Being a total newbie, I have been reading about logic gates. I got some doubts about flip flops RS(with NOR gates or NAND gates). The flip flops are said to keep a previous state so:

  1. Why an AND gate for example does not mantain its output through time? I mean, if the two inputs are for example 1, the output will be 1 and if I dont change the two inputs, when I read the ouput I will always have 1. So, this is not a way to memorize a value? I am missing something but I dont know what

  2. How electronically speaking, I can read from an output of a gate to know If I have 1 or 0 in its output? I mean, with a multimeter I can read manually the output of a gate but how is done electronically, for example, for reading from an output of a flip flop?

  3. Why is always needed the complemented Q? Its because for reading the Q value and know if I have 1 or 0 I have to read the complemented Q?



1 Answer 1

  1. The idea of a flip-flop is a single input may change and the output does not. That's why you have two inputs.

  2. The output is a voltage level. Inputs are sensitive to voltage levels. "Reading" means interpreting the voltage level as either "high" or "low".

  3. The complemented output isn't a must. It however comes in very handy in many situations.

  • \$\begingroup\$ Thanks. If I understood ok, the idea of a flip flop is to be less sensitive to accidental changes in its inputs in order to preserve the value of the output. Looking at the truth table of a NAND gate, its true that to change the Q from 1 to 0 or the opposite way, the two inputs have to change its values but with values 1 for S and 0 for reset, if the set changes its value to 0, the Q will be unknown so in this case, just one change in one input can alter the value of its output \$\endgroup\$ Dec 31, 2018 at 15:25
  • \$\begingroup\$ It's not about accidental changes but about memorizing a previous state. Once you set the flip-flop, it becomes insensitive on the set input. Once you reset it, it becomes insensitive on the reset input. \$\endgroup\$
    – Janka
    Dec 31, 2018 at 15:48
  • \$\begingroup\$ Ok, but when you say an input "may change", may change because I want to or because...? \$\endgroup\$ Dec 31, 2018 at 17:08
  • \$\begingroup\$ You want the circuit to become insensitive to the set input once set. You don't want the output to change depending on the value of the set input. That's how you turn a short pulse on the set input into a steady output value. And same for the reset input. \$\endgroup\$
    – Janka
    Dec 31, 2018 at 17:55

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