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TPS65921: keypad i/f with external pull-ups

Gunter Schmer
Genius 13667 points
Other Parts Discussed in Thread: TPS65950, TPS65921

Hi,

 

the TRM shows that for the row inputs on the keypad interface there are internal pullup resistors. If one were to use additional external pull-ups, which voltage should they be pulled to?

 

Thanks,

--Gunter

 

  • 0
    TI__Genius 13667 points

    Hi,

     

    additionally the Mistral DM37xx EVM is using external 4.7K resistors pulling to VIO_1v8. There is a note in the TRM that if a smaller keypad area is used, the unused row/column inputs are recommended to be pulled up. Should a good design use external pullups to be safe from false scanning? And secondly what voltage should the pins be pulled to? It is not clear from the TRM/datasheet.

     

    Thanks,

    --Gunter

  • 0 in reply to Gunter Schmer
    TI__Genius 13667 points

    Hi,

     

    the feedback was that there are internal PU resistors and external are not needed. My followup question is that in the TPS65950 datasheet there is the following line on page 130:

    "The keyboard interface can be used with a smaller keyboard area than 8 × 8. To use a 6 × 6 keyboard,

    KBR(6) and KBR(7) must be tied high to prevent any scanning process distribution."

     

    So that raises the question again, whether external PU resistors could be needed. Could someone clarify?

     

    Thanks,

    --Gunter

  • 0 in reply to Gunter Schmer
    TI__Genius 15100 points

    External resistors are not needed. I will have to fix the document. I believe the document was providing a very conservative approach to avoid false scanning as you have pointed out.

  • 0 in reply to Gandhar Dighe
    TI__Expert 6715 points
    Hi Team,

    This issue has come up again and we are interested in knowing the nominal value of the internal pullups:

    From the customer ---------------------
    Do you know what the nominal value is for the internal pull ups on the kbd_r_x pins?
    We found that using a stiffer pull down resistor (10K > 100ohms) on the input pins, we get the correct binary value when reading the port.
    We would also like to know what is the maximum logic LOW threshold value of the input pins? I was not able to find this information in the TRM or Datasheet.
    This will allow us to set the pull down value to meet the threshold levels, with some margin, and minimize power consumption.
    ---------------------

    I was also not able to find this information in the datasheet.

    Thank you!

    Regards,
    ~John
  • 0 in reply to John Fedak
    TI__Expert 6715 points
    Hi Team,

    An additional question has come up:
    ------------------------------
    Is it possible to disable the internal pull ups on the KBD_R pins?
    The TPS65921 register manual shows two registers (GPPUPDCTR2 and GPPUPDCTR3) which are described to control pullup/pulldowns, but the fields are reserved.
    Due to a backwards compatibility issue with units already in the field, we need to explore alternate solutions.
    ------------------------------

    Please let me know!

    Regards,
    ~John