# SR Latch Understanding

this is my first post here as I am just getting into this electronic engineering stuff. My question concerns the SR flip-flop or latch, the NOR gate version. I have been reading a copy of http://www.amazon.co.uk/Digital-Techniques-Tutorial-Electronic-Engineering/dp/0412549700/ref=sr_1_1?ie=UTF8&qid=1376179565&sr=8-1&keywords=t.j.+stonham.

The circuit I am looking at is below:

simulate this circuit – Schematic created using CircuitLab

R and S are inputs, Q is the output of the second NOR gate, Q' is the previous value of Q, and X is the output of the first NOR gate.

I understand that Q is a function of R and S and the previous output Q' but there is something I cannot get my head around.

The truth table for the function is

R    S    Q'  |  Q
0    0    0   |  0
0    0    1   |  1
0    1    0   |  1
0    1    1   |  1
1    0    0   |  0
1    0    1   |  0
1    1    0   |  0 /*don't care*/
1    1    1   |  0 /*don't care*/


What I don't understand is a comment in the book. It says that if the two 'don't care' inputs are disregarded then the output of the right-most NOR gate is the complement of the left-most NOR gate.

Now, I have of course seen the cross coupled NOR gates diagram of the SR latch which has a NOT Q output, so I know I am doing something wrong somewhere, but I cannot see where.

For each of the cases in the truth table, this holds true, Q is the complement of X. But when R=1, S=0 and Previous Q=1 this is not the case.

When R=1, Q is always 0. But Previous Q=1, so X is also 0. X and Q are the same in the case.

EDIT:

Having thought about this, could the answer be that the 'problem' state I am considering is an unstable state; i.e. when NOR2 outputs 0, that 0 will also change NOR1 to 1, which would solve the problem.

Although that thinking led me to another problem, and that is that surely then, for a split (milli)second the outputs of NOR1 and NOR2 are the same before the change. The only problem I can see with this thinking is that I am tracking the changes in the circuit fairly slowly, when in fact the change should be instantaneous, i.e. that as soon as NOR2 changes to 0, NOR1 changes to 1 instantaneously.

EDIT II:

I am going to try to clarify tho confusion with my question here, and express the problem more concisely.

To clarify the variables: S - Set as usual R - Reset as usual Q - Output as usual Previous Q - The initial value of Q X - The output of NOR1

The problem I was having can be looked at as a transfer between two states.

The first state:

simulate this circuit

Here R and S are 0, Previous Q is 1, so X is 0, which gives Output Q as 1, which is Previous Q, the circuit maintains itself as expected (R and S were both zero).

But in the next circuit I switch R to 1:

simulate this circuit

So now Output Q and X are the same, they are not the complement of each other.

This is where I think I am going wrong:

This last state is an unstable state, and so will not remain as such. Previous Q will instantaneously turn to0 and hence NOR1 will output 1, giving X as 1, the complement of Output Q.

I think the problem in my thinking was that I was looking at it as if it would take some time interval for previous Q to reach NOR1 and change X to 1, when in fact it would occur at the exact same time as Output Q changing to 0.

Hopefully that has cleared things up.

• The electronic parts button above the textarea will let you insert an actual schematic. Commented Aug 11, 2013 at 0:38
• That's not the usual meaning (or location) of Q'... Commented Aug 11, 2013 at 1:00
• I am not very familiar with notation, what is the usual notation for the previous value of Q? Commented Aug 11, 2013 at 1:13
• The one I'm used to is Q₋₁. Commented Aug 11, 2013 at 1:31
• Q' is the output of NOR1 gate, if you look closely your Q and Q' are shorted and therefore the whole truth table is wrong. Commented Aug 11, 2013 at 10:35

First a slight correction to your diagram. The NOT Q output is the output of the NOR1 gate, not the input you have shown.

When both inputs are LOW (0) the latch holds it state.

With S, R = 0, 0

If Q = 1, then NOR1 input is 0,1 and its output (NOT Q) is 0 keeping Q = 1 If Q = 0, then NOR1 input is 0,0 and its output (NOT Q) is 1 keeping Q = 0

i.e. NO CHANGE

If S = 1 (R = 0) then the output of NOR1 (NOT Q) will be 0 and the output of NOR2 (Q) will be 1 - the latch is SET, Q is HIGH

If R = 1 (S = 0) then the output of NOR2 (Q) will be 0 and the output of NOR1 (NOT Q) will be 1 - the latch is RESET, Q is LOW

The problem comes when both S and R inputs are taken HIGH at the same time and forms a race condition. This logical condition (S = 1, R = 1) is to be avoided as the output cannot be determined.

• Sorry perhaps my notation is confusing, I will edit it, I've also just realised that I forgot to label X, the output of NOR1. Commented Aug 11, 2013 at 12:34

In Flip Flops with SR inputs,,S has priority over Q and R has priority over Qbar, which creates the don't care condition for the other inputs.