How do you hold a state in a fully CMOS logic circuit?

Question

I can use the P and N MOSFET transistors to create boolean operators (AND, OR, NOT and so on..) They are basically signal analyzers: if the input signal is taken away, the circuit stop working.

How do I design a circuit that, given an input starts producing an output -and keeps it even if the input is taken away-? Can i build one just by using resistors, capacitors and MOS transistors?

Answer 1

Yes, it's called a flip-flop.

The simplest is two transistors cross-coupled with resistors.

If you have NAND gates, just feed the output of the first to one of the inputs of the second, and the output of the second to one of the inputs of the first. The remaining inputs are used to set or reset the FF.

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Answer 2

You will need a set/reset latch or a flip/flop. You can make a set/reset latch out of to nand gates. Alternatively, you can use a flip/flop as well.

Answer 3

Define the input with a pull up resistor. Never allow a CMOS circuit to stay in an undefined state.