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Chap. 5 Flip-Flops and Related Devices

Chap. 5 Flip-Flops and Related Devices. Introduction Combinational Circuit The output levels at any instant of time are dependent on the levels present at the inputs at that time

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Chap. 5 Flip-Flops and Related Devices

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  1. Chap. 5 Flip-Flops and Related Devices • Introduction • Combinational Circuit • The output levels at any instant of time are dependent on the levels present at the inputs at that time • Any prior input-level conditions have no effect on the present outputs because combinational logic circuits have no memory • Most Digital Systems = Combinational circuits + Memory elements • General digital system that combines combinational logic gates with memory device : Fig. 5-1 • The external outputs are a function of both its external inputs and the information stored in its memory elements • The most important memory element = Flip-Flop • F/F is made up of an assembly of logic gates • Even though a logic gate, by itself, has no storage capability, several can be connected together in ways that permit information to be stored • Output State of F/F : Fig. 5-2 • Normal output (Q) : 0 or 1 1 = HIGH = Set • Inverted output(Q) : 1 or 0 0 = LOW = Clear = RESET • F/F = Latch = Bistable multivibrator

  2. 1 1 0 1 1 1 1 0 1 1 Q 1 1 1 0 0 1 1 0 1 0 1 0 1 1 0 1 0 0 1 0 0 0 1 0 • 5-1 NAND Gate Latch • NAND gate latch(or Latch) • Constructed from two NAND gates : Fig. 5-3 • Setting the Latch • Both cases Q ends up HIGH : Fig. 5-4 • Clearing the Latch • Both cases Q ends up LOW : Fig. 5-5 • Simultaneous Setting and Clearing • Set = Clear = 0 • Q = = 1 : Undesired condition • Set = Clear = 1 • No change • Summary • Truth table : Fig. 5-6(b) Fig. 5-3 2 possible resting state when SET=RESET=1 Fig. 5-4 Pulsing SET input to 0 Fig. 5-3 참고 1 1 Fig. 5-5 Pulsing CLEAR input to 0

  3. 0 0 0 1 Q • Alternate Representations : Fig. 5-7 • Ex. 5-1)Determine Q output inFig. 5-8 • Ex. 5-2)Switch debouncing circuit inFig. 5-9 • 5-2 NOR gate Latch • Ex. 5-3) Determine Q output in Fig. 5-11 • Ex. 5-4) what happen if the light beam is momentarily interrupted in Fig. 5-12 • Q will remain HIGH and the alarm will remain ON even if phototransistor return to ON( Set=0, Clear=0 : no change) • F/F State on Power-Up • When power is on, not possible to predict the starting state of a F/F’s output • Output depend on factors such as internal propagation delays, parasitic capacitance, and external loading Fig. 5-7 Alternate Representation Resting Input = 0 * Invalid Q = = 0 Fig. 5-10 (a) NOR gate latch, (b) truth table, (c) simplified block symbol * Inactive Stage(Resting ) NAND latch : S=C=1 NOR latch : S=C=0

  4. 5-3 Troubleshooting Case Study • Ex. 5-5) Describe & analyze the circuit in Fig. 5-13 • Ex. 5-4) what are the possible faults(refer to Tab. 5-1) • Possible faults(Switch position A에서 Q=1이여야 함) • Internal open at Z1-1 : 0이 입력되지 않음 • Component failure in NAND gate Z1 • Internally shorted to ground at Z1-3, Z1-4, and Z2-2 • 5-4 Clock Signals and Clocked F/Fs • Async/Synchronous System • Asynchronous System : The output of logic circuits can change state any time one or more of the input change • Synchronous System : The exact times at which any output can change states are determined by a signal commonly called the clock • Synchronous circuits are easier to design and troubleshoot because the circuit outputs can change only at specific instants of time. • Clock Signal = rectangular pulse train or square wave(Fig. 5-14) • Positive-Going Transition(PGT), Negative-Going Transition(NGT) • The synchronizing action of the clock signals is accomplished through the use of clocked flip-flops

  5. Clocked Flip-Flops : Fig. 5-15 • 1. Clocked FFs have a clock input(CLK, CK, or CP) • In most clocked FFs, the CLK input is edge-triggered : NGT or PGT • 2. Clocked FFs have one or more control inputs • The control inputs will have no effect on Q until the active clock transition occurs(=Synchronous control inputs) • 3. In summary, • The control inputs control the WHAT : Output state(DATA 0 or 1) will go to • The clock input determines the WHEN : actually triggers the change • Setup and Hold Times • Setup time(5 - 50 ns) • minimum time that control input must remain at constant value before the transition. • Hold time(0 - 10 ns) • minimum time that control input must not change after the positive transition • 5-5 Clocked S-C F/F • Clocked S-C F/F • Waveform analysis in Fig. 5-17 : positive going edge transition 50 % Control Input Clock Input Set-Clear F/F Positive clock transition ts th

