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SN74HCS125: Interfacing 5V input to 3.3V GPIOs (5V tolerant)

Neet
Intellectual 391 points
Part Number: SN74HCS125
Other Parts Discussed in Thread: ESDS314, SN74HC125

Hello Community,

We have a application where we want to interface the output of SN74HCS125 operating at 5V VCC to the MCU digital input pins which are 5V tolerant. The signal shall be from external PWM generator upto 4kHz maximum.

  • I want to confirm do I need to add any current limiting resistor between the output of SN74HCS125 & the inputs of the MCU?
  • The total trace length between the MCU & the buffer output pins shall be less than 4"

Kindly suggest the robust measures to implement this use case.

Regards

  • 0
    Guru 244360 points

    5 V tolerant inputs do not have positive clamping diodes, so a series resistor would not have any effect. You can connect the signal directly.

  • 0 in reply to Clemens Ladisch
    Intellectual 391 points

    Hello Clemens,

    Thank you for the feedback, do we need to be concerned about the high current drive of the buffer in this case? CMOS by default has high impedance, yet if we might need some measures to make this configuration robust your inputs shall be helpful - this is a reliability centered use case.

    We have already added ESDS314 on the inputs of the SN74HC125. Adding a RC filter after the Buffer will it be of any help or is it better suited before the buffer in this configuration?

    Kindly guide.

  • +1 in reply to Neet
    Guru 244360 points

    What is the distance between the buffer and the MCU? Do you expect some noise to be coupled into this trace? If not, all filtering should be before the buffer.