lecture 19: clock distribution - bashirullah lab...c l c in . 4 example: dec alpha 21164 clocking...
TRANSCRIPT
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Lecture 19: Clock Distribution
• Clock distribution trends • Distribution networks • Clock Power • Clock Skew • Timing Definitions
• Source: Ch 7 – J. Rabaey notes, Weste and Harris Notes, S. Russu, ISSCC,
Clocking
• Synchronous systems use a clock to keep operations in sequence – Distinguish this from previous or next – Determine speed at which machine operates
• Clock must be distributed to all the sequencing elements – Flip-flops and latches – Domino circuits and memories
• On practical chips, the RC delay of the wire resistance and gate load is very long – Variations in this delay cause clock to get to different elements at
different times – This is called clock skew
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Clock definition and parameters
• Skew – Spatial variation of the clock
signal as distributed throughout the chip
– Global vs. local skew • Clock Jitter
– Temporal variation of the clock with respect to a reference edge
• Duty Cycle Variation – 50/50 Design target
The clock is a periodic synchronization signal used as a time reference for data transfers in synchronous digital systems
[S. Russu. SoC 2004]
Clock Distribution Trends
[S. Russu. SoC 2004]
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Clock Distribution Networks
Recall: Buffer Sizing • Recall Example: drive 64-bit datapath with unit inverter
• N= • F= • f=
1 1 1 1
8 4
16 8
2.8
23
64 64 64 64
Initial Driver
Datapath Load
N:f:D:
16465
2818
3415
42.815.3
Fastest
1 2 N
1 f fN
N Ff =
CL CIN
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Example: DEC Alpha 21164 Clocking
• 300MHz, 9M transistors • 2 distributed driver channels
– 3.75nF clock load – 58 cm final driver width – 20Watts (out of 50W)
trise = 0.35ns tskew = 150ps
tcycle= 3.3ns
Clock waveform
Location of clock driver on die
pre-driver
final drivers
Clock Drivers
Alpha Processors
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3D Skew Visualization
Del
ay (p
s)
A B
3D Skew Visualization
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Clock power breakdown example • 30% of power dissipation in clock network • Most of the power is in the final clock buffers, grid and flip-flops
A B
Review: Timing Definitions
t CLK
t D
t c 2 q
t hold t su
t Q DATA
STABLE
DATA STABLE
Register
CLK
D Q
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Review: Timing Definitions
• TCQ:
• Tsetup (U):
• Thold:
• Tskew:
Clock Skew
• Global – long path error – Data is launched at point A and
received at point C
A B
C
TSKEW
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Clock Skew
• Local – short path error – Data is launched at point A and
received at point B
A B
C
TSKEW
Summary
• Clock distribution networks • Clock skew and jitter • Timing definitions