  6. Q Q • The clock input = Trigger input • Negative-going edge transition : Fig. 5-18 • Internal circuitry of the edge-triggered S-C F/F • Edge-triggered S-C F/F : Fig. 5-19 • 1. NAND Latch • 2. Pulse-steering : NAND gate에 모두 1이 입력되면 SET=0 이 되고 Q=1 • 3. Edge-detector : Fig. 5-20 • 5-6 Clocked J-K F/F • Clocked J-K F/F : Fig. 5-21 • Toggle Mode : J = K = 1(S-C F/F에서는 Invalid) • Negative-going edge transition : Fig. 5-22 • Internal circuitry of the edge-triggered J-K F/F : Fig. 5-23 • Q=0, = 1인 상태에서 J=K=1이 입력되면 • NAND 1 의 입력은 모두 1이고 따라서 출력은 0이 되고 Q =1로 Toggle • NAND 2의 입력은 1, 1, 0이고 따라서 출력은 1이 되고 =0으로 Toggle • 5-7 Clocked D F/F • Clocked D F/F : Fig. 5-24 Parallel Data Transfer : Fig. 5-27 • Implementation of the D F/F : Fig. 5-25, 26 Jack-King F/F Data F/F

  7. 5-8 D Latch : Transparent Latch • D Latch : Fig. 5-28 • Edge detector is not used : EN(Enable) input 사용 • Ex. 5-8) Determine waveform Q in Fig. 5-29 • 5-9 Asynchronous Inputs • Asynchronous Inputs(= override inputs) • Used to set the FF to the 1 or clear the FF to the 0 state at any time, regardless of the conditions at the other inputs • Clocked J-K F/F with asynchronous inputs : Fig. 5-30 • Designations for Asynchronous Inputs • PRE(Preset), CLR(Clear) • SD(Direct SET), RD(Direct RESET) • Ex. 5-8) Determine the Q output in Fig. 5-31 • 5-11 F/F Timing Considerations • Setup/Hold Time • Propagation Delays : Fig. 5-35 (Typ. MAX Few - 100 ns) • tPLH : Delay going from LOW to HIGH, tPHL : HIGH to LOW D Latch is not edge Triggered, (Level Triggered) Use the overbar to indicate the active LOW CLK Q tPLH tPHL

  8. CLK • Maximum Clock Frequency :fMAX(Typ. Max 20 to 35 MHz) • Clock Pulse HIGH and LOW Times : Fig. 5-36(a) • The minimum time duration that the CLK must remain LOW before it goes HIGH tW(L), and HIGH before it returns LOW tW(H) • Asynchronous Active Pulse Width : Fig. 5-36(b) • The minimum time duration that a PRESET or CLEAR input must be kept in its active state in order to reliably set or clear the FF • tW(L) for active-LOW asynchronous inputs • Clock Transition Times • Manufacturer usually do not list a maximum transition time requirement • Generally less than 50 ns for TTL, and less than 200 ns for CMOS • Actual ICs : Tab. 5-2(TTL : 7474, 74LS112, CMOS : 74C74, 74HC112) • Ex. 5-10) Determine following from Tab. 5-2 • (a) tPLH = 25 ns for 7474, (b) tPHL = 41 ns for 74HC112, (c) tW(L) for 74LS112, active-LOW CLR input, (d) 7474, Hold time is needed(non-zero hold time), (e) All F/F, Setup time is needed(No non-zero setup time)

