june 20101 clock data q-flop flop dataq clock flip-flop is edge triggered. it transfers input data...
DESCRIPTION
June Sequencing Elements Timing NotationsTRANSCRIPT
June 2010 1
clock
data
Q-flopFl
opdata Q
clock
Flip-flop is edge triggered. It transfers input data to Q on clock rising edge.
Memory Elements
June 2010 2
Flop
clk
Flop
clk
Combinational Logic
TC
Static Sequencing by Flip-Flops
One flip-flop is used on each cycle boundary. Tokens advance from one cycle to the next on rising edge.
June 2010 3
Sequencing Elements Timing Notations
Logic Propagation Delaypdt
Flop Clock-to- Propagation Delaypcqt
Q
Flop Clock-to- Contamination Delayccqt
Q
Logic Contamination Delaycdt
June 2010 4
hold
Flop Hold Timet
setup
Flop Setup Time
t
Flop -to- Contamination Delacdqt
D Q
Flop -to- Propagation Delaypdqt
D Q
June 2010 5
Combinational LogicA Y
Flop
clk
D Q
A
Y
clk
D
Q
cdt
pdt
setuptholdt
ccqt pcqt
June 2010 6
Max-Delay Constraints
Q1
D2
TC
clk setuptpcqt
pdt
FF1
clk
FF2
clk
Combinational LogicQ1 D2
setup setup
sequencing overhead
c pcq pd pd c pcqT t t t t T t t
Min-Delay Constraints
Logic circuits cannot be too fast.
Such malfunction is called race condition, hold time
failure or min-delay failure.
Otherwise the input data to next sequential circuit will
change while it is still holding its current data.
June 2010 8
Combinational LogicFF
1
clk
Q1FF
2
clk
D2
Q1
D2
clkccqt
holdt cdt
hold hold ccq cd cd ccqt t t t t t
July 2010 9
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.
June 2010 10
FF1
clk
FF2
clk
Combinational LogicQ1 D2
setup skew
sequencing overhead
pd c pcqt T t t t
Q1
D2
setuptpcqt
pdt
clk
TC
skewt