Page 159 - DCAP108_DIGITAL_CIRCUITS_AND_LOGIC_DESIGNS

Digital Circuits and Logic Design

Notes the negative transition of the clock pulse. Some master-slave flip-flops change output state on the
positive transition of the clock pulse by having an additional inverter between the CP terminal
and the input of the master.

Figure 9.8: Timing Relationship in a Master Slave Flip-Flop

9.4 Triggering of Flip-Flop

The state of a flip-flop is changed by a momentary change in the input signal. This change is called a
trigger and the transition it causes is said to trigger the flip-flop. The basic circuits of Figure 9.1 and
Figure 9.2 require an input trigger defined by a change in signal level. This level must be returned
to its initial level before a second trigger is applied. Clocked flip-flops are triggered by pulses.
The feedback path between the combinational circuit and memory elements in Figure 9.9 can
produce instability if the outputs of the memory elements (flip-flops) are changing while the
outputs of the combinational circuit that go to the flip-flop inputs are being sampled by the clock
pulse. A way to solve the feedback timing problem is to make the flip-flop sensitive to the pulse
transition rather than the pulse duration.
The clock pulse goes through two signal transitions: from 0 to 1 and the return from 1 to 0.
As shown in Figure 9.9 the positive transition is defined as the positive edge and the negative
transition as the negative edge.
Figure 9.9: Definition of Clock Pulse Transition

The clocked flip-flops already introduced are triggered during the positive edge of the pulse, and
the state transition starts as soon as the pulse reaches the logic-1 level. If the other inputs change
while the clock is still 1, a new output state may occur. If the flip-flop is made to respond to the
positive (or negative) edge transition only, instead of the entire pulse duration, then the multiple-
transition problem can be eliminated.
9.5 Timing Signal

(Computer science) A pulse generated by the clock of a digital computer to provide synchronization
of its activities.
(Electronics) Any signal recorded simultaneously with data on magnetic tape for use in identifying
the exact time of each recorded event.

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