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floating (unconnected) inputs

autodeskguest

In the good old days of TTL, a floating (unconnected) input was treated as

a HI. For CMOS, I've read (Xilinx website and others) that a floating input

was a NO-NO. It seems that the input can float to a HI or LO, or in the

middle where the input draws more power. So, from what I've read, inputs

need to be connected to a HI, LO, or a signal. I'd like to hear from other

people on this subject.

-Dave

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  • On 04/10/15 15:37, David Pollum wrote:

    In the good old days of TTL, a floating (unconnected) input was treated as

    a HI.

     

    Well, no. In TTL, a floating input is "very probably" high, but it's

    highly recommended to avoid them. Pull all unused inputs up.

     

    For CMOS, I've read (Xilinx website and others) that a floating input

    was a NO-NO. It seems that the input can float to a HI or LO, or in the

    middle where the input draws more power.

     

    Yes, floating inputs on CMOS are indeterminate and cause high power

    consumption. They're also susceptible to EMI and can seriously junk your

    circuit.

     

    So, from what I've read, inputs

    need to be connected to a HI, LO, or a signal. I'd like to hear from other

    people on this subject.

     

    Simple.

    - NEVER leave an input floating, regardless of what technology

    - For TTL, tie high through a resistor

    - For CMOS, tie them high or low through a resistor depending on logic

    - It is acceptable to tie directly to ground