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TCA8418: Pull-ups or no pull-ups for used and unused rows & columns?

Brian Castro
Prodigy 30 points
Other Parts Discussed in Thread: TCA8418
  • Part Number: TCA8418

The datasheet states:

1. TCA8418 has internal pull-ups so no external pull-up  resistors are necessary

2. Add external pull-up resistors to any unused rows & columns.

So, why do I need external pull-ups on unused rows and columns if they already have them internal to the device? Shouldn’t I be able to set all the unused rows and columns as inputs and enable the internal pull-ups? If external pull-ups ‘are’ needed shouldn’t I be able to configure all the unused rows & columns as inputs and use one pull-up resistor for all?


Furthermore, why does the EVAL board violate 2) above? There are no external pull-ups on all the unused rows and columns. 

I guess my real question is what is best practice for this IC? Do I use external pull-up resistors for unused rows & columns? Also for active rows that are inputs is there any need to add external pull-up resistors?

  • 0
    Prodigy 30 points

    I may have found some answers in another thread but I would still like confirmation. 

    1. The pull-ups on unused pins are not necessary if the IC is configured/initialized properly (internal pull-ups enabled). Hence why the EVAL board doesn't have them . 

    2. The internal pull-ups are relatively weak 100k. If have long traces or otherwise high trace capacitance it won't hurt to add an external 10k pull-up to active rows. 

  • +1 in reply to Brian Castro
    TI__Guru 72751 points

    Hi Brian,

    Assumption below is that you are using the device as a keypad scanner (as opposed to a GPIO expander).

    Brian Castro said:
    1. The pull-ups on unused pins are not necessary if the IC is configured/initialized properly (internal pull-ups enabled). Hence why the EVAL board doesn't have them . 

    For our device, rows are inputs and columns are outputs (outputs HI-Z or LOW, both states don't require pull ups) so rows should have pull ups (our device has them internal by default unless you disable them).

    Brian Castro said:
    2. The internal pull-ups are relatively week 100k. If have long traces or otherwise high trace capacitance it won't hurt to add an external 10k pull-up to active rows. 

    Sometimes the circuits/switches customers have are very capacitive and can cause errors with the detection algorithm for the rows. Having external pull ups help the rows reach the ViH level before the next column strobe occurs and prevents 'ghost presses' in the case of a high capacitive load. In short, yes external pull up resistors help, I recommend placing them as DNP (do not populate) if you are unsure if you need them or not.

    -Bobby

  • 0 in reply to BOBBY
    Prodigy 30 points

    confirmed. thanks!