Sequential Logic

1. Sequential Logic

2. Block diagram of Sequential Circuit Inputs Combinational Circuit Outputs Next State Present State Storage Elements

3. Definition • A combinational circuit and storage elements are interconnected to form sequential circuit. • Storage elements are capable of storing binary information. • It defines the state. • Inputs together with the present state of storage elements determine the output. • Determine values used to determine the next state of storage elements.

4. Definition ctd… • Outputs in a sequential circuit are a function not only of inputs. • Present state of stored elements. • Next state of storage elements is also a function of inputs and present state of stored elements. • Thus, a sequential circuit is specified by a time sequence of inputs, internal states and outputs

5. Two types • Synchronous • Synchronous sequential circuits can be defined from the knowledge of its signals at discrete instants of time. • Asynchronous • Asynchronous sequential circuits depends upon the inputs at any instant of time and the order in continuous time in which the inputs change.

6. Synchronous sequential circuits • Employs signals that effect storage elements only at discrete instants of time. • Synchronization is achieved by a timing device called a clock generator. • Clock generator produces a periodic train of clock pulses. • Synchronous sequential circuits use clock pulses as inputs to storage elements are called clocked sequential circuits.

7. Storage can be constructed from logic with delay connected in a closed loop. • A property - there must be no inversion. • A buffer is usually implemented using two inverters.

8. Clocked sequential diagrams Inputs Combinational Circuit Outputs Flip-Flops Clock pulses

9. Latches • A storage element can maintain a binary state indefinitely, until directed by an input signal to switch states. • Difference between latches and flip-flops • No of inputs they possess • Manner which the inputs affect the binary state • Most basic storage elements are latches

10. S-R Latch

11. S-R Latch • Two cross-coupled NOR gates. • Derived from • Replacing inverters with NOR gates.

12. S-R Latch • Two inputs • S - set • R – Reset • When • Q = 1 and Q’ = 0 Set State • Q = 0 and Q’ = 1 Reset State Outputs Q and Q’ are normally the complements of each other

13. Function Table

14. S’R’ Latch

15. Two cross-coupled NAND gates.

16. Function Table

17. SR latch with control inputs

18. Function Table

19. Eliminate undesirable undefined state in SR latch is to ensure that inputs S and R are never equal to 1 at the same time.

20. D Latch

21. Function Table

22. Flip-flops • The simplest form of sequential circuit. • Variety of flip flops, all of which share two properties. • One is …. • A bistable device. • Exists in one of two states • In the absence of input, remains in the state. • Thus, can function as a 1 – bit memory.

23. 2nd one • Has two outputs. • Which are always complements of each other. • Generally labeled Q and Q’.

24. Flip Flops • Latches are asynchronous, which means that the output changes very soon after the input changes. • Most computers today are synchronous, which means that the outputs of all the sequential circuits change simultaneously to the rhythm of a global clock signal. • A flip-flop is a synchronous version of the latch.

25. Master-Slave Flip-Flop • There are several fundamental types of flip-flops. • In addition there are minor variations depending on the number of inputs and how they control the state of the flip-flop. • Very simple type of flip-flop called a D-flip-flop.

26. A master-slave D-flip-flop is built from two SR-latches and some gates

27. SR Master – Slave Flip-Flop