5 V (peak) to be
    transmitted in either direction.

    So, it is ok for it to output 3v3 when input with 5V.

    Could I get away by reversing the A, B, applying 5V to A, as opposed to B?

    Anyway, 4066 is quite an old part, and if upgrading to TMUX1511 will solve a lot of problems, I could do that as well, but for the time being I could take reverse the a and b with a tool.

    Best Regards,


  • Hello,

    My main usage is switching on/off rx/tx from microcontroller such as tm4c123, and maybe also switching on/off pwms of 20khz frequency. my main concern with switching the part is availability/cost.

  • TMUX1511 offers powered up protection up to 3.6V. I need 5V.  By the way the 4066 solution does work when controls are on.

    Best Regards,


  • Hello Can,

    Sorry for the confusion but I think I understand the problem now. Like Clemens said the 4066 cannot pass signals higher than vdd and it is not recommended to have signals higher than vdd on a signal pin even if is off/low/Hi-z.  For this application because your mux is powered on at  the start i don't believe power off protection is a critical feature here.

    Instead you need a device like the TMUX1511 which support signals on the io pins that are beyond the supply and stays hi z even when the signal path is off( control logic Low)  Note device must have supply to stay hi z with 5v on pin.

    Finally, all ti analog switches can pass signals bidirectionally so sending the signal from a to b or b to a doesn't matter!

    Please let me know if you have any additional questions!