Page 117 - Handout Digital Electronics

The SR flip flop Excitation Table

Present state Next state SR flip flop inputs
Q(t) Q(t+1) S R
0 0 0 x
0 1 1 0
1 0 0 1
1 1 x 0

The SR excitation table works as follows: Transition from state 0 (Q = 0) to next state Q = 0, the S input
must be zero (0), R input is a don’t care, meaning whether a 0 or a one is used, the output remains the
same. The designer will however choose to use a 0 or a 1 or whatever is at his/her disposal during the
design stage. Transitions from state Q = 0 to state Q = 1 and from state Q =1 to state Q = 0 are straight
forward and easy to understand. Transition from State Q = 1 to next state Q = 1, R must be equal to zero
(0) and S is a don’t care, meaning a zero or a one can be used. If a zero is chosen when in state one, it
means S = 0, R = 0, thus no change. A no change means if you are in state 1 and S = 0, R = 0, you will
remain in state 1. The same applies if you are in state 0.

The JK flip flop Excitation Table

Present state Next state JK flip flop inputs
Q(t) Q(t+1) J K
0 0 0 x
0 1 1 x
1 0 x 1
1 1 x 0

The JK flip flop is slightly complicated by the toggling input combination. So, it has don’t care conditions
in each line of the JK inputs. The first and the last are just like in the SR excitation table. This is however
not surprising since the JK flip flop is a modification of the SR flip flop. The second and third input
combinations make use of the toggling input combination of the JK flip flop. The best way to produce the
SR and JK excitation tables is to use their state diagrams.

Lecture 14 Video Lecture

Watch the complete explanation of this chapter here:
https://www.youtube.com/watch?v=6bMaqAmXWjA

Lecture 14 Online Quiz

Test your understanding by completing the quiz:
https://forms.office.com/Pages/DesignPageV2.aspx?origin=NeoPortalPage&subpage=design&id=6It6CPsfU
EqeY6fdRNP5cqrl2mW71GRAiQAluYy6dIVUNTlKVjJTWkNDRkZRN0UzWVFaTjg1TVlUTi4u

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