CAD for VLSI Design-2 Lecture-9

CAD for VLSI Design-2 Lecture-9 - Transcript

CAD for VLSI Design II
Lecture 9 V Kamakoti and Shankar Balachandran

Overview of this Lecture
Delay elements in CMOS circuits Cont d

Clock Frequency
The limiting factor on the clocking rate is the propagation delay through the IFL block

Changes on the Q s must propagate through the IFL before they can affect the next state

Timing Violations
Tc l k tC Q max t f f max t s umax

The clock period Tclk has a lower bound of tff max If the clock period is equal to tff max tC Q max then the flip flop state changes can violate setup times Remedy Use faster flip flops decrease tC Q Use faster gates decrease tff Use a slower clock increase clock period Tclk

Clock Skew
The previous discussion assumes that clock signals arrive at all flip flops simultaneously this is not a good assumption since it is not true in practice Because of different wire lengths over which the clock signals travel and the load at the destination there is a slight difference in clock arrival times at different flip flop inputs Clock skew tskew is the difference in time between triggering edges seen at different flip flops Clock skew affects minimum Tclk

Timing Analysis Example
For the circuit given below determine all the sequential circuit timing parameters

For a D flip flop use tsu 2ns th 15ns and tC Q 20ns For a NAND gate use tp max 10ns and tp min 3ns

tif max 3t p max nand 30ns tif min 2t p min nand 6ns t ff max 2t p max nand 20ns t ff min 2t p min nand 6ns tc max 2t p max nand 20ns tc min 2t p min nand 6ns
Tsu tsu max tif max tc min 2 30 6 26ns Th th max tif min tc max 20 15 6 29ns Tclk tC Q max t ff max tsu max 20 20 2 42ns f clk max 1 42ns 23 8MHz tskew max tC Q min t ff min th max 20 6 15 11ns
For a D flip flop use tsu 2ns th 15ns tC Q 20ns For a NAND gate use tp max 10ns tp min 3ns



1 Phase Clock w Level Triggering

t w td q min tlmin th max t skewmax t skewmax

and

tlmin t w td q min th max t skewmax
d q min

t



t w t sumax or

tlmin t w th max



References
UTD 3325 Course Material University of Texas at Dallas

Questions and Answers
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