Digital Fundamentals Tenth Edition Floyd Flipflops Floyd Digital

Digital Fundamentals Tenth Edition Floyd Flip-flops Floyd, Digital Fundamentals, 10 th ed sunwq@sjtu. edu. cn © 2008 Pearson Education Shanghai Jiao Tong University

One-Shots The one-shot or monostable multivibrator is a device with only one stable state. When triggered, it goes to its unstable state for a predetermined length of time, +V then returns to its stable state. For most one-shots, the length of time in the unstable state (t. W) is determined by an external RC circuit. REXT CEXT Q CX Trigger RX/CX Q Trigger Q t. W Floyd, Digital Fundamentals, 10 th ed sunwq@sjtu. edu. cn Shanghai Jiao Tong University

One-Shots Nonretriggerable one-shots do not respond to any triggers that occur during the unstable state. Retriggerable one-shots respond to any trigger, even if it occurs in the unstable state. If it occurs during the unstable state, the state is extended by an amount equal to the pulse width. Retriggerable one-shot: Trigger Retriggers Q t. W Floyd, Digital Fundamentals, 10 th ed sunwq@sjtu. edu. cn Shanghai Jiao Tong University

One-Shots An application for a retriggerable one-shot is a power failure detection circuit. Triggers are derived from the ac power source, and continue to retrigger the one shot. In the event of a power failure, the one-shot is not triggered an alarm can be initiated. Missing trigger due to power failure Triggers derived from ac Retriggers Q Retriggers Power failure indication t. W Floyd, Digital Fundamentals, 10 th ed sunwq@sjtu. edu. cn Shanghai Jiao Tong University

The 555 timer Inventor: Hans Camenzind in Signetcis, later absorbed by Philips Semiconductors, now NXP. Many billions was sold. 25 Microchips that shocked the world, IEEE Spectrum, May 2009 Floyd, Digital Fundamentals, 10 th ed sunwq@sjtu. edu. cn Shanghai Jiao Tong University

The 555 timer Comparator↓ Latch↓ Positive Input VI+ S R Q 0 0 NC VINegative input 0 1 when VI+>VI 0 when VI+<VI- Very high input impedance. 1 0 1 1 1 0 BJT↓ b Floyd, Digital Fundamentals, 10 th ed c BJT State Voltage on c HIGH On Same as e LOW Off Disconnected from e sunwq@sjtu. edu. cn b e Shanghai Jiao Tong University

The 555 timer Inputs SR Latch Output vthreshold vtrigger S R vo Q 1 >2/3 Vcc >1/3 Vcc 0 1 LOW On <2/3 Vcc >1/3 Vcc 0 0 No Change <2/3 Vcc <1/3 Vcc 1 0 HIGH Off >2/3 Vcc <1/3 Vcc 1 1 HIGH Off Floyd, Digital Fundamentals, 10 th ed sunwq@sjtu. edu. cn Shanghai Jiao Tong University

The 555 timer can be configured in various ways, including as a one-shot. A basic one shot is shown. +VCC R 1 C 1 RESET DISCH VCC THRES OUT TRIG CONT GND (1) Initially, a static HIHG on Trigger results in a static LOW on the output. Floyd, Digital Fundamentals, 10 th ed sunwq@sjtu. edu. cn Shanghai Jiao Tong University

The 555 timer as one shot (2) When a negative going pulse appears, comparator B output a HIGH, setting Q and turning off Q 1. Then C 1 will start to charge. Floyd, Digital Fundamentals, 10 th ed sunwq@sjtu. edu. cn Shanghai Jiao Tong University

The 555 timer as one shot (3) After sometime, comparator A output a HIGH, resetting Q and the circuit returns to steady state Floyd, Digital Fundamentals, 10 th ed sunwq@sjtu. edu. cn Shanghai Jiao Tong University

The 555 timer as one shot The 555 timer can be configured in various ways, including as a one-shot. A basic one shot is shown. The pulse width is determined by R 1 C 1 and is approximately t. W +V = 1. 1 R 1 C 1. CC R 1 The trigger is a negative-going pulse. C 1 RESET DISCH VCC THRES OUT TRIG CONT GND t. W = 1. 1 R 1 C 1 V(t)=VCC [1 -exp(-t/RC)] Floyd, Digital Fundamentals, 10 th ed sunwq@sjtu. edu. cn Shanghai Jiao Tong University

The 555 timer as one shot Determine the pulse width for the circuit shown. t. W = 1. 1 R 1 C 1 = 1. 1(10 k. W)(2. 2 m. F) = 24. 2 ms +VCC +15 V R 1 10 k. W RESET DISCH VCC THRES OUT TRIG CONT GND C 1 t. W = 1. 1 R 1 C 1 2. 2 m. F Floyd, Digital Fundamentals, 10 th ed sunwq@sjtu. edu. cn Shanghai Jiao Tong University

The 555 timer The 555 can be configured as a basic astable multivibrator with the circuit shown. Floyd, Digital Fundamentals, 10 th ed sunwq@sjtu. edu. cn Shanghai Jiao Tong University

The 555 timer as an astable multivibrator The 555 can be configured as a basic astable multivibrator with the circuit shown. Floyd, Digital Fundamentals, 10 th ed sunwq@sjtu. edu. cn Shanghai Jiao Tong University

