Sequential Logic Flip-Flops and Related Devices

1. Sequential LogicFlip-Flops andRelated Devices Dr. Rebhi S. Baraka rbaraka@iugaza.edu.ps Logic Design (CSCI 2301) Department of Computer Science Faculty of Information Technology The Islamic University of Gaza

2. Outline • Basic Definitions • Latches • Edge-Triggered Flip-Flops • Master-Slave Flip-Flops

3. What is Sequential Logic • Sequential logic is a type of logic circuit whose output depends not only on the present input but also on the history of the input. • The basic memory element in sequential logic is the Flip-Flop. Outputs Inputs Combinational circuit Memory elements

4. Types of Sequential Logic • Synchronous logic or clocked logic : there is a clock signal, and the internal memory changes only on a clock edge. Combinational circuit Outputs Inputs Memory elements Clock pulses

5. Types of Sequential Logic • Asynchronous logic: The behavior of the circuit depends upon the input signals at any instant of time and the order in which the inputs change. The storage elements used are time-delay devices, i.e. the time it takes for the signal to propagate through the device.

6. sequential Logic Devices (Multivibrators) • Bistable: bistable devices have two stable states, called SET and RESET. • Monostable: monostable devices (one shot) have only one stable state. • Astable: astable devices have no stable state and used as oscillator.

7. Bistable Devices • Bistable devices have two stable states, called SET and RESET. • They can retain either of these states indefinitely, making them useful as storage devices. • Two categories of bistable devices: • The Latch • The Flip-Flop The basic difference between latches and flip-flops is the way in which they are changed from one state to the other.

8. Latches • The latch is a bistable device that can reside in either of two states by means of a feedback arrangement. • The main difference between a latch and a flip-flop is in the method used for changing their state. • Latches operate with signal levels making. We study: • S-R (Set-Reset) latch • Gated S-R latch • Gated D latch

9. S-R Latch • An active-HIGH input S-R (Set-Reset) latch is formed with two cross-coupled NOR gates as shown below

10. S-R Latch The function table of an active-HIGH input S-R latch summarizes its operation.

11. Latch • An active-LOW input (Set-Reset) latch is formed with two cross-coupled NAND gates (respectively negative-OR gate ).

12. Latch The function table of an active-LOW input latch summarizes its operation.

13. The three modes of operation (SET, RESET, no-change) and the invalid state

14. Example: If the S’ and R’ waveforms are applied to the inputs of the active-Low S’-R’ latch, determine the waveform that will be observed on the Q output. Assume that Q is initially LOW.

15. The gated S-R latch • A gated latch requires an enable input, EN (as show in the next slide). • The S and R inputs control the state to which the latch will go when a HIGH level is applied to the EN input. • The latch will not change until EN is HIGH, but as long as it remains HIGH, the output is controlled by the state of the S and R inputs. • The invalid state occurs when both S and R are simultaneously HIGH.

16. The gated S-R latch

17. Determine the Q waveform if the inputs are applied to the gated S-R latch that is initially RESET. Example:

18. The gated D Latch • It differs from the S-R latch in having only one input in addition to EN. • This input is called the D (data) input (see next slide). • When the D input is HIGH and the EN input is HIGH, the latch will set. • When the D input is LOW and the EN input is HIGH, the latch will reset. • The output Q follows the input D when En is HIGH.

19. The gates D Latch with sample waveform

20. Flip-Flops • Flip-Flops are synchronous bistable devices • Synchronous means that the output changes state only at a specified point on a clock (CLK). • The clock is designated as a control input C. • We study two types of flip-flops: • Edge-Triggered Flip-Flops • Master-Slave Flip-Flops

21. Edge-Triggered Flip-Flops • An Edge-Triggered Flip-Flop changes state either at the positive edge (rising edge) or at the negative edge (falling edge) of the clock pulse. • Three types: • S-R Edge-Triggered Flip-Flop. • D Edge-Triggered Flip-Flop. • J-K Edge-Triggered Flip-Flop.

