Flip-Flops

Tom Kelliher, CS 240

Mar. 31, 2008

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Assignment due Friday.

Assignment

Read 5-4.

From Last Time

Sequential circuits and latches.

Outline

1. The problem with latches, again. Review of clocked SR latch.

2. Analysis of master-slave JK flip-flop.

3. Analysis of edge-triggered D flip-flop.

4. Characteristic tables.

Coming Up

Sequential circuit analysis.

Problems with Latches

1. Level sensitivity, transparency.

   Must use two-phase, etc. clocking.

2. Solution: Flip-Flops, which remove transparency and permit use of a single clock signal

Clocked SR latch:

Analyze Q, !Q with these input waveforms. Assume Q low initially.

JK Flip-Flop

Master-Slave device: ensures no transparency. While master (leading latch) is transparent, slave is latched and vice-versa.

When J and K are both high, toggles in a controlled manner.

Diagram:

Analyze P, !P, Q, and !Q with these input waveforms. Assume Q low initially.

Changes state on edge, but not edge-triggered: one's catching.

D Flip-Flop

Edge-triggered: samples input only during a clock transition.

Rising edge triggered D flip-flop. Figure 6-13 uses eight NAND gates and three inverters (verify for yourself). A slight improvement:

Analyze Q, !Q, !R, !S, T, and U with these input waveforms. Assume Q low initially.

Any one's catching?

Characteristic Tables

Compact way of representing flip-flop behavior.

1. JK flip-flop:

   J	K	$\bf Q(t + 1)$	Operation
   0	0	$Q(t)$	No change
   0	1	0	Reset
   1	0	1	Set
   1	1	$\overline{Q(t)}$	Complement

   Clock edge is implied in the transition from $t$ to $t + 1$.

2. D flip-flop:

   D	$\bf Q(t + 1)$	Operation
   0	0	Reset
   1	1	Set

   Always loads. To control loading, use this circuit:

   

   Gating the clock signal leads to problems.

Tom Kelliher