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Keypad Electrical Connections

Chris Read
Genius 3345 points
Other Parts Discussed in Thread: TPS65950

I want to be able to use electrical signals to short a row and a column line together.  The TPS65950 documentation always shows simply a mechanical switch between the row and the column, but I want to use an electrical switch such as a FET.  There isn't much guidance in the documentation about doing this.  Does anyone have a suggestion about what to use?  pFET?  nFET?  something better/cheaper/more flexible?

 

Thanks!

  • 0
    TI__Genius 15100 points

    Hi Chris,

    There arent any specific recommendations for this. You can build any logic circuit that would satisfy the switching operation for detecting a key press.

     

    Regards,

    Gandhar.

     

     

  • 0 in reply to Gandhar Dighe
    Intellectual 650 points

    I am working on the same issue.  I tried using 2 74LVC244 parts with 4 column outputs to the 4 OE/ to mux 16 switches onto 4 rows.  What I found was that in hardware mode the column selects all go low at once while it's waiting for a keypress, so the driver outputs fight on the LVC244 parts.  We were able to get this work with software mode by scanning the columns individually.  Next rev. we'll use a different circuit with open drain outputs.

    Also, in the TRM SWCU050E section 10.5.1 where it describes s/w mode, I believe step 3(b) should read "Enable the first column to set the corresponding bit to 0 in the KBC_REG register." instead of 1.

  • 0 in reply to Andrew Dyer
    TI__Genius 15100 points

    Hi Andrew,

    Thanks for sharing. I will look at the steps in TRM.

     

    Regards,

    Gandhar.

     

  • 0 in reply to Gandhar Dighe
    Genius 3345 points

    Is there at least a maximum resistance that is allowable?  Alternatively, can you provide any further information about the size of the pullup on the row, and the voltage thresholds for detection on the row and column?


    Thanks,

    Chris