Transcript Sequential Circuits

Memory Elements
data
Flop
clock
Q
clock
data
Q-flop
Flip-flop is edge triggered. It transfers input data to Q on clock rising edge.
June 2010
1
Static Sequencing by Flip-Flops
TC
Combinational Logic
Flop
clk
Flop
clk
One flip-flop is used on each cycle boundary. Tokens advance from one
cycle to the next on rising edge.
June 2010
2
Sequencing Elements Timing Notations
t pd
Logic Propagation Delay
tcd
Logic Contamination Delay
t pcq
Flop Clock-to-Q Propagation Delay
tccq
Flop Clock-to-Q Contamination Delay
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3
t pdq
Flop D-to-Q Propagation Delay
tcdq
Flop D-to-Q Contamination Dela
tsetup
Flop Setup Time
thold
Flop Hold Time
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4
A
Combinational
Logic
A
t pd
Y
tcd
Y
clk
clk
thold
D
Flop
tsetup
Q
D
tccq
t pcq
Q
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5
Max-Delay Constraints
Q1
D2
Combinational Logic
FF2
clk
FF1
clk
TC
clk
tsetup
t pcq
Q1
t pd
D2

Tc  t pcq  t pd  tsetup  t pd  Tc  tsetup  t pcq

sequencing overhead
June 2010
6
Min-Delay Constraints
Logic circuits cannot be too fast.
Otherwise the input data to next sequential circuit will
change while it is still holding its current data.
Such malfunction is called race condition, hold time
failure or min-delay failure.
FF1
clk
Q1
Combinational
Logic
clk
tccq
Q1
clk
D2
FF2
thold
tcd
D2
tccq  tcd  thold  tcd  thold  tccq
June 2010
8
Clock Skew
• Clock should theoretically arrive simultaneously to all
sequential circuits.
• Practically it arrives in different times. The differences
are called clock skews.
• Clock skew consists of the following components:
– Systematic is the portion existing under nominal conditions. It
can be minimized by appropriate design.
– Random is caused by process variations like devices’ channel
length, oxide thickness, threshold voltage, wire thickness, width
and space. It can be measured on silicon and adjusted by delay
components.
July 2010
9
Q1
D2
Combinational Logic
FF2
clk
FF1
clk
TC
clk
tsetup
t pcq
tskew
Q1
t pd
D2

t pd  Tc  tsetup  t pcq  tskew
June 2010
sequencing overhead

10