Ground state for IC inputs

Discussion in 'The Projects Forum' started by lokgotkent, Oct 16, 2010.

  1. lokgotkent

    Thread Starter New Member

    Sep 22, 2010
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    As far as I know the inputs for ICs are not necessarily in Ground state when they are left floating. However, I would only be interested to input a voltage when the switch is turned on. How can I achieve the ground state while the switch is not turned on?
     
  2. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    1,571
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    pulled low with a 10k resistor
     
  3. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
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    10K is the most common for this although with CMOS ICs you can use 100K or anything inbetween.
     
  4. lokgotkent

    Thread Starter New Member

    Sep 22, 2010
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    So what you mean is by shorting the input to both 'signal line' and 'ground with a large resistor in between'? By doing this, if signal line is 'high', it does not affect the 'ground'? Please correct me if I am wrong. Must the resistance be 10k? I thought it depends on the input voltage from signal line?
     
  5. Rbeckett

    Member

    Sep 3, 2010
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    All input and output from microcontrollers must be tied high or low, allowing either to float will yeild unexpected results such as random numbers and fibberish on the debug or display screens. If you will take a look at tieing inputs/outputs the explantions will clarify it much better than I can explain it with my limited background. Also decoupling caps and back EMF diodes are a cheap investment in the overall safety and longevity of any project that you might vuild with any micro processor. Have a good long read of the supplied datasheets for the chips you use, there is a ton of info you will need to know in those few pages, such as sink and source, min and max volts, temps and frequencies. Always a good idea to read and thoroughly understand them first, then build the circuit within those guidelines and all will be good.
    Bob
     
  6. windoze killa

    AAC Fanatic!

    Feb 23, 2006
    605
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    @rbeckett - Not sure where you got the idea he was talking about micro controllers from. He seems to be asking about ICs in general.

    @OP - It would be a good idea to read the datasheet for the IC you are using. Some are already pulled low internally and some are pulled high. If you need to pull it low then you connect the input pin to one side of a 10K resistor and the other side of the resistor to ground. You will need to do this to every input you need to pull low. If you want the input to be high then you will need to connect the pin to VCC (or whatever else you are calling the power rail) with a 10K resistor. BUT as I said read the datasheet to find out if it needed. No use wasting components if they aren't needed.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    All INPUTS to CMOS ICs must have a current path to either Vdd or ground, or circuit performance will be unpredictable. You can use resistors to create a current path.

    Unused OUTPUTS should be left disconnected. If you directly short an output to either Vdd or ground, you may damage the device.
     
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