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TCA8418E: TCA8418E

sonMEE
Intellectual 460 points

Part Number: TCA8418E

Hi,

The manual of TCA8418E states that "If unused, connect to VCC through a pullup resistor".

However, Figure 20. of TCA8418E-EVM Schematic shows unused is floating.

First, Which is correct?

Second, why I don't connect it to ground instead of pull-up resistor, it reduces the cost and the space?

Third, if I must connect them to Vcc through pull-up resistors, can I connect all unused to One common resistor?

Fourth, does the switches matrix need pull-up resistors?

Best regards

  • 0
    TI__Guru 62920 points
    Hi Amin,

    In general it is a good practice to make sure that digital inputs remain in a fully-high or fully-low state and avoid intermediate voltage (since these can result in excessive current consumption). For the TCA8418E, there are multiple valid ways of accomplishing this:

    1. Follow the datasheet recommendation and pull the GPIOs high through a resistor.
    2. Leave the GPIOs floating. These pins have an internal pull-up resistance that is enabled by default, and this will be sufficient to maintain a high-level voltage at the pin. You should be careful not to disable this internal pull-up while the GPIO is configured as an input, though.
    3. Pull the GPIOs to ground. In this case, you may want to disable the internal pull-up resistances so that they do not increase the supply current drawn by the IC.

    Pull-up resistors are not required for the functionality of the IC, but some external switches do need pull-up resistances (for example, they may be configured so that the line is normally pulled high but can be shorted to ground via a key press).

    Regards,
    Max
  • 0 in reply to Miguel Robertson
    Intellectual 460 points
    Thanks Max,
    On page 40, indicates no external pull-up resistors are needed. Which means the matrix switches don't need pull-up.
    Another question, is there a TQFP package type version? I haven't try to set the layout and Figure 40 , the text indicates " a 4 layer board is required". If the signal traces need 4 layers, that will force me to have a 6 layer board.
  • 0 in reply to sonMEE
    TI__Guru 62920 points
    Hi Amin,

    No, there is only a WCSP package option for this device. This unfortunately requires a few layers to break out the signals. Figure 40 shows an implementation with 3 routing layers, although it may be possible to simplify this if some of the GPIOs are unused.

    Max
  • 0 in reply to Miguel Robertson
    Intellectual 460 points
    Thanks Max,
    Yes, I'm going to use 4 rows and 6 columns. Which may be fine.