multiple power sources on CMOS?

Thread Starter

Gadersd

Joined Dec 8, 2012
98
I have a micro controller connected to a CMOS shift register. The shift register is powered by a separate battery than the micro controller. The micro controller powers on before the shift register. Would the floating vcc on the shift register cause problems when the micro controller powers on?
 

BobTPH

Joined Jun 5, 2013
3,111
Not if you set all the outputs going to the CMOS SR to low until the SR is powered. If one is high, it might end up powering the CMOS chip through protection diodes.

Bob
 

crutschow

Joined Mar 14, 2008
27,009
You can also put a resistor (1kΩ-10KΩ) is series with the SR inputs to limit any current during the time the SR is not powered.
 

Thread Starter

Gadersd

Joined Dec 8, 2012
98
I would like to place a 10K pull down resistor on the shift register input and a 10k series resistor from the input to the micro controller. I am worried that this won't supply the 4.1 volts needed to turn on the input. What can I change to make this work?
 
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