1. Jeff Yi CS 147

2. Circuits  Combinatorial – Circuit that only relies on inputs.  Sequential - Circuit determined by input as well as the previous state of the circuit.

3. Types of sequential circuits  Synchronous sequential circuit – Uses a clock to determine when a events occur.  Asynchronous sequential circuit – Uses the previous circuit changes to determine when the next event occurs.

4. Feedback  Sequential circuits use the previous state as input by using feedback.  After passing through a flip-flop, the result is put back as input along with any other inputs.

5. Clock  A clock sends signals in high and low levels in intervals for sequential circuits.  Clocks have high and low levels as well as rising and falling edges.

6. Clock Examples

7. Flip-Flop  Flip-flops are a synchronous sequential circuit that is edge-triggered.  Latches are a synchronous sequential circuit that is level-triggered.

8. Types of Flip-flops  SR flip-flop uses 2 inputs: S (set) and R (reset).  JK flip-flop is a variation of SR where J is set and K is reset.  D flip-flop uses a data input of 1 or 0, where 1 sets the flip-flop and 0 resets it.  T flip-flop uses a toggle input of 1 to change states

9. SR Flip-flop  Has an S input to set and an R input to reset.  When S=R=0, then no change is made.  When S=1 and R=0, the state is changed to set.  When S=0 and R=1, the state is reset.  However, when S=R=1, the output is undefined.

10. SR Flip-flop Truth Table

11. SR Flip-flop (NOR)

12. SR Flip-flop (NAND)

13. JK Flip-flop  Variation of SR Flip-flop  Named after Texas Instruments engineer, Jack Kirby.  J is the set input and K is the reset input  Combinations of J and K are the same as the SR Flip-flop except for when both are 1.  In the case where J=K=1, the output simply toggles between set and reset.

14. JK Flip-flop

15. D Flip-flop  Uses data input to set S or R as 1.  When D=1, S=1 and R=0.  When D=0, R=1 and S=0.  This prevents both S and R from both being 1.

16. D Flip-flop

17. T Flip-flop  Uses a single toggle input for the JK Flip-flop.  When T=1, then whatever the state is, it switches to the opposite.  When T=0, nothing changes.

18. T Flip-flop

19. Questions?

20. References  Lecture Notes  Anthony Des Reis, Assembly Langugae And Computer Architecture Using C++ And Java, Thompson,2004.  Null and Lobur, The Essentials of Computer Organization and Architecture, Second Edition  “Flip-flops”. http://wearcam.org/ece385/lectureflipflops/fli pflops/