Lec1

1. EET 3350 Digital Systems Design
John Wakerly
Chapter 7: 7.1 – 7.2
Sequential Circuits
Flip-Flops
1

2. Combinational Logic Circuits
• Output set depends
only on the current n m
input set
• If the input(s) change,
the output(s) may
change
• History of the input
behavior is not binary values,
important to the future an n-bit binary
output behavior number
binary values,
an m-bit binary
number
2

3. Sequential Logic Circuits
• Output depends on
current input and
n m
history of past inputs
• Information about the
past behavior (history)
of the input(s) is held in
memory
• Memory is finite,
holding “just enough”
information as defined
Size of memory depends
by design specification on how much history we
need to know.
3

4. Sequential Logic Circuits
• Some of the CL outputs
are used to control the
n m
memory elements
• Outputs from the
memory elements are
provided as inputs to
the CL circuit
– this is a form of feedback
• Memory may also have
an external “clock”
signal to control when
changes can occur
4

5. Sequential Logic Circuits
• The addition of the
“extra” inputs and
n m
outputs doesn’t change
the basic concepts
associated with the CL
part of the circuit
– it just has more signals
• Our approach to
designing the CL circuit
is the same as always
• Need to design the
memory part
5

6. Sequential Logic Circuits
• Memory output is
considered the circuit’s
n m
“current state” – a
numerical label
• State embodies all the
information about the i j
past needed to define the
current output
– State variables, one or
more bits of information
– Present state, now, waiting binary values, an i-bit or
– Next state, next, with clock j-bit binary number
6

7. Describing Sequential Circuits
• State diagram
– Graphical version of states, inputs, transitions and
outputs
Input Sequential Output
Logic
1-bit Circuit 1-bit
Clock
SL machine to recognize that the last four inputs have been 1
Seems to be an SL machine to recognize the falling edge
7

8. Describing Sequential Circuits
• State table
– For each current-state, specify next-state(s) as a
function of the present inputs
– For each current-state, specify the output(s) as a
function of the present inputs
8

9. Describing Sequential Circuits
• State table
– Alternate format
More about states, tables and diagrams later.
9

10. Describing Sequential Circuits
• State table
– Alternate format
More about states, tables and diagrams later.
9