CH 4 - Flipflops

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Published on 20 Apr 2013
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SEQUENTIAL CIRUCITS
Something to consider
o Computer specs us terms like “8GB of Ram” and “2.2GHz
processors”.
RAM = Random Access Memory; 8GB = 8 Billion ints
2.2 GHz = 2.2 billion clock pulses per second
1 hertz = 1 operation per second
o What does this mean in circuity?
How do you use circuits to store values?
What is the purpo
se of a clock signal?
Currently a particular input cause particular output
o Consider a same input has 3 different outputs ex. Tickle Me Elmo
Two kinds of circuits
o Combinational Cirucits (past material)
Circuits where the output values are entirely dependent and
predictable from the input values
o Sequential Circuit (new)
Circuits that depend on both the inputs and the previous state
of the circuit
Sequential circuits
o This creates circuits whose internal state can change over time
the same input values can result in different outputs
o Purpose: memory values, react to changing inputs
Creating Sequential circuits
o Essentially, sequential circuits are a result of having feedback in the
circuit
o Some gates don’t have useful results when outputs are fed back on
inputs
A
QT
QT+1
0
0
0
0
1
0
1
0
0
1
1
1
A
QT
QT+1
0
0
0
0
1
1
1
0
1
1
1
1
In these truth tables QT and QT+1 represent the values of Q at a
time T, and a point in time immediately after T+1
o Others have more interesting characteristics, which lend themselves
to storage devices
A
QT
QT+1
0
0
1
0
1
1
1
0
1
1
1
0
A
QT
QT+1
0
0
1
0
1
0
1
0
0
1
1
0
o NAND and NOR feedback circuits (unlike AND and OR which gets
stuck) the output QT+1 can be changed, based on A
However, gates like these that feed back on themselves could
center an unsteady state
Latches
o If multiple gates of these types are combined, you can get more
steady behavior
o These circuits are called latches
o
Latch, behavior:
Assume that
  and
  at start
The
input sets the output
to 1 which sets the output
to 0
Setting
  keeps the output at 1, which maintains
both output values
Assume that
  and
  at start, and then
set to 0
This sets output to 1, which sets the output
to 0
Setting
  keeps the output value
at 0 which
maintains both output values
Note how the circuit remembers its signal going from 10 or 01
to 11


0
0
X
X
1
1
0
1
X
X
1
0
1
0
X
X
0
1
1
1
0
1
0
1
1
1
1
0
1
0
Going from 00 to 11 produces unstable behavior
o  Latch, behavior:
In this case the circuit remember previous output when going
from 10 or 01 to 00


0
0
0
1
0
1
0
1
1
0
1
0
1
0
X
X
0
1
1
1
X
X
1
0
1
1
X
X
0
0
Going from 11 to 00 produces unstable behavior
The output sginals don’t change instantaneously
o More on instability
Unstable behavior occurs when a
latch goes from 00 to 11
or a  latch goes from 11 to 00
The singals don’t change simultaneously so the outcome
depends on which singal changes first
Because of the unstable behavior 00 is considered a forbidden
state in NAND-based
latches, and 11 is considered a
forbidden state in NOR-based  latches
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