The 555 timer The 555 can be configured as a basic astable multivibrator with the circuit shown. In this circuit C 1 charges through R 1 and R 2 and discharges through only R 2. The output +V frequency is given by: CC R 1 The frequency and duty cycle are set by these components. Floyd, Digital Fundamentals, 10 th ed RESET DISCH VCC R 2 THRES OUT C 1 TRIG CONT GND sunwq@sjtu. edu. cn Shanghai Jiao Tong University

The 555 timer Given the components, you can read the frequency from the chart. Alternatively, you can use the chart to pick components for a desired frequency. +VCC C 1 (m. F) R 1 RESET DISCH VCC R 2 THRES OUT C 1 TRIG CONT GND f (Hz) Floyd, Digital Fundamentals, 10 th ed sunwq@sjtu. edu. cn Shanghai Jiao Tong University

Homework • pp. 410 – 28, 30 Floyd, Digital Fundamentals, 10 th ed sunwq@sjtu. edu. cn Shanghai Jiao Tong University

Floyd, Digital Fundamentals, 10 th ed sunwq@sjtu. edu. cn Shanghai Jiao Tong University

Selected Key Terms Latch A bistable digital circuit used for storing a bit. Bistable Having two stable states. Latches and flip-flops are bistable multivibrators. Clock A triggering input of a flip-flop. D flip-flop A type of bistable multivibrator in which the output assumes the state of the D input on the triggering edge of a clock pulse. J-K flip-flop A type of flip-flop that can operate in the SET, RESET, no-change, and toggle modes. Floyd, Digital Fundamentals, 10 th ed sunwq@sjtu. edu. cn Shanghai Jiao Tong University

Selected Key Terms Propagation The interval of time required after an input signal delay time has been applied for the resulting output signal to change. Set-up time The time interval required for the input levels to be on a digital circuit. Hold time The time interval required for the input levels to remain steady to a flip-flop after the triggering edge in order to reliably activate the device. Timer A circuit that can be used as a one-shot or as an oscillator. Floyd, Digital Fundamentals, 10 th ed sunwq@sjtu. edu. cn Shanghai Jiao Tong University

1. The output of a D latch will not change if a. the output is LOW b. Enable is not active c. D is LOW d. all of the above Floyd, Digital Fundamentals, 10 th ed sunwq@sjtu. edu. cn Shanghai Jiao Tong University © 2008 Pearson Education

2. The D flip-flop shown will a. set on the next clock pulse b. reset on the next clock pulse c. latch on the next clock pulse D CLK Q d. toggle on the next clock pulse Floyd, Digital Fundamentals, 10 th ed sunwq@sjtu. edu. cn Shanghai Jiao Tong University © 2008 Pearson Education

3. For the J-K flip-flop shown, the number of inputs that are asynchronous is PRE a. 1 b. 2 Q J c. 3 CLK d. 4 Q K CLR Floyd, Digital Fundamentals, 10 th ed sunwq@sjtu. edu. cn Shanghai Jiao Tong University © 2008 Pearson Education

4. Assume the output is initially HIGH on a leading edge triggered J-K flip flop. For the inputs shown, the output will go from HIGH to LOW on which clock pulse? a. 1 b. 2 c. 3 d. 4 Floyd, Digital Fundamentals, 10 th ed CLK J K 1 sunwq@sjtu. edu. cn 2 3 4 Shanghai Jiao Tong University © 2008 Pearson Education

5. The time interval illustrated is called a. t. PHL b. t. PLH 50% point on triggering edge CLK c. set-up time d. hold time Q 50% point on LOW-to. HIGH transition of Q ? Floyd, Digital Fundamentals, 10 th ed sunwq@sjtu. edu. cn Shanghai Jiao Tong University © 2008 Pearson Education

6. The time interval illustrated is called a. t. PHL b. t. PLH c. set-up time D CLK d. hold time Floyd, Digital Fundamentals, 10 th ed ? sunwq@sjtu. edu. cn Shanghai Jiao Tong University © 2008 Pearson Education

7. The application illustrated is a a. astable multivibrator HIGH b. data storage device c. frequency multiplier d. frequency divider Floyd, Digital Fundamentals, 10 th ed sunwq@sjtu. edu. cn QA J fin CLK K QB J fout CLK K Shanghai Jiao Tong University © 2008 Pearson Education

Output lines Q 0 8. The application illustrated is a a. astable multivibrator Q 1 b. data storage device c. frequency multiplier d. frequency divider Q 2 Parallel data input lines Q 3 Clock Clear Floyd, Digital Fundamentals, 10 th ed sunwq@sjtu. edu. cn Shanghai Jiao Tong University © 2008 Pearson Education

9. A retriggerable one-shot with an active HIGH output has a pulse width of 20 ms and is triggered from a 60 Hz line. The output will be a a. series of 16. 7 ms pulses b. series of 20 ms pulses c. constant LOW d. constant HIGH Floyd, Digital Fundamentals, 10 th ed sunwq@sjtu. edu. cn Shanghai Jiao Tong University © 2008 Pearson Education

10. The circuit illustrated is a +VCC a. astable multivibrator b. monostable multivibrator c. frequency multiplier d. frequency divider Floyd, Digital Fundamentals, 10 th ed sunwq@sjtu. edu. cn R 1 RESET DISCH VCC R 2 THRES OUT C 1 TRIG CONT GND Shanghai Jiao Tong University © 2008 Pearson Education

Answers: Floyd, Digital Fundamentals, 10 th ed 1. b 6. d 2. d 7. d 3. b 8. b 4. c 9. d 5. b 10. a sunwq@sjtu. edu. cn Shanghai Jiao Tong University