  9. - 현재 그림은 정상 동작 - CLK 입력 전에 Q1 = 1 이며, CLK 입력과 동시에 J2 = 1이고 따라서 Q2 = 1 • 5-12 Potential Timing Problem in FF Circuits • Potential Timing Problem : Fig. 5-37 • J2 input of Q2 will be changing as it receives the same NGT( ). This could lead to an unpredictable response at Q2 • 해결책 : tPHL must be greater than Q2’s hold time requirement • Hold time이 적다 = CLK 후에도 control input을 계속 유지시킬 필요 없음 • Fortunately, all modern edge-triggered FFs have hold time requirements that are 5 ns or less; most have tH = 0(clock transition과 동시에 control input이 바뀌어도 상관이 없다) • For these FFs, situation like that in Fig. 5-37 will not be a problem • 가정 : FF’s hold time requirement is short enough to respond reliably • The FF output will go to a state determined by the logic levels present at its synchronous control inputs just prior to the active clock transition • if we apply this rule to Fig. 5-37, J2 = 1, K2 = 0 • Ex. 5-11) Determine the Q output in Fig. 5-38 • Clock transition의 이전 입력 값을 갖는다 CLK 입력과 동시에 J2에는 1(Q1) 이 유지되어야 하지만 J1 = K1 = 1에 따라 Toggle되어 CLK 입력과 동시에 곧바로 J2 = 0이 되어 J2의 Hold time을 만족 시킬 수 없다

  10. 5-13 Master/Slave FFs • Master/Slave FF • 2개의 F/F을 사용(Slave 와 Master F/F)하며 negative-edge transition 사용 • 위와 같이 사용하는 이유 : Timing Problem 해결(Sec 5-12) • Timing Problem의 해결 방법 • Negative Edge triggered F/F : 현재 사용 • Master/Slave F/F 사용 : 과거에 사용 • 예제 • 7470 : J-K Edge triggered F/F • 7471 : J-K Master/Slave F/F • 5-14 FF Application • Unclocked FFs • Switch debouncing(Ex. 5-2), Event storage(Ex. 5-4) • Clocked FFs • We will briefly introduce the more common applications in the following sections

  11. 5-15 FF Synchronization • Asynchronous signal input • A human operator’s actuating input switch at some random time • A FF can be used to synchronize the effect of an asynchronous input • Partial Pulse : Fig. 5-39 • The operator actuates or releases the switch are essentially random, This can produce partial clock pulses at output X • A method for preventing the appearance of partial pulses : Fig. 5-40 • 5-16 Detecting an Input Sequence • Detecting an Input Sequence : Fig. 5-41 • An output is to be activated only when the inputs are activated in a certain sequence • HIGH output only if A goes HIGH and then B goes HIGH some time later • 5-17 Data Storage and Transfer • Register • A data(binary number, BCD number,..) are generally stored in groups of FFs called registers FF Synchronization

  12. Data Transfer • The data transfer involves the transfer of data from one FF or register to another • The logic value stored in FF A is transferred to FF B upon the NGT of the TRANSFER pulse • Synchronous data transfer : Fig. 5-42 • Asynchronous data transfer : Fig. 5-43 • Transfer Enable = 0 : PRE=CLR=1, 통상적인 FF으로 동작 • Transfer Enable = 1 : A=1 이면 B=1, A=0 이면 B=0 • Parallel Data Transfer : Fig. 5-44 • The contents of X1, X2, and X3 are transferred simultaneously into Y1, Y2, and Y3(Upon application of the PGT of the TRANSFER pulse) • Parallel transfer does not change the contents of source register • 5-18 Serial Data Transfer : Shift Registers • Shift Register : Fig. 5-45 • A group of FFs arranged so that the binary numbers stored in the FFs are shifted from one FF to the next for every clock pulse • Hold Time Requirement • In shift register, the FFs must have a very small or zero hold time requirement Sec. 5-12 Timing Problem과 동일

  13. 01 11 1/0 • Serial Transfer between Registers : Fig. 5-46 • Ex. 5-13) The contents of each FF after sixth shift pulse in Fig. 5-38 ? • The registers are filled up with zeros(zero inserted) • Shift-Left Operation • 역으로 배치(Shift 방향에 따른 장단점은 없으며, 응용 특성에 따라 선택) • Parallel versus Serial Transfer • Parallel transfer : Speed • All of the information is transferred simultaneously upon the occurrence of a single transfer command pulse • Serial transfer : economy and simplicity • The complete transfer of N bits requires N clock pulses • 5-19 Frequency Division and Counting • 3 bit binary counter : Fig. 5-47 • The FFs change state(toggle) whenever the pulses are applied • Each FF divides the frequency of its input by 2 • Counting Operation : Fig. 5-48(State Table) • State Transition Diagram : Fig. 5-49 • Graphical representation of state table • Circle(state), Line(transition), I/O(input/output) 여러 개의 Transmission wire 필요 N 개 FF은 1/2N 까지 분주 가능 clock

