lecture 11

7.2.2 The edge triggered latches: Flip-flops
We have studied both S-R and D latch circuits with an enabling input. The latch
responds to input’s data only when the enabling input E is activated. In many digital
applications however, it is desirable to limit the responsiveness of a latch circuit to a
very short period of time instead of the entire duration that the enabling input is
activated. Another method of enabling a multivibrator circuit is called edge triggering.
For this method, the circuit’s data inputs have control only during the time during which
the enabling input is transitioning from one state to another. There are two types of
edge triggering

If the enabling input functions when transitioning from low state to high state
(from 0 to 1), the latch is said to be positive edge triggered.
- If the enabling input functions when transitioning from high state to low state,
the latch is said to be negative edge triggered.
Any latch whose enable input is edge triggered is a flip-flop. So we can say that
a flip flop is an edge triggered latch. An S-R flip-flop is an edge triggered S-R latch and
a D flip-flop is an edge triggered D latch.