Fanout calculations of CMOS MCU to 74HC IC's

Discussion in 'General Electronics Chat' started by dfro, Jul 26, 2010.

  1. dfro

    Thread Starter Active Member

    Feb 6, 2006
    37
    0
    I am interested in calculating how many parallel 74HC IC input pins can be driven from a single CMOS microcontroller output pin.

    All of the fanout calculations I have read about seem to be always based on driving 74LS (bipolar) device input pins.

    Here are my calculations:

    MCU output current limit = 20mA
    74HC input current limit = 1 uA

    Fanout = .020/.000001 = 20,000

    Answer: I can drive 20,000 parallel 74HC IC input pins from a single MCU output pin.

    Am I correct?

    Thanks,
    Dave
     
  2. dfro

    Thread Starter Active Member

    Feb 6, 2006
    37
    0
    I found my answer here:

    www.fairchildsemi.com/an/AN/AN-314.pdf

    It says there is essentially no fanout limitation for CMOS to CMOS connections.

    I am sure capacitance plays a role at some number of inputs, though.
     
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