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High-Speed Serial Interface Circuits and Systems
Woo-Young ChoiDept. of Electrical and Electronic Engineering
Yonsei University
Lecture 10: Bit Errors
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W.-Y. Choi2High-Speed Serial Interface (2020/2)
Bit Errors
- Delivery of digital bits from Tx to Rx without error
Tx Rx
Serializer...
123
n
Tx Driver
PLL FIREqualizer
Channel CTLE FFE/DFE
Clock Recovery
Deserializer ...
123
n
- How to quantify ‘error’?- What causes bit errors?
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W.-Y. Choi3High-Speed Serial Interface (2020/2)
Bit Errors
() = • Most important performance metric
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W.-Y. Choi4High-Speed Serial Interface (2020/2)
Bit Errors
Assuming Gaussian distribution
(1/ 0)P =
(0 /1)P =
20200
( )1 exp22
sv v dvg ss p
¥ é ù--ê úë û
ò 00
12 2
sverfc gs
æ ö-= ç ÷ç ÷
è ø
21211
( )1 exp22
sv v dvg
ss p-¥
é ù--ê úë û
ò 11
12 2
sverfc gs
æ ö-= ç ÷ç ÷
è ø
22( ) exp( )x
erfc x y dyp
¥
= -ò
Assuming equal probability for 1 and 0
BER = 0 1
0 1
14 2 2
s sv verfc erfcg gs s
é ùæ ö æ ö- -+ê úç ÷ ç ÷ç ÷ç ÷ê úè øè øë û
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W.-Y. Choi5High-Speed Serial Interface (2020/2)
Bit Errors
BER = 0 1
0 1
14 2 2
s sv verfc erfcg gs s
é ùæ ö æ ö- -+ê úç ÷ ç ÷ç ÷ç ÷ê úè øè øë û
Select g so that BER is minimized
1 0 0 1
1 0
s sv vs sgs s
+=
+It can be shown BER minimized with
It can be also shown that the optimal BER12 2
Qerfc æ ö= ç ÷è ø
1 0
1 0
s sv vQs s-
=+
è Signal-to-Noise Ratio
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W.-Y. Choi6High-Speed Serial Interface (2020/2)
Bit Errors
Optimal BER12 2
Qerfc æ ö= ç ÷è ø
1 0
1 0
s sv vQs s-
=+
2exp( / 2)~2Q
Q p-
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W.-Y. Choi7High-Speed Serial Interface (2020/2)
Bit Errors
Optimal BER12 2
Qerfc æ ö= ç ÷è ø
1 0
1 0
s sv vQs s-
=+
What causes bit errors?
(1) ISI
... ...
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W.-Y. Choi8High-Speed Serial Interface (2020/2)
Bit Errors
Optimal BER12 2
Qerfc æ ö= ç ÷è ø
1 0
1 0
s sv vQs s-
=+
What causes bit errors?
(1) ISI
Larger bandwidth always better?
Smaller ISI but more white noises delivered to Rx
Optimal bandwidth depends on noise types, typically 0.7~0.8 of data rate
In addition, more bandwidth needs more power consumption
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W.-Y. Choi9High-Speed Serial Interface (2020/2)
Bit Errors
Optimal BER12 2
Qerfc æ ö= ç ÷è ø
1 0
1 0
s sv vQs s-
=+
What causes bit errors?
(2) Sampling Time Noises: Jitters
CR should provide optimal sampling time
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W.-Y. Choi10High-Speed Serial Interface (2020/2)
Bit Errors
Optimal BER12 2
Qerfc æ ö= ç ÷è ø
1 0
1 0
s sv vQs s-
=+
What causes bit errors?
(2) Sampling Time Noises: Jitters
Data duty cycle important
Data duty distortion
è Timing margin reduction
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W.-Y. Choi11High-Speed Serial Interface (2020/2)
Bit Errors
Optimal BER12 2
Qerfc æ ö= ç ÷è ø
1 0
1 0
s sv vQs s-
=+
What causes bit errors?
(3) Coupling Noises
(a) Power Supply Noises
– IR drop in power-supply rail
Device#1
Device#2
Device#3
VDD
I1 I2 I3VDD1 VDD2 VDD3
• Use as wide supply rails as possible
• Avoid serial power supply configuration
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W.-Y. Choi12High-Speed Serial Interface (2020/2)
Bit Errors
Optimal BER12 2
Qerfc æ ö= ç ÷è ø
1 0
1 0
s sv vQs s-
=+
What causes bit errors?
