Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Resistor value for Enable/Disable

  1. Jul 1, 2011 #1
    How is the resistor value for an IC Enable/disable pin decided.
    I always use 10K. But not sure how to calculate it .
    Do you connect Enable bar directly to ground or thru a resistor.
     
  2. jcsd
  3. Jul 1, 2011 #2

    berkeman

    User Avatar

    Staff: Mentor

    You can calculate the range of allowable pullup/pulldown resistor values based on the output current of the input (bias current) and the input logic high/low threshold voltage values. So the resistor value must be low enough so that the input logic voltage level is met, while the input bias current is passing through the resistor. That gives you an upper bound on the reisistor value, and you'll choose something to give you 2x or 4x margin or so.

    The reason to use a resistor to tie off an input (that is always at high or low) is for testability at the In-Circuit-Test (ICT) stage in manufacturing. That's the "bed of nails" test that checks continuity and basic component values on your PCB assembly.
     
  4. Jul 2, 2011 #3
    I'm a bit confused. An example would help.

    How would the resistor help for ICT?
    They can have a test point without resistor.
     
  5. Jul 2, 2011 #4

    berkeman

    User Avatar

    Staff: Mentor

    I'll try to post an example later today if I can get back to the PF. For ICT, if an input is hard grounded, you cannot tell if the IC pin is connected or not. All the ICT test point on that net can detect is the hard ground. If you connect the input to ground with a pulldown resistor, you can measure the value of the resistor, and also detect that the IC pin is connected (because of the internal ESD protection diodes to Vcc and Ground).
     
  6. Jul 5, 2011 #5

    berkeman

    User Avatar

    Staff: Mentor

    Sorry for the delay -- crazy busy weekend.

    So for a simple example, say you want to use a pulldown resistor to tie off an unused input to a 74LS00 quad NAND gate. From the old TI TTL databook (yes, the yellow one), you get:

    Vil = 0.8V max

    Iil = -0.4mA max at Vil = 0.4V

    So for the second set of numbers, you could use a resistor R = 0.4V / 0.4mA = 1k Ohm, but it would be best to use a smaller resistor by 2x or so to have margin on Vil.

    For a more modern example, we can check the 75175 line driver that you mentioned in your PM to me.

    http://pdf1.alldatasheet.com/datasheet-pdf/view/5748/MOTOROLA/SN75175D.html

    Vil = 0.8V max

    Iil = -100uA at Vil = 0.4V

    So the second set of numbers gives you R = 4k Ohms.
     
  7. Jul 5, 2011 #6
    Thanks. This is for unused pins only, correct?
    What about resistor values for Enable/disable?
     
  8. Jul 5, 2011 #7

    berkeman

    User Avatar

    Staff: Mentor

    The Enable input should have the same characteristics as the general inputs, I would think. Just double-check the datasheets to be sure.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Resistor value for Enable/Disable
  1. Resistor Value Codes (Replies: 2)

Loading...