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EE 459/500 – HDL Based Digital
Design with Programmable Logic
Lecture 16
Timing and Clock Issues
Overview
Sequential system timing requirements
Impact of clock skew on timing
Impact of clock jitter on timing
Clock distribution
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2
Clocked Synchronous State Machine
Flip-Flop Timing Parameters
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Latch Timing Parameters
State Machine Timing
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Satisfying Timing Requirements
The period must be long enough for the data to propagate through the registers and logic and to be set up at the destination register before the next rising edge of the clock. Satisfied by making T long enough. Cycle time: TCLK > tc-q + tlogic + tsu
The hold time at the destination register must be shorter than
the minimum propagation delay through the logic network. This requirement is independent of system clock; manufacturer’s minimum delay specifications are needed. Guarantee that minimum combinational logic delay is larger than hold time. Race margin: thold < tc-q,cd + tlogic,cd
Clock Uncertainties
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4
3
Power Supply
Interconnect
5 Temperature
6 Capacitive Load
7 Coupling to Adjacent Lines
1 Clock Generation
Devices
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Clock Nonidealities
Clock Skew
• Spatial variations in equivalent clock edges
• Mostly deterministic
Clock Jitter
• Temporal variations in consecutive clock edges
• Mostly random
Pulse Width Variation
Clock Skew and Clock Jitter
Clk
Clk
tSK
tJS
Clock skew and jitter can affect the cycle times
Clock skew can cause race conditions
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Overview
Sequential system timing requirements
Impact of clock skew on timing
Impact of clock jitter on timing
Clock distribution
Clock Skew
Bad design
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Clock Skew
Clock Skew
R1
D QCombinational
Logic
In
CLK tCLK1
R2
D Q
tCLK2
tc - qtc - q, cdtsu, thold
tlogictlogic, cd
Assume the following timing parameters are available:
Contamination or minimum delay (tc-q,cd) and maximum
propagation delay (tc-q) of the register
Setup (tsu) and Hold (thold) times for registers
Contamination delay (tlogic,cd) and maximum delay (tlogic) of the
combinational logic
The positions of the rising edges of clocks CLK1 and CLK2 (tCLK1
and tCLK2) relative to a global reference. Ideally tCLK1 = tCLK2.
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Positive Clock Skew
Launching edge arrives before the
receiving edge
Minimum clock cycle:
T+ tc-q + tlogic + tsu
R1
D QCombinational
Logic
In
CLK tCLK1
R2
D Q
tCLK2
tc - qtc - q, cdtsu, thold
tlogictlogic, cd
Negative Clock Skew
Receiving edge arrives before the
launching edge
Minimum clock cycle:
T+ tc-q + tlogic + tsu
R1
D Q Combinational
Logic
In
CLK
t CLK1
R2
D Q
t CLK2
t c - q t c - q, cd t su, t hold
t logic t logic, cd
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Positive and Negative Clock Skew
Impact of Clock Skew on Timing:
Cycle Time (Long Path)
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Impact of Clock Skew on Timing:
Race Margin (Short Path)
Overview
Sequential system timing requirements
Impact of clock skew on timing
Impact of clock jitter on timing
Clock distribution
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Clock Jitter
CLK
-tji tter
TC LK
t j itter
CLK
InCombinational
Logic
tc-q , tc-q, cd
t log ict log ic, cd
tsu, thold
REGS
tjitter
TCLK - 2tjitter tc-q + tlogic + tsu
2tjitter+ thold < tc-q,cd + tlogic,cd
Impact of Clock Jitter on Timing:
Cycle Time (Late-Early Problem)
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Impact of Clock Jitter on Timing
Impact of Clock Skew and Jitter:
Cycle Time (Late-Early Problem)
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Impact of Clock Skew and Jitter:
Race Margin (Early-Late Problem)
Combined Impact of Clock Skew and Jitter
Minimum clock cycle (cycle time)
• TCLK > tc-q + tlogic + tsu - + 2tjitter
Positive skew improves performance
Negative skew reduces performance
Jitter reduces performance
Minimum logic delay (race)
• tlogic,cd + tc-q,cd > thold + + 2tjitter
Skew reduces race margin
Jitter reduces acceptable skew
Notes:
Absolute delay through a clock distribution path is not important
What matters is the relative arrival time at the register points at
the end of each path
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Overview
Sequential system timing requirements
Impact of clock skew on timing
Impact of clock jitter on timing
Clock distribution
Dealing with Clock Skew and Jitter
Balance clock paths (tree distribution)
Don’t use gated clocks
Use negative skew to eliminate race conditions (at the cost of performance): • Add up the components that result in the time
budget - the period must be greater than this value
TCLK > tc-q + tlogic + tsu - ( <0)
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Clock Distribution
Clock Distribution
Distribute clock in a tree fashion
H-Tree
CLK
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More Realistic H-Trees
Example: EV6 (Alpha 21264) Clocking
600 MHz – 0.35 micron CMOS
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Spartan-6 FPGA
Spartan-6 FPGA Global Clock Network
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Spartan-6 FPGA I/O Clock Network
Spartan-6 FPGA Clock Management Tile (CMT)
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Digital Clock Manager (DCM)
Eliminating Clock Skew
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Eliminating Clock Skew
Quadrant Phase Shifting
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Fine Phase Shifting
Summary
Clock skew and clock jitter – increasingly
important issues with technology downscaling
CAD tools (e.g., ISE WebPack) take care of
many issues automatically
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References and Credits
Chapter 10 of:
• Jan M. Rabaey, Anantha Chandrakasan, Borivoje
Nikolic, Digital Integrated Circuits, 2nd Edition,
Prentice Hall, 2003.
Spartan-6 FPGA Clocking Resources:
• http://www.xilinx.com/support/documentation/user
_guides/ug382.pdf
Appendix A: Asynchronous Inputs
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Asynchronous Inputs: Multiple
Synchronizers
