The main way to store and retrieve things!
Goal: Change the output of the storage only when the clock changes.
- So when the clock turns on, the data might change, but the output does not change.
The solution is easy! Have 1 Clocked (Gated) SR Latch (this is what sets and resets the value) and feed it directly into another clocked SR latch (with Q = S and ) such that when Q is set, the other latch will also be set. However, the BIG difference is that the second latch’s clock takes the negation of the first latch’s clock. They are NOT on the same timings! (opposite timings, weirdly)
Not on same timings = no oscillation issue
- When C=1, the first latch updates its value. When C=0, the second latch takes the first latch’s value and stores that while also outputting it.
For the input to propagate to the output, both latches need to be active once
- The second latch is effectively a D-latch because R will be the negation of S (by nature of and )
- Flip-flops are Edge-Triggered
Now all we got to do is fix the S=R=1 problem again by making that first latch a D latch.
Edge-Triggered D flip-flop
- This is a negative-edge triggered flip-flop (like the above SR one)
- Means its output Q changes to D on the falling-edge — when C goes from 1 to 0
Positive-edge triggered flip-flops
Take the above, but negate C. That is all. Why this works? When C goes from 0 to 1 (rising), the second latch is active, updating the output.
- This is “THE” flip-flop!