ECE 497 JS Lecture - 18 Noise in Digital Circuits

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1 ECE 497 JS Lecture - 18 Noise in Digital Circuits Spring 2004 Jose E. Schutt-Aine Electrical & Computer Engineering University of Illinois jose@emlab.uiuc.edu 1

2 Announcements Thursday April 15 th Speaker: Prof. Andreas Cangellaris NO CLASS TUESDAY APRIL 20 th 2

3 Different Types of Crosstalk Crosstalk Between Capacitive Lines Primarily on chip Major effect is increase in delay Crosstalk Between Transmission Lines Distributed and wave effects Approximate as near and far end crosstalks Signal return Crosstalk Imperfect ground reference Unbalanced currents 3

4 Coupling to Floating Line A V A V A V A C C B V B V B C CO Important when high-swing signal passes near a low-swing pre-charged signal (e.g. RAM) k c = C O CC + C C k c is capacitive coupling coefficient 4

5 Coupling to Driven Line A V A V A V A C C R O B V B V B C CO Transient decays with a time constant τ = R ( C + C ) xc o C o 5

6 Capacitive Crosstalk Countermeasures Signals on adjacent layers should be routed in perpendicular directions Avoid floating signals Make rise time as large as possible Crosstalk can be made common mode by routing true and complement lines close to each other Provide shielding by placing conductors tied to GND and reference 6

7 Example (Dally & Poulton 6-7) IC package modeled as lumped 5 nh inductor will house 128 full swing (3.3V) outputs into 50-Ω lines with 1 ns rise time How many return pins are needed if drop across returns must be less than 300 mv? How many pins if rise time is reduced to 3 ns? V t 7

8 Solution Assume current ramp same as voltage ramp V 3.3 I = 128 = 128 = 8.44A R 50 I I L V = 300mV L 300 = 0.355nH t t L 0.355nH n n 141 pins When rise time is 3 ns L I n V t n 46.9 At least 46 pins 8

9 Integrated Circuit Wiring Metal 5 Metal 4 Metal 3 Metal 2 Metal 1 Substrate Vertical parallel-plate capacitance 0.05 ff/µm 2 Vertical parallel-plate capacitance (min width) 0.03 ff/µm Vertical fringing capacitance (each side) 0.01 ff/µm Horizontal coupling capacitance (each side)

10 Coupled Transmission Lines w s h ε r V 1 V 2 I 1 C m I 2 C s C s L m 10

11 Telegraphers Equations for Coupled Transmission Lines Maxwellian Form V = L I + L I z t t V = L I + L I z t t I = C V + C V z t t I = C V + C V z t t

12 Crosstalk noise depends on termination 50 Ω 50 Ω 50 Ω 12

13 Crosstalk depends on signal rise time 50 Ω 50 Ω t r = 1 ns t r = 7 ns 13

14 Crosstalk depends on signal rise time 50 Ω t r = 1 ns t r = 7 ns 14

15 Coupling Coefficients V 1 V 2 I 1 C C I 2 C s C s M Capacitive coupling coefficient: k cx = C S CC + C C Inductive coupling coefficient: k lx = M L 15

16 Line A Near End Crosstalk u v Line B w x d x y z Approximate quantity Assumes that load is terminated with characteristic impedance of single isolated line Sum of contributions to reverse traveling wave that arrives at point x during period equal to time of flight k rx = ( k + k ) cx 4 1x 16

17 Line A Far End Crosstalk u v Line B w x d x y z Approximate quantity Assumes that load is terminated with characteristic impedance of single isolated line Time derivative of signal on line A scaled by forward-coupling coefficient and coupling time k fx = k cx 4 k 1x 17

18 Transmission-Line Crosstalk Line A u v Line B w x d x y z V A (x) V A (y) V B (x) V B (y) 18

19 TL Crosstalk Countermeasures High-swing signals should not be routed on lines immediately Match k lx and k cx to eliminate far end crosstalk If k fx is nonzero, avoid long parallel lines Terminate with Zs Make rise time as long as possible 19

ECE414/514 Electronics Packaging Spring 2012 Lecture 6 Electrical D: Transmission lines (Crosstalk) Lecture topics

ECE414/514 Electronics Packaging Spring 2012 Lecture 6 Electrical D: Transmission lines (Crosstalