SEQUENTIAL LOGIC. Satish Chandra Assistant Professor Department of Physics P P N College, Kanpur

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1 SEQUENTIAL LOGIC Satish Chandra Assistant Professor Department of Physics P P N College, Kanpur

2 OSCILLATORS Oscillators is an amplifier which derives its input from output. Oscillators Sinusoidal Oscillators Non-Sinusoidal Oscillators Ringing Oscillators Blocking Oscillators Multivibrators Dr. Satish Chandra, P P N College, Kanpur 2

3 MULTIVIBRATORS A Multivibrator is a regenerative circuit with two active devices, designed so that one device conducts while the other cut off. It can store binary numbers, count pulses, synchronize arithmetic operations and perform other essential functions in digital systems. Multivibrators Astable Multvibrator Monostable Multivibrator Bistable Multivibrator or Flip-Flop Dr. Satish Chandra, P P N College, Kanpur 3

4 FLIP-FLOPS Flip-Flops (FF) is another name for a bistable multivibrator one whose output voltage is either low or high, a 0 or 1. The output stays low or high; to change it, the circuit must be driven by an input called a trigger. Until the trigger arrives, the output voltage remains low or high indefinitely Dr. Satish Chandra, P P N College, Kanpur 4

5 FLIP-FLOPS Flip-Flops (FF) R S FF T FF D FF J K FF Master/Slave J K FF Dr. Satish Chandra, P P N College, Kanpur 5

6 FLIP-FLOPS The cross-coupling from each collector to the opposite base. The cross-coupling results in positive feedback. If Q 1 is saturated, the low Q 1 collector voltage will force Q 2 to cut off. Similarly, if Q 2 get saturated, it will force Q 1 to go into cut off. So there are two stable operating conditions: Q 1 saturated, Q 2 cut off; or Q 1 cut off, Q 2 saturated Dr. Satish Chandra, P P N College, Kanpur 6

7 RS FLIP-FLOP To control the state of a FF, we have to add trigger inputs. If a high voltage is applied to the S (SET) input, then Q 1 saturates; this forces Q 2 into cut off. Likewise, a high voltage can be applied to the R (RESET) input; this saturates Q 2 and forces Q 1 into cut off Dr. Satish Chandra, P P N College, Kanpur 7

8 RS FLIP-FLOP Applying a high voltage to the S input is called setting and results ; y = 1 Applying a high voltage to the R input is called as resetting and results ; y = 0 So, a FF is a natural circuit for generating a variable and its compliment Dr. Satish Chandra, P P N College, Kanpur 8

9 RS FLIP-FLOP R S Y Y 0 0 LAST VALUE FORBIDDEN Dr. Satish Chandra, P P N College, Kanpur 9

10 RS FLIP-FLOP NOR gate IC equivalent NOR gate The output of each NOR gate drives one of the inputs on the other NOR gate. Also the S and R inputs allows us to set or reset the output y Dr. Satish Chandra, P P N College, Kanpur 10

11 RS FLIP-FLOP A high S input sets y to 1. A high R input reset y to 0 If R and S are both low, the output remains latched in its last state. Contradictory state of R and S both high at the same time is still forbidden. An RS FF is therefore, also called a latch Dr. Satish Chandra, P P N College, Kanpur 11

12 RS FLIP-FLOP RS FF, or latches, are said to be transparent, i.e.. any change in input at R and S is transmitted immediately to the output at Y and Y. R S Y 0 0 LAST VALUE FORBIDDEN Dr. Satish Chandra, P P N College, Kanpur 12

13 RS FLIP-FLOP NOR gate RS FF NAND gate RS FF Dr. Satish Chandra, P P N College, Kanpur 13

14 Clocked RS FLIP-FLOP When the Clock is low both the AND gates are disabled, which means the output Y remains in the last state it was in. When the Clock input goes high, however, both AND gates are enabled. This allows the S and R inputs to reach the RS FF Dr. Satish Chandra, P P N College, Kanpur 14

15 Clocked RS FLIP-FLOP For the FF to operate properly, there must be a transition from low to high on the Clock input. Clocking is important in large digital systems where large number of FF may be interconnected. The single clock is applied to all FF simultaneously; so that they all changes states in unison Dr. Satish Chandra, P P N College, Kanpur 15

16 T FLIP-FLOP The output of FF toggles between set and reset condition on arrival of each clock pulse. Toggle means to switch to the opposite state. So it is called Toggle or T FF. The RS FF can be converted to function as T FF with slight modification Dr. Satish Chandra, P P N College, Kanpur 16

17 T FLIP-FLOP The S input of RS FF is connected to Y and R input is connected to Y output. Since Y and Y cannot be in same state simultaneously, the output of T FF goes high on alternate clock signals Dr. Satish Chandra, P P N College, Kanpur 17

18 T FLIP-FLOP Since the output frequency of the pulses is the half the frequency of the input pulses. Thus, the T FF operates as frequency divider. If Y = 0 and Y = 1, then R = 0 and S = 1, so the output is set on arrival of pulse. If Y = 1 and Y = 0, then R = 1 and S = 0, so the output is reset on arrival of pulse Dr. Satish Chandra, P P N College, Kanpur 18

19 D FLIP-FLOP The RS FF has two data inputs, S and R. Generating two input signals to drive a FF is a disadvantage in many application. The forbidden condition of both R and S high may occurs undesirably. D FF is a circuit that needs only a single data input Dr. Satish Chandra, P P N College, Kanpur 19

20 D FLIP-FLOP Delay (D) FF prevents the value of D from reaching the Y output until a clock pulse occurs. When the clock is low both AND gates are disabled therefore D can change value without affecting the value of Y. When the clock is high both AND gates are enabled. Y is forced to equal the value of D. CLK D Y 0 X LAST VALUE Dr. Satish Chandra, P P N College, Kanpur 20

21 EDGE-TRIGGERED D FLIP-FLOP An RC circuit at the input of a D latch. The RC time constant is much smaller than the clock pulse width. The C charged fully when the clock goes high (leading edge) produces a narrow positive spike. The trailing edge results in a narrow negative spike Dr. Satish Chandra, P P N College, Kanpur 21

22 EDGE-TRIGGERED D FLIP-FLOP The positive spike enables the AND gates for an instant; but the negative spikes does nothing. This referred to as positive-edge triggering. This kind of operation is called edge triggering because the FF responds only when the clock is in transition between its two voltage states. CLK D Y 0 X LAST VALUE X LAST VALUE Dr. Satish Chandra, P P N College, Kanpur 22

23 EDGE-TRIGGERED D FLIP-FLOP Dr. Satish Chandra, P P N College, Kanpur 23

24 EDGE-TRIGGERED D FLIP-FLOP When power is first applied, FFs come up in random states. To reset it operator has to send clear signal all FFs. Some systems requires to preset certain FFs. OR gates allow us to slip in a high PRESET or a high CLEAR when desired Dr. Satish Chandra, P P N College, Kanpur 24

25 EDGE-TRIGGERED D FLIP-FLOP A high PRESET forces Y to set and a high CLEAR forces Y to reset. The Preset and Clear are asynchronous inputs, as they activate the FF with out use of clock pulse. The D input of FF is a synchronous input, as D activate only when a clock edge occurs Dr. Satish Chandra, P P N College, Kanpur 25

26 EDGE-TRIGGERED D FLIP-FLOP a) Positive-edge triggered b) Negative-edge triggered c) Negative-edge triggered with inverted preset and clear Dr. Satish Chandra, P P N College, Kanpur 26

27 J K FLIP-FLOP JK FF is a very important FF normally used to build counter. The JK FF is a modification of T FF as it is also clock driven FF. J and K are called control inputs because they determine what the FF does when clock pulse arrives Dr. Satish Chandra, P P N College, Kanpur 27

28 J K FLIP-FLOP The RC circuit has a short time constant, thus converting the rectangular clock pulse into narrow spikes. When J & K are low, both AND gates are disabled. Therefore, clock pulses have no effect. The output Y retains its last value. CLK J K Y X 0 0 LAST VALUE LAST VALUE Dr. Satish Chandra, P P N College, Kanpur 28

29 J K FLIP-FLOP When J is low and K is high, the upper gate is disabled. When Y is high, the lower gate can pass a reset trigger on arrival of next positive clock edge. This forces Y to become low, i.e. reset the FF. When J is high and K is low, the lower gate is disabled. When Y is low but Y is high, therefore the upper gate passes a set trigger on arrival of next positive clock edge. This forces Y to become high, i.e. set the FF. When J and K are high, both gates are enabled. If Y is high, the lower gate passes a reset trigger on next positive edge. When Y is low, the upper gate passes a set trigger on next positive edge. Either way, Y changes to the complement of the last state Dr. Satish Chandra, P P N College, Kanpur 29

30 EDGE-TRIGGERED JK FLIP-FLOP a) Positive-edge triggered b) Negative-edge triggered c) Positive-edge triggered with preset and clear d) Negative-edge triggered with inverted preset and clear Dr. Satish Chandra, P P N College, Kanpur 30

31 J K MASTER/SLAVE FLIP-FLOP Widely used as the main counting device. Free from race problem i.e., toggling more than once during a negative clock edge. The master (M) is positiveedge-triggered and the slave (S) is negative-edgetriggered. Therefore the master (M) responds to its J and K inputs before the slave (S) Dr. Satish Chandra, P P N College, Kanpur 31

32 J K MASTER/SLAVE FLIP-FLOP If J = 1 and K = 0, then M sets on the + clock edge. The high Y of the M drives the J input of the S, so when clock edge hits, the S sets, copying M. If J = 0 and K = 1, then M resets on the + clock edge. The low Y of the M drives the J input of the S, so when clock edge hits, the S resets, copying M Dr. Satish Chandra, P P N College, Kanpur 32

33 J K MASTER/SLAVE FLIP-FLOP If the M s J and K inputs are high, it toggles on the + clock edge and the S toggles on the clock edge. Therefore, no matter what the Master does, the slave copies it Dr. Satish Chandra, P P N College, Kanpur 33

34 Sequential Logic COUNTERS Dr. Satish Chandra, P P N College, Kanpur 34

35 COUNTERS A counter has the ability to count. It is one of the most useful and versatile subsystems in a digital systems. A counter driven by a clock can be used to count the number of clock cycles. Since the clock pulses occurs at known intervals, the counter can be used as an instrument for measuring time and therefore period or frequency. There are basically two different types of counters Synchronous counters Asynchronous counter Dr. Satish Chandra, P P N College, Kanpur 35

36 COUNTERS The ripple counter is simple and straightforward both in operation and construction and usually requires a minimum of hardware. It does, however, have a speed limitation. Each FF is triggered by the previous FF, and thus the counter has a cumulative settling time. Counter such as these are called serial or asynchronous. In parallel or synchronous counter, each FF is triggered by the clock (in synchronisation), and thus settling time is simply equal to the delay time of a single FF. But that increases the hardware Dr. Satish Chandra, P P N College, Kanpur 36

37 3 BIT BINARY RIPPLE COUNTER The three FFs are in cascade. The clock pulse drives the A FF. Output of the A FF drives the B FF. The B FF in turn drives the C FF. All the J and K inputs are tied to +Vcc. Thus each FF will change state with a negative transition at its clock input Dr. Satish Chandra, P P N College, Kanpur 37

38 3 BIT BINARY RIPPLE COUNTER COUNT C B A Dr. Satish Chandra, P P N College, Kanpur 38

39 4 BIT BINARY RIPPLE COUNTER The three FFs are in cascade. The clock pulse drives the A FF. Output of the A FF drives the B FF. The B FF in turn drives the C FF, which then drives the D FF. All the J and K inputs are tied to +Vcc. Thus each FF will change state with a negative transition at its clock input. Overall propagation delay time is the sum of the individual delays Dr. Satish Chandra, P P N College, Kanpur 39

40 4 BIT BINARY RIPPLE COUNTER COUNT D C B A Dr. Satish Chandra, P P N College, Kanpur 40

41 3 BIT BINARY DOWN RIPPLE COUNTER COUNT C B A Dr. Satish Chandra, P P N College, Kanpur 41

42 3 BIT BINARY DOWN RIPPLE COUNTER The system clock is still used at the clock input to FF A, but the complement of A, is used to drive FF B; likewise, complement of B is used to drive FF C. FF A simply toggles with each edge as before. But FF B will toggle each time A goes high. Similarly, FF C triggered by B and so C will toggle each time B goes high. Counter contents are reduced by one count with each clock transition. Thus, counter is operating in count down mode Dr. Satish Chandra, P P N College, Kanpur 42

43 3 BIT BINARY UP-DOWN COUNTER It is a combination of the two counters; up and down. If the count-down control line is low and the count-up control line high, the counter will have count up waveform. If the count-down is high and count-up is low; the counter will then be in count-down mode and have count down waveform Dr. Satish Chandra, P P N College, Kanpur 43

44 BINARY RIPPLE COUNTER The total number of counts through which the counter can progress is given by 2 n, where n is the total number of FF used. It is said to have a natural count of 2 n. Thus, we can build counters which count through 2, 4, 8, 16, 32, etc., states by using proper number of FFs. A 3 FF counter is referred to as a modulus-8 (mod-8) counter since it has 8 states. Similarly, a 4 FF counter is a mod-16 counter The modulus of a counter is the total number of states through which the counter can progress Dr. Satish Chandra, P P N College, Kanpur 44

45 MODIFIED RIPPLE COUNTER It is often desirable to construct counters which have moduli other than 2, 4, 8, 16, etc. A smaller-modulus counter can always be constructed from a larger-modulus counter by skipping states. Such counters are said to have a modified count. The correct number of FFs is determined by choosing the lowest natural count which is greater than the desired modified count. For example, a mod-7 counter requires 3 FFs, since 8 is the lowest natural count greater than the desired modified count of Dr. Satish Chandra, P P N College, Kanpur 45

46 RIPPLE COUNTER USING FEEDBACK One method used to cause a counter to skip count is to feedback a signal from some FF to some previous FF. Suppose we want to implement a mod-7 counter. Since only one count is to be skipped, it would be convenient to let the counter advance through it natural sequence and then reset it one count early Dr. Satish Chandra, P P N College, Kanpur 46

47 MOD-7 COUNTER COUNT C B A Dr. Satish Chandra, P P N College, Kanpur 47

48 MOD-6 COUNTER Suppose we want to implement a mod-6 counter. Since two count is to be skipped, it would be convenient to let the counter advance through it natural sequence and then reset it two count early Dr. Satish Chandra, P P N College, Kanpur 48

49 MOD-6 COUNTER COUNT C B A Dr. Satish Chandra, P P N College, Kanpur 49

50 DECODING GATES A decoding gate can be connected to the CLR input of all FFs in such a way that the output of the AND gate will be high only when the counter contents are equal to a given state, after which we want to skip all the states. All eight gates necessary to decode the eight states of the 3-bit counter are shown Dr. Satish Chandra, P P N College, Kanpur 50

51 SYNCHRONOUS COUNTER The ripple counter is simplest to build, but there is a limit to its highest operating frequency. Each FF has a delay time and in a ripple counter these delay times are additive. The total settling time for the counter is approximately the delay time times the total number of FFs. The speed limitation can be overcome by the use of synchronous or parallel counter. Now every FF is triggered by the clock. Thus, they all make their transitions simultaneously Dr. Satish Chandra, P P N College, Kanpur 51