  14. STATE 1 0 VT- VT+ VOLT • MOD Number • MOD Number indicates the number of states • N Flip-flops = 2N different state, and count up to 2N - 1 • Ex. 5-14) What will be the state after 13 pulses(현재는 101) in Fig. 5-49 • Ex. 5-15) 6 Flip-flop arrangement of Fig. 5-47 • 5-20 Microcomputer Application • Transfer binary data of internal register to external register X : Fig. 5-50 • 1) Place the binary number onto its data output lines • 2) Place the proper address code on its address output lines • 3) Generate the clock pulse CP(Write signal) • Ex. 5-16) a) What is address decode logic ? : 11111110 b) address code = 11111111일 때 X = ? : X will not change(그대로 0110) • 5-21 Schmitt-Trigger Devices • Schmitt-Trigger Inverter : Fig. 5-51 • Schmitt-trigger type of input is designed to accept slow-change signals and produce an oscillation- free output

  15. One-Shot tp • 5-22 One-Shot(=Monostable Multivibrator) • One-Shot : Fig. 5-52(a) • 1) Once triggered by trigger input(T), Q = Opposite state • 2) “1” remains for a fixed period of time tp(Determined by tp = 0.69RC) • 3) After a time tp, the OS outputs return to their resting state(“0”) • Non-retriggerable One-Shot : Fig. 5-52(b) • Retriggerable One-Shot : Fig. 5-53 • Actual Devices : Fig. 5-54 • 74121/221 : Single/Dual non-retriggerable one-shot • 74122/123 : Single/Dual retriggerable one-shot • 5-23 Analyzing Sequential Circuits • Analyze a sequential circuits(FFs + Gates) in the following example • Ex. 5-17) Determine the waveform at X, Y, Z, and W for 8 clock cycles • Counter stops counting at X=1, Y=0, and Z=0(W=0 : no change) Quasi-stable State 보통 “0” 에서 “1”

  16. 5-24 Clock Generator Circuits • Multivibrator • Bi-stable multivibrator : Flip-flops have two stable state • Mono-stable multivibrator : One-shots have one stable state(“0”) • Astable = Free-running multivibrator : no stable state • Schmitt-Trigger Oscillator : Fig. 5-56 • 555 Timer Used as an Astable Multivibrator : Fig. 5-57 • Ex. 5-17) Calculate the frequency and the duty cycle of the 555 timer • Crystal-Controlled Clock Generators • Output frequency = Crystal’s resonant frequency • Clock Generator Circuit : Fig. 5-58 • Using TTL inverter : R = 300 - 1500 Ohm, 최대 20 MHz • Using CMOS inverter : R = 100 K Ohm, 최대 10 MHz • 5-25 Troubleshooting FF circuits • Open Inputs : Ex. 5-19 • K0가 Open 되어 J0 = K0 = 1 로 Toggle 됨(TTL open = 1) “1” = Quasi-Stable State * R depends on the type of crystal used and its frequency(Graph로 제공됨)

  17. Q Q • Shorted Outputs : Ex. 5-20 • D(Z2-2)에 0이 입력되며, 따라서 Q(Z2-5) = 0 이어야 정상 • Possible Circuit Faults • Z2-5 or Z1-4 is internally shorted to Vcc • Z2-5 or Z1-4 is externally shorted to Vcc • Z2-4 is internally or externally shorted to GROUND(Preset : Q = 1) • Z2 internal failure • In case of Z2 internal failure • 1) Check Z2’s Vcc and GROUND : O.K. • 2) Unsoler Z2, and Check it’s amplitude, frequency, pulse width, and transition times (by using oscilloscope) : O.K. • 3) Replace it with new one, but the new chip behaves in exactly the same way • 4) Finally he detects a solder bridge between pins 6 and 7 of Z2 • 5) Remove the solder bridge and then the circuit functions correctly • Explain how this fault produced the operation observed • The Q and outputs are internally cross-coupled so that the level on one will affect the other • A constant LOW at would keep a LOW at one input of NAND gate so that Q would have to stay HIGH regardless of the J or K 현재는 Q = 1 Rule out “1” “0” Both outputs should be checked for faults, even those that are not connected to other devices

  18. Clock Skew • A clock signal arrives at the CLK inputs of different FFs at different times(propagation delay가 원인) • The skew can cause a FF to go to a wrong state : Fig. 5-61 • Q2 는 CLOCK 1에서 Q1=0 이 입력되어 계속 Q2=0 이 되어야 함(그러나 그림에서는 CLOCK 2 이후에 Q2=1이 되어 오동작) • 해결 방법 • Problems caused by clock skew can be eliminated by equalizing the delays(the active transition arrives at each FF at approximately the same time) 각각의 Clock Input에서의 Propagation Delay를 계산

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