Experiments 3.1: Sequential Logic - S-R Latch and a Gated S-R Latch

This video demonstrates two circuits: First - an S-R Latch that is created using two NOR gates. It can be set or reset using the two inputs on the left hand side and shows how a basic flip-flop works. Second - a gated S-R latch that adds an enable input to the circuit, which must be pressed before the state can be changed. This is one of the first circuits in the EE223 Introduction to Digital and Analogue electronics module at Dublin City University. The associated materials are at: http://www.eeng.dcu.ie/~molloyd/EE223/

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[warpp2]

Hello, i have a question : What kid of resistance are you using for the push button?

[DerekMolloyDCU]

@warpp2 The resistor on the buttons is about 10k Ohm or above. Any value around that should be fine.

[superfrankpt]

What is the state when we power on the circuit?

I assume that because the input in both NORs are the same the outputs will be the same too! Won't that violate the "Q" "Not Q" complementary!?

[DerekMolloyDCU]

@superfrankpt Good point! Yes, it will violate this condition and would be unpredictable and invalid.

[superfrankpt]

@DerekMolloyDCU so what happens when we power on the circuit? Must we provide some initial state?

I'm just a begginers trying to understand mor about logic gates :)

[DerekMolloyDCU]

@superfrankpt Yes, and this is not ideal. You would have to iterate over all of the flip-flops and maybe set them to 'reset' before they are used. The S-R flip-flop is more a theoretical concept and is not used commonly in digital circuits. It is more likely that you would use a J-K flip-flop which doesn't have the undefined stage.

[warpp2]

@DerekMolloyDCU Ahh thx i will try that, becous somethings it dont works ect in your previous and I've got your videos yesterday to watch and I got one that said to 470 Ohms and it works. waht I have seen is that if you dont use the resistor, the led is not entirely stable.

[GalaticTG]

You have two unused nors on that chip. If you configured them as T flipflops and you had another chip like that you could make a 4-bit binary counter.

[bradster1200]

Good Video, Wikipedia didn't explain it nearly as well as you did.