52 SYNCHRONOUS COUNTER Keep the J and K inputs of each FF high, such that the FF will toggle with any negative transition at its clock input. Then use AND gates to gate every second clock to FF B, every forth clock to FF C, and so on. Since the clock pulses are gated to each individual FF, this configuration is referred to as gate logic Dr. Satish Chandra, P P N College, Kanpur 52

53 MOD-8 PARALLEL BINARY COUNTER The clock is applied directly to FF A which responds to a negative transition at the clock input and toggles. The FF A will change state with each negative clock transition. Whenever A is high, AND gate X is enabled and a clock pulse is passed through the gate to the clock input of FF B. The FF B will change state with every alternate negative clock transition. Since AND gate Y is enabled and will transmit the clock to FF C only when both A and B are high, the FF C changes state with every forth negative clock transition Dr. Satish Chandra, P P N College, Kanpur 53

54 MOD-8 PARALLEL BINARY COUNTER COUNT C B A Dr. Satish Chandra, P P N College, Kanpur 54

55 MOD-7 PARALLEL BINARY COUNTER The parallel counter can be used as a basis for building counters of other moduli. It is necessary to find some means of eliminating desired states from the natural count sequence. In Mod-7 counter, all the FFs are high during count 7. In changing from count 7 to 0, all FF change to the 0 state Prevent FF A from being reset during this transition, with out affecting the FF B and FF C. The counter would progress from count 7 to 1. Thus, count 0 would be skipped. A Mod-7 counter would be formed Dr. Satish Chandra, P P N College, Kanpur 55

56 MOD-7 PARALLEL BINARY COUNTER COUNT C B A Dr. Satish Chandra, P P N College, Kanpur 56

57 MOD-6 PARALLEL BINARY COUNTER COUNT C B A Dr. Satish Chandra, P P N College, Kanpur 57

58 MOD-5 PARALLEL BINARY COUNTER COUNT C B A Dr. Satish Chandra, P P N College, Kanpur 58

59 PARALLEL UP/DOWN COUNTER Dr. Satish Chandra, P P N College, Kanpur 59

60 PARALLEL UP/DOWN COUNTER COUNT C B A Dr. Satish Chandra, P P N College, Kanpur 60

61 PARALLEL UP/DOWN COUNTER In count-up mode. A FF must toggle every time all prior FF are in a 1 state, and the clock makes a transition. In the count-down mode, FF toggles must occur when all prior FF are in a 0 state. In the count-up mode, the system clock is applied at the count-up input, while the count-down input is held low. In the count-down mode, the system clock is applied at the count-down input while holding the count-up input low Dr. Satish Chandra, P P N College, Kanpur 61

62 Sequential Logic SHIFT REGISTERS Dr. Satish Chandra, P P N College, Kanpur 62

63 SHIFT REGISTERS A register is simply a group of FF that can be used to store a binary number. There must be one FF for each bit in the binary number. A register used to store an 8-bit binary number must have eight FFs. The FFs must be connected in such a way that the binary number can be entered (shifted) into the register and taken (shifted) out from the register. A group of FFs connected to provide either or both of these functions is called shift register Dr. Satish Chandra, P P N College, Kanpur 63

64 SHIFT REGISTERS There are two ways to shift data into a register (serial or parallel) and similarly two ways to shift the data out of the register. This leads to the construction of four basic register types; Serial In Serial Out SISO 54/74L91 8 bits Serial In Parallel Out SIPO 54/ bits Parallel In Serial Out PISO 54/ bits Parallel In Parallel Out PIPO 54/ bits Dr. Satish Chandra, P P N College, Kanpur 64

