digital-logicflipfloplatchlogic-gates
I was asked to find the answer for this, it is a pretty vague question:
On a D Flip Flop, what is the Q' output? Look this up and write your answer in your pre-lab report.
I am trying to understand the circuit, it looks like Q' is what retains the previous value of Q when the clock is high. Can someone explain more about this to me?
Montgomery Phister's textbook "Logical design of digital computers" from 1958 (written in 1957) introduces the D flip-flop as "the delay memory element".
This doesn't prove anything but it suggests that already in 1957, at least, there was tradition to suggest that the D means "delay".
As a side-note to put things into historical perspective, here's a schematic for a flip-flop from the same book, using radio tubes and 200 volt anode voltage.
You are using c as an active high reset, then setting the value to c rather than 1'b0. It is active high so c is always 1 when this code executes. The solution to this is the reset condition to 0 not c:
always @(posedge clk or posedge c) begin
if (c) begin
x <= 1'b0; //Reset Condition
end else begin
//x <= ...;
end
end
Your code makes use of an initial and asynchronous reset. This does not really represent any type of hardware you have to choose one or the other, you can not have both.
The waveform in the question does not show the async reset being applied at the start of the simulation. you should apply it for at least 1 clock cycle and release it on the posedge of your clock.
Best Answer
Very fundamental thing. Flip-flops are composed of logic circuits that have cross coupled feedback such that they "hold" the last established state. These bi-stable circuits are often composed of inverting gate elements that invert twice around their cross coupled feedback to realize the latched state.
Simply the Q output is the output of one side of the bi-stable circuit. The Q' is the other output which, in the case of cases where inverted logic elements are used, ends up being the logical inversion of the Q output.
So if Q = 1 the Q' = 0. Likewise of Q = 0 the Q' = 1.