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SN74HCS157: unused inputs

Vincenzo Pizzolante
Mastermind 23701 points
Part Number: SN74HCS157

hi,

section 9.2.1.2 Input Considerations of DS states:

"[...] Unused inputs must be terminated to either VCC or ground. These can be directly terminated if the input is completely unused, or they can be connected with a pull-up or pull-down resistor if the input is to be used sometimes, but not always. A pull-up resistor is used for a default state of HIGH, and a pull-down resistor is used for a default state of LOW. The resistor size is limited by drive current of the controller, leakage current into the SN74HCS157, as specified in the Electrical Characteristics, and the desired input transition rate. A 10-kΩ resistor value is often used due to these factors.[...]"

Is it not just sufficient to pull them to GND in case (as for example in fig.11, also misleading)?

thanks a lot in advance

KR

Vincenzo

  • +1
    TI__Guru* 96782 points

    Hi Vincenzo,

    It looks like there's a typo in figure 11-1 -- it should say "Unused inputs tied to GND" since these are obviously shorted to ground-- either way would be fine though. The important thing is to terminate unused inputs, and leave unused outputs disconnected.

    The statement you quoted includes your answer at the start:

    Unused inputs must be terminated to either VCC or ground. These can be directly terminated if the input is completely unused...

    In other words - you can directly connect any unused input to either VCC or GND.

  • 0 in reply to Emrys Maier
    TI__Mastermind 23701 points

    hi Emrys,

    clear.

    unfortunately we already manufactured the pcb with unused in/outs left NC.

    Would it have malfunctions or will it be still functional?

    what should we expect in such situation?

    thanks a lot 

    KR

    Vincenzo

  • 0 in reply to Vincenzo Pizzolante
    Guru 277170 points

    Because of the Schmitt-trigger inputs, there will be no damage, and other channels will not be affected. However, floating inputs will still increase the power consumption.