Logic Gates - Pullup Resistor Inverted Inputs

126

01:03:33

The problem with using one-way switches to activate CMOS ICs is that floating inputs wreak havoc on CMOS circuits via shoot-through current.  Depending on available parts, one solution is to use pullup resistors.  This method inverts the inputs of the logic gates while forcing the input to be either HIGH or LOW - no floating values.  This schematic uses a NOR to simulate an AND (upper left), a NAND to simulate an OR (upper right), an AND to simulate a NOR (bottom left), and an OR to simulate a NAND (bottom right).

published 4 years ago

HIGreg

4 years ago

This is a really good demonstration of De Morgan's theorem: a gate is equivalent to inverting inputs, output, and changing the gate between AND and OR. This can be done by physically changing the inputs, ouput, and gate. Or it can be done "in your mind." This mental trick can help reduce chip count if you're ever in a situation to implement a gate or two with a quad NAND chip. If you find yourself with signals where 0v is a logical 1, then using the "opposite gate" is useful too. This is one reason that NAND and NOR are considered the universal gates: with only NANDs (or only NORs) you can implement inverters, NAND, NOR, AND, and OR gates. De Morgan's Theorem comes in handy!

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