This document discusses sequential circuits and their operation. It explains that sequential circuits have memory that stores the history of past inputs, and their outputs depend on both the current inputs and stored past inputs. The memory in sequential circuits is made up of flip-flops that are controlled by both combinational logic outputs and a clock signal. Sequential circuits can be described using state diagrams or state tables that define the next state and outputs based on the current state and inputs.

1. EET 3350 Digital Systems Design
John Wakerly
Chapter 7: 7.1 – 7.2
Sequential Circuits
Flip-Flops
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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
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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.
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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
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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
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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
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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
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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
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9. Describing Sequential Circuits
• State table
– Alternate format
More about states, tables and diagrams later.
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10. Describing Sequential Circuits
• State table
– Alternate format
More about states, tables and diagrams later.
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