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Floating the inputs of Logic devices
TI recommends holding all unused or unconnected input and control pins at a valid logic level. Unconnected inputs tend to float at the input threshold level of the device causing excess supply current (ICC). There are two options to maintain unconnected input pins at a valid logic level:
1) By using pull-up (to VCC) or pull-down (to GND) resistors at the input. An output may be left floating except in the case of transceivers, where inputs and outputs use the same pin and must be held at a valid logic level.
2) By using TI Logic Devices with the Bus-Hold feature, for e.g., SN74LVCH16244A. The bus-hold feature prevents inputs from floating and holds them at the last known valid (HIGH or LOW) logic level.
Exceptions to this rule are FET switch-type functions and SSTV, SSTU devices. I/Os on FET switch-type functions can be left floating. SSTV and SSTU devices have reset circuitry to disable the input buffers. The inputs on these devices can be left floating only if RESET is held LOW.
Please see application report Implications of Slow or Floating CMOS Inputs, literature number SCBA004.
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