# Sequential Circuits : Part I Read Sections 5-1, 5-2, 5-3.

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Sequential Circuits : Part I Read Sections 5-1, 5-2, 5-3

2 Topics Sequential Circuits ♦ Latches ♦ Flip Flops

3 Sequential Circuits Definition: State of system is “stored information” Present state and inputs, determine outputs and next state

4 Types of Sequential Circuits Synchronous ♦ State changes are synchronized by one or more clocks Asynchronous ♦ Each state change occurs independently of other changes

5 Clocking of Synchronous Changes of the state enabled by a clock

6 Comparison Synchronous ♦ Easier to analyze ♦ Choose the clock so that changes are only allowed to occur before next clock pulse Asynchronous ♦ Potentially faster ♦ Harder to analyze Will look mostly at synchronous

7 Basic Storage Fig. 5-2 Logic Structures for Storing Information ____________________________________ YES.YESYESYESYESYESYESYESYESYESYESYES

8 Basic Storage Apply low or high for longer than t pd Feedback will hold the value of the input Fig. 5-2 Logic Structures for Storing Information ____________________________________ YES.YESYESYESYESYESYE SYESYESYESYESYES

9 Basic Storage Apply low or high for longer than t pd Feedback will hold the value of the input Fig. 5-2 Logic Structures for Storing Information ____________________________________

10 SR (set-reset) Latches Basic storage made from gates Requirement: outputs be the complements of each other S & R both 0, Latch in “resting” state Have to keep both from 1 at same time Fig. 5-4 SR Latch with NOR Gates

11 Simulation Of SR Behavior When both S and R go to 0 after 11, Q & Q_b take on unknown values; depends on circuit delays and slight differences in the times at which S & R change values

12 Latch YES. yes. YES.YES.YES.YES.YES. YES YES.

13 Latch

14 Add Control Input (SR ) Input, C, controls when state can change Good Morning. Good Morning. Good Morning. Good Morning

15 Add Control Input (SR ) Input, C, controls when state can change Is there a latch with no undefined state?

16 D-type Latch No undefined (illegal) state

17 D-type Latch No undefined (illegal) state

18 Flip-Flops Two major types ♦ Master-Slave Two stage Output not changed until clock disabled (low) ♦ Edge triggered Change happens when clock level changes

19 Master-Slave Flip-Flop Either master or slave is enabled, not both Master Latch Slave Latch

20 Timing Diagram Illegal State (b) FF in wrong state due to 1’s catching (a) Q should be 0 since Q was 0 before the clock pulse and both S & R are 0 just before the clock goes to 0

21 Note: New inputs appear at latches are not sent to output until clock low Changes at input of FF when clock high trigger next state

22 D-Type Positive-Edge Triggered FF

23  Master-Slave: Postponed output indicators  Edge-Triggered: Dynamic indicator Standard Symbols for Storage Elements

24 Direct Inputs Set/Reset independent of clock ♦ Direct set or preset ♦ Direct reset or clear Often used for power-up reset

25 Next State Diagrams Registers