delay locked loop 1
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DLLs and PLLs 1
Delay Locked Loopsand
Phase Locked Loops
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DLLs and PLLs 2
Motivation: System
B3B1
B2
FF1G1 FF2
B4
U1 U2
CLK
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DLLs and PLLs 3
AnalysisSetup Time Calculation
The setup time at FF2 is calculated from the common point CLK.Use the maximum delay to set up FF2:D (G1 output)
Use the minimum delay to drive FF2:CLK (B4 output)
To drive the data we add (max)
B1
B2FF1 (CLK -> Q)G1FF2 (tsu)
If the CLK at FF1 would occur at exactly the same time as CLKon the board, then the delays for B1 and B2 would effectivelybe zero.
Note that this technique relies on the max delays beingeliminated being greater then losing the minimum delays in theclock path.
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Setup Time Flow
B3B1
B2
FF1G1 FF2
B4
Use max for data path
U1 U2
CLK
Use min for clock path
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AnalysisHold Time Calculation
The hold time at FF2 is calculated from the common point CLK.Use the minimum delay to set up FF2:D (G1 output)
Use the maximum delay to drive FF2:CLK (B4 output)
To drive the clock we add (max)
B3
B4FF2 (th)
If the CLK at FF2 would occur at exactly the same time as CLKon the board, then the delays for B3 and B4 would effectively
be zero.
Since the delay through U1 and U2/G1 can not be zero, then ifFF2:th = 0, we can design the system as if flip-flops in U2have a zero hold time at the device’s input pins. That istypically how we designed with “good” SSI and MSI devices.
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Hold Time Flow
B3B1
B2
FF1G1 FF2
B4
Use min for data path
U1 U2
CLK
Use max for clock path
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Motivation: Clock Control
• Locking to external signals• Phase control
• Frequency Multiplication• Frequency Division
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Basic Phase Locked Loop
PhaseDetector
Low
PassFilter
VoltageControlled
Osc.
Analog Circuit
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Example: PLL Multiplication
PhaseDetector
Low
PassFilter
Voltage
ControlledOsc.
Divide by
n
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Example: PLL Multiplication
PhaseDetector
Low
PassFilter
Voltage
ControlledOsc.
Divide by
n
Single Event UpsetHeavy Ion or Proton
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DLL Another Technique
Goal: System Clock Matches Clock at F-F
FF1
U1
CLK
Ideal0 ns
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Buffer Tree Delays Not Negligible
CLK
FF1:CLK
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DLLs and PLLs 13
Buffer Tree Delays Not NegligibleSo Add A Delay
CLK
FF1:CLK
Delayed CLK
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DLLs and PLLs 14
Buffer Tree Delays Not NegligibleAdd A Bit More Delay
CLK
FF1:CLK
Delayed CLK
More Delay
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DLLs and PLLs 15
Buffer Tree Delays Not NegligibleAdd Just A Bit More Delay
CLK
FF1:CLK
Delayed CLK
More Delay
A Bit More
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DLLs and PLLs 16
What Did We Do?
CLK
FF1:CLK
Delayed CLK
More Delay
A Bit More
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DLLs and PLLs 17
DLL Another Technique
Insert “Proper” Delay
FF1
U1
Goal: Insert Delay to Make The Difference Zero If We DelayThings “Enough” To Line Up Corresponding Edges of Different Cycles.
CLK
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DLLs and PLLs 18
DLL Principle Of Operation
Delay Line
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DLLs and PLLs 19
DLL Principle Of Operation
Single Event UpsetHeavy Ion or Proton
Delay Line
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DLLs and PLLs 20
Manufacturers’ Selections
• Actel
– AX: PLL – ProAsic, ProAsic+ ???????
• Xilinx Virtex: DLL• Chip Express
– QYH500: DLL – CX2000: PLL – CX3000: PLL
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DLLs and PLLs 21
Virtex Architecture Overview
IOB = I/O BlockDLL = Delay-locked loopBRAM = Block RAM
(4,096 bits ea.)CLB = Configurable Logic
Block
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DLLs and PLLs 22
Virtex DLL
• Uses discrete digital delay line
• Latency: 1 to 4 clock periods• 4 Quadrature phases
• Can double frequency – quadruple if two DLL’s used
• Divide by 1.5, 2, 2.5, 3,4,5,8, or 16• Deskew board clock • Multiple FPGAs, other devices
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DLLs and PLLs 23
Board Level Clock De-Skewing
Note: Can include
multiple Virtex devices.
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DLLs and PLLs 24
Chip Express QYH500 DLL
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DLLs and PLLs 25
Chip Express QYH500 DLL
Single Event UpsetHeavy Ion or Proton
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DLLs and PLLs 26
AX PLL Architecture
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DLLs and PLLs 27
AX PLL
• Fout = Fin * i / j – 1 ≤ i,j ≤ 64
• Programmable delays – 250 ps steps, Range ± 3.75 ns
• Can cascade up to 8 PLLs
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DLLs and PLLs 28
AX PLL: External Components
+1.5V
Board FPGA
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DLLs and PLLs 29
Key Parameters
• Minimum and Maximum Frequencies
• Tolerance on input signals – Frequency – Jitter
• Lock Time• Output Phase Offset• Output Jitter
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DLLs and PLLs 30
Timing Analysis
CLK
With a crystal clock oscillator, the time from rising edge to
rising edge (or falling edge to falling edge) is quite stable,with crystal clock oscillators having relatively low jitter andgood short term stability.
l
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DLLs and PLLs 31
Timing AnalysisMax
Min
CLK
• With DLL and PLL, one must check the jitter specifications carefully, to assure that worst-case timing
must be met.• Analysis must include the time before the loop locks.• Analysis must include the effects of SEU’s on the loop’s
control circuits.