65 SHIFT REGISTERS Dr. Satish Chandra, P P N College, Kanpur 65

66 SHIFT REGISTERS Dr. Satish Chandra, P P N College, Kanpur 66

67 SISO The data is allowed to flow straight through the register and out of the other end. Since there is only one output, the data leaves the shift register one bit at a time in a serial pattern. Serial-in, serial-out shift registers delay data by one clock time for each stage, therefore, acts as a temporary storage device or it can act as a time delay device Dr. Satish Chandra, P P N College, Kanpur 67

68 SISO The SISO shift register is the simplest as it has only three connections, the serial data input which determines what enters the left hand FF, the serial data output which is taken from the output of the right hand FF and the sequencing clock signal. The logic circuit diagram below shows a generalized serial-in serial-out shift register Dr. Satish Chandra, P P N College, Kanpur 68

69 SIPO A serial-in/parallel-out shift register is similar to the serial-in/serial-out shift register in that it shifts data into register at serial-in pin and shifts data out at the serialout pin. It is different in that it makes all the FFs available as outputs. Therefore, a serial-in/parallel-out shift register converts data from serial format to parallel format. If four data bits are shifted in by four clock pulses at datain, the data becomes available simultaneously on the four Outputs Q, R, S, T after the fourth clock pulse Dr. Satish Chandra, P P N College, Kanpur 69

70 SIPO Dr. Satish Chandra, P P N College, Kanpur 70

71 PISO The Parallel-in/Serial-out shift register acts in the opposite way to the serial-in/parallel-out. The data is loaded into the register in a parallel format in which all the data bits enter their inputs simultaneously, to the parallel input pins of the register. The data is then read out sequentially in the normal shiftright mode from the register at T representing the data present at Q to T. This data is outputted one bit at a time on each clock cycle in a serial format Dr. Satish Chandra, P P N College, Kanpur 71

72 PISO It is important to note that with this type of data register a clock pulse is not required to parallel load the register as it is already present, but four clock pulses are required to unload the data. As this type of shift register converts parallel data, such as an 8-bit data word into serial format, it can be used to multiplex many different input lines into a single serial DATA stream which can be sent directly to a computer or transmitted over a communications line Dr. Satish Chandra, P P N College, Kanpur 72

73 PISO Dr. Satish Chandra, P P N College, Kanpur 73

74 PISO Dr. Satish Chandra, P P N College, Kanpur 74

75 PIPO The final mode of operation is the Parallel-in/Parallel-out Shift Register. This type of shift register also acts as a temporary storage device or as a time delay device similar to the SISO. The data is presented in a parallel format to the parallel input pins and then transferred together directly to their respective output pins by the same clock pulse. Then one clock pulse loads and unloads the register. The PIPO shift register has only three connections, the parallel input which determines what enters the FF, the parallel output and the sequencing clock signal Dr. Satish Chandra, P P N College, Kanpur 75

76 PIPO Dr. Satish Chandra, P P N College, Kanpur 76

77 PIPO Dr. Satish Chandra, P P N College, Kanpur 77

78 SISO, SIPO & PISO Dr. Satish Chandra, P P N College, Kanpur 78

79 SHIFT REGISTERS Then to summarise a little about Shift Registers A simple Shift Register can be made using only D-type FFs, one FF for each data bit. The output from each FF is connected to the D input of the FF at its right. Shift registers hold the data in their memory which is moved or shifted to their required positions on each clock pulse. Each clock pulse shifts the contents of the register one bit position to either the left or the right Dr. Satish Chandra, P P N College, Kanpur 79

80 SHIFT REGISTERS The data bits can be loaded one bit at a time in a series input (SI) configuration or be loaded simultaneously in a parallel configuration (PI). Data may be removed from the register one bit at a time for a series output (SO) or removed all at the same time from a parallel output (PO). One application of shift registers is in the conversion of data between serial and parallel, or parallel to serial. Shift registers are identified individually as SIPO, SISO, PISO, PIPO, or as a Universal Shift Register with all the functions combined within a single device Dr. Satish Chandra, P P N College, Kanpur 80

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