CMPUT 329 Fall 2003 Topic E Clock Skew

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CMPUT 329 - Fall 2003 Topic. E: Clock Skew and Clock Gating José Nelson

CMPUT 329 - Fall 2003 Topic. E: Clock Skew and Clock Gating José Nelson Amaral CMPUT 329 - Computer Organization and Architecture II 1

Timing With Propagation Delays CMPUT 329 - Computer Organization and Architecture II 2

Timing With Propagation Delays CMPUT 329 - Computer Organization and Architecture II 2

Clock Skew y. Clock signal may not reach all flip-flops simultaneously. y. Output changes

Clock Skew y. Clock signal may not reach all flip-flops simultaneously. y. Output changes of flip-flops receiving “early” clock may reach D inputs of flip-flops with “late” clock too soon. CMPUT 329 - Computer Organization and Architecture II 3

Clock Skew Reasons for slowness: (a) wiring delays (b) capacitance (c) incorrect design CMPUT

Clock Skew Reasons for slowness: (a) wiring delays (b) capacitance (c) incorrect design CMPUT 329 - Computer Organization and Architecture II 4

Clock-skew calculation xtffpd(min) + tcomb(min) - thold - tskew(max) > 0 x. First two

Clock-skew calculation xtffpd(min) + tcomb(min) - thold - tskew(max) > 0 x. First two terms are minimum time after clock edge for a D input to change x. Hold time is earliest time that the input may change x. Clock skew subtracts from the available hold-time margin x. Compensating for clock skew: • Longer flip-flop propagation delay • Explicit combinational delays • Shorter (even negative) flip-flop hold times CMPUT 329 - Computer Organization and Architecture II 5

Example of bad clock distribution CMPUT 329 - Computer Organization and Architecture II 6

Example of bad clock distribution CMPUT 329 - Computer Organization and Architecture II 6

Clock distribution in ASICs z. This is what a typical ASIC router will do

Clock distribution in ASICs z. This is what a typical ASIC router will do if you don’t lay out the clock by hand. CMPUT 329 - Computer Organization and Architecture II 7

“Clock-tree” solution x. Often laid out by hand x. Wide, fast metal (low R

“Clock-tree” solution x. Often laid out by hand x. Wide, fast metal (low R ==> fast RC time constant) CMPUT 329 - Computer Organization and Architecture II 8

Gating the clock y. Definitely a no-no x. Glitches possible if control signal (CLKEN)

Gating the clock y. Definitely a no-no x. Glitches possible if control signal (CLKEN) is generated by the same clock x. Excessive clock skew in any case. CMPUT 329 - Computer Organization and Architecture II 9

If you really must gate the clock. . . CMPUT 329 - Computer Organization

If you really must gate the clock. . . CMPUT 329 - Computer Organization and Architecture II 10