Introduction to Sequential Logic; Latches

Tom Kelliher, CS 240

Mar. 28, 2008

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Assignment

Read 5-3.

From Last Time

Finished Xilinx introduction lab.

Outline

1. Sequential logic.

2. SR latch.

3. D latch.

Coming Up

Flip-flops

Sequential Logic

1. Combinational logic is nice but...

2. Sequential logic: introduces notion of memory.

3. Synchronous vs. asynchronous circuits.

   There will always be some asynchronous elements in a circuit which interfaces to the real world environment.

4. Clock: frequency, period, edges, duty-cycle.

   Non-overlapping clocks.

5. How can we achieve memory?
   

   This is the basic idea, to be modified for actual use.

6. General model of a sequential circuit:
   

SR Latch

1. Active low inputs.

2. Schematic:
   

3. Operation: three valid, one invalid input.

4. Asynchronous.

Clocked SR Latch

1. Schematic:
   

   Behavior.

2. Transparent when clock is high.

   Latched when clock is low.

3. Problem with use in circuits: double clocking.

   A solution: non-overlapping clocks. (Achieved with master-slave flip-flops.)

D Latch

1. SR latches inconvenient when storing data from, say, an ALU.

2. D latch stores data directly:
   

   (Think of this as a logic primitive.)

Tom Kelliher