Sequential circuits vs. combinational

Comparison to combinational

Combinational circuits implement Boolean functions

Input ---> output, no memory

Only data inputs and control inputs determine the output

Example: Coke (um, Pepsi) machine

Assume:

price 75 cents, machine only accepts quarters

Action:

deposit quarter ---> no output

deposit quarter ---> drink delivered

Notice that the output was not always the same for the same input

This is not a Boolean function (combinational circuit)

Memory was used to determine output along with input

Example: linked list object

call list.size() method

What is input? empty

Does it always return same value?

No, its output is the current length of the linked list

Sequential circuit

Inputs: values x, labelled with subscripts

Outputs: values z, labelled with subscripts

Uses clock, unlike combinational circuit

State

made up of devices called flip-flops

k flip-flops store a k-bit number representing the current state

values denoted by q with subscripts

Output z computed from

inputs x

state q

Needed:

store current state

update to new state

Circuit elements

Combinational logic

Clock

State storage

Example

state

input

output

new state