(3) Coupling Noises
(a) Power Supply Noises
– Inductive coupling due to bonding wires
• As short bonding wire as possible
• Better packages
( )( ) di tv t Ldt
=
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W.-Y. Choi13High-Speed Serial Interface (2020/2)
Bit Errors
Optimal BER12 2
Qerfc æ ö= ç ÷è ø
1 0
1 0
s sv vQs s-
=+
What causes bit errors?
(3) Coupling Noises
(a) Power Supply Noises
– Power supply itself
• Good filtering
InterfaceChip
DC-DCConverter
Digital Signal Processing
Chip
220V AC
Bead BeadCAP CAP
LC-typeSupply filter
(Ferrite bead, Ferrite choke)
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W.-Y. Choi14High-Speed Serial Interface (2020/2)
Bit Errors
Optimal BER12 2
Qerfc æ ö= ç ÷è ø
1 0
1 0
s sv vQs s-
=+
What causes bit errors?
(3) Coupling Noises
(a) Power Supply Noises
– Power supply itself
• Differential signaling
Internal VDD reduced
Large on-chip capacitor needed
• On-Chip Supply Regulator
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W.-Y. Choi15High-Speed Serial Interface (2020/2)
Bit Errors
Optimal BER12 2
Qerfc æ ö= ç ÷è ø
1 0
1 0
s sv vQs s-
=+
What causes bit errors?
(3) Coupling Noises
(b) Cross Talk
– Neighboring channels (on-chip, on-board) cause capacitive and inductive coupling
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W.-Y. Choi16High-Speed Serial Interface (2020/2)
Bit Errors
Optimal BER12 2
Qerfc æ ö= ç ÷è ø
1 0
1 0
s sv vQs s-
=+
What causes bit errors?
(3) Coupling Noises
(b) Cross Talk
Near-End Cross Talk (NEXT) Far-End Cross Talk (FEST)
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W.-Y. Choi17High-Speed Serial Interface (2020/2)
Bit Errors
TD
2TD
~Tr
~Tr
far end crosstalk
Near end crosstalk
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W.-Y. Choi18High-Speed Serial Interface (2020/2)
Bit Errors
Optimal BER12 2
Qerfc æ ö= ç ÷è ø
1 0
1 0
s sv vQs s-
=+
What causes bit errors?
(3) Coupling Noises
(b) Cross Talk
– Neighboring channels (on-chip, on-board) cause capacitive and inductive coupling
• On-Board
Enlarge space between channels
Adjust transition timing for different lanes
Compensate coupling at Tx side
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W.-Y. Choi19High-Speed Serial Interface (2020/2)
Bit Errors
Optimal BER12 2
Qerfc æ ö= ç ÷è ø
1 0
1 0
s sv vQs s-
=+
What causes bit errors?
(3) Coupling Noises
(b) Cross Talk
– Neighboring channels (on-chip, on-board) cause capacitive and inductive coupling
• On-ChipSeparate sensitive signals
Shield signal lines with grounds
Use different metal layers for adjacent lanes
(Minimum spacing within a metal layer much smaller than metal
layer separation)
Perpendicular routing for vertical metal layers (Manhattan routing)
Limit maximum parallel routing distance
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W.-Y. Choi20High-Speed Serial Interface (2020/2)
Bit Errors
Optimal BER12 2
Qerfc æ ö= ç ÷è ø
1 0
1 0
s sv vQs s-
=+
What causes bit errors?
(4) Noises in semiconductor devices
(Summary of MOSFET noises in https://www.nikhef.nl/~jds/vlsi/noise/sansen.pdf)
è Homework (Due 11/16 2 pm)Hand-in one-page summary of MOSFET noises
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W.-Y. Choi21High-Speed Serial Interface (2020/2)
Bit Errors
How to measure BER?
PRBS (Pseudo Random Bit Sequence)
24-1 bit PRBS
2 1b
N
ffD =-
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W.-Y. Choi22High-Speed Serial Interface (2020/2)
Bit Errors
2N-1 bit PRBS generation
29-1 bit PRBS generation
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W.-Y. Choi23High-Speed Serial Interface (2020/2)
Bit Errors
... ...
Measure errors in Rx
How to measure BER?
Add PRBS generator in Tx
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W.-Y. Choi24High-Speed Serial Interface (2020/2)
Bit Errors
... ...
Measure error rate in Rx
How to measure BER?
Add PRBS generator in Tx
BER contour