More Sequential Circuits, plus Architecture

More Sequential Circuits, plus Architecture

S

Q

R

COS126: General Computer Science ?

Sequential vs. Combinational Circuits

Combinational circuits. Output determined solely by inputs. Can draw solely with left-to-right

signal paths.

Sequential circuits.

S

Output determined by inputs

AND previous outputs.

Feedback loop.

R

Q

2

SR Flip-Flop

SR Flip-Flop. S = 1, R = 0 (set) S = 0, R = 1 (reset) S = R = 0 S = R = 1

"Flips" bit on. "Flops" bit off. Status quo. Not allowed.

S

Q

R

Implementation

SR flip flop

S Q

R

Interface

3

Clocked SR Flip-Flop

Clocked SR Flip-Flop. Same as SR flip-flop except S and R only active when clock is 1.

SR flip flop

S

S

clk

Q

R

R

Implementation

Clocked SR flip flop

S clk Q

R

Interface

Q

clk R S

4

Clocked D Flip-Flop

Clocked D Flip-Flop. Output follows D input while clock is 1. Output is remembered while clock is 0.

Clocked SR flip flop

D

S

clk

clk Q

R

Implementation

Q clk

D

Clocked D flip flop

D Q clk

Interface

5

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