1. ECE 3130 – Digital Electronics and Design Lab 5 Latches and Flip-Flops Fall 2012 Allan Guan

2. What are latches and flip-flops? Sequential circuits that store information (i.e. memory elements – output depends not only on present inputs but also previous inputs) Latches – output responds to input immediately as long as the enable signal is asserted Flip flops – output responds only to the rising and falling edge of the enable signal Allan Guan

3. Today’s lab Build and test an SR latch from NAND gates Build an test a D latch from the SR latch Build and test a DFF from the D latch – Master-slave implementation (MS) – Positive gate trigger implementation (PGT) Allan Guan

4. SR Latch Truth TableNAND Implementation The inputs are active-low (hence the bar on top of S and R). This means that the output will change when the input is pulsed low. The latch is set (that is, storing logic 1) when Q = 1 and nQ = 0. The latch is reset (that is, storing logic 0) when Q = 0 and nQ = 1. StatenSnRQnQ Rest1101 Set0110 1110 Reset1001 1101 Undefined0011 11?? Allan Guan

5. Can you make an SR Latch from NOR Gates? Show step-by-step procedure how to replace NAND gates with NOR gates. Do the inputs/outputs remain the same? Allan Guan

6. Output Waveforms for SR Latch (NAND Implementation) Allan Guan

7. D Flip-Flop Truth Table ClockDQnQn Q n+1 0X00 0X11 10X0 11X1 SR Latch Implementation This is an SR latch! The output follows D as long as the Clock signal is 1. Otherwise, it holds its value. Allan Guan

8. Output Waveforms for D Latch Allan Guan

9. D Flip-Flop (DFF) The value of D is stored on either the rising or falling clock edge. The figure below shows a positive-edge triggered DFF. Clock D Q Allan Guan

10. Master-Slave DFF The “slave” DFF only changes when the “master” DFF changes. The “slave” DFF clock is inverted, hence, this is negative-edge triggered. For positive-edge triggering, switch the clock inputs between the “master” and the “slave”. Allan Guan

11. Output Waveforms for Master-Slave DFF (Negative-Edge Triggered) Allan Guan

12. PGT Schematic Allan Guan

13. PGT Waveforms Allan Guan

14. Can you explain the output waveform of the PGT? Why did we use an odd number of inverters to make a PGT? Could we have used an even number of inverters and replaced the AND gate by NOR, NAND, or OR gates to obtain a similar output? Allan Guan

15. PGT-Driven DFF Schematic Allan Guan

16. PGT-Driven DFF Waveforms Allan Guan

17. Analysis Include output waveforms for all the circuits in today’s lab in your report. So far, you have seen that you obtain glitches or peaks in your output during transition of input from one state to another. After doing this lab, can you suggest a way to get rid of those glitches and peaks? Keeping the above question in mind can you state one probable use of D flip flops in digital circuits? Allan Guan