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

 

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.