22. Edge-Triggered Flip-Flops

23. S-R Edge-Triggered Flip-Flops • S and R inputs here are called synchronous inputs because data on these inputs are transferred to the output only on the triggering edge of the clock pulse. • When S is HIGH and R is LOW, the Q output goes HIGH on the triggering edge of the clock pulse, and the flip-flop is set. • When S is LOW and R is HIGH, the Q output goes LOW on the triggering edge of the clock pulse, and the flip-flop is reset. • When both S and R are LOW, the output does not change. • When both S and R are HIGH, an invalid condition exists.

24. S-R Edge-Triggered Flip-Flop Logic Symbol Function (truth) Table

25. S-R Edge-Triggered Flip-Flop Sample Waveform

26. Method of Edge-Triggering

27. Method of Edge-Triggering • The pulse transition detector produces a very short duration spike (of few nanoseconds) on the positive-going transition of the clock pulse. • This is caused by the small delay on one input to the NAND gate.

28. Flip-flop making a transition from the RESET state to the SET state on the positive-going edge of the clock pulse.

29. Flip-flop making a transition from the SET state to the RESET state on the positive-going edge of the clock pulse.

30. Edge-triggered D flip-flop • The D flip-flops is useful when a single bit is to be stored.

31. Edge-triggered D flip-flop

32. Edge-triggered D flip-flop Given the waveforms for the D and the clock, determine the Q output waveform if the flip-flop starts out RTESET.

33. Edge-triggered J-K flip-flop • The J-K flip-flop differs in that it has no invalid state. • This is achieved by the connecting Q output to Gate 2 and connecting Q’ to Gate 1 as shown below

34. Edge-triggered J-K flip-flop

35. Edge-triggered J-K flip-flop

36. Edge-triggered J-K flip-flop Given the waveforms for the J, K and the clock. Determine the Q output waveform if the flip-flop starts out RTESET.

37. Edge-triggered J-K flip-flop Given the waveforms for the J, K and the clock. Determine the Q output waveform if the flip-flop starts out RTESET.

38. Asynchronous Preset and Clear Inputs • These are inputs that affect the state of the flip-flop independent of the clock. • They are labeled preset (PRE) and clear (CLR), or direct set (SD) and direct reset (RD)

39. Asynchronous Preset and Clear Inputs (Logic Diagram) for the J-K Flip-Flops • Active LOW preset (PRE) and clear (CLR) inputs. • The inputs are connected so that they override the effect of the synchronous inputs, J, K, and the clock.

40. Example

41. Master-Slave Flip-Flops • In this kind of flip-flops data are entered into the flip-flop at the leading at the leading edge of he clock pulses, but the output does not reflect the input state until the trailing edge. • The pulse-triggered master-slave flip-flop does not allow data to change while the clock pulse is active.

42. The Pulse-Triggered Master-Slave J-K Flip-Flop • It is composed of two sections: • The master section which is a gated latch • The Slave section which is a gated latch clocked on the inverted clock pulse and is controlled by the outputs of the master section • The master section responds to the J and K inputs at the leading edge of the clock • The slave section responds to the Q and Q’ of the master section of the trailing edge of the clock.

43. Flip-Flop Operating Characteristics Self study! • Propagation delay times • Set-up time • Hold time • Maximum clock frequency • Pulse widths • Power dissipation

44. Flip-Flop Applications • Parallel data storage • Frequency division • Counting

45. Flip-Flop Applications: parallel data storage • Store several bits of data from parallel lines simultaneously in a group of flip-flops. • As shown in the next slide, the asynchronous reset (R) inputs are connected to a common CLR line, which initially resets all the flip-flops. • This is an example of a basic register used for data storage.

47. Flip-Flop Applications: Frequency division • Dividing (reducing) the frequency of a periodic waveform. • When a pulse waveform is applied to the clock input of the J-K flip-flop with a toggle mode, the Q output is a square wave with one-half the frequency of the clock input. • Thus, a single flip-flop can be applied as a divide-by-2 device, i.e., resulting in an output that changes at half the frequency of the clock waveform.

48. The J-K flip-flop as a divide-by-2 device. Q is one-half the frequency of CLK.

49. Example of two J-K flip-flops used to divide the clock frequency by 4. QA is one-half and QB is one-fourth the frequency of CLK.

50. Example: Develop the fout waveform for the given circuit when an 8 KHz square wave input is applied to the clock input of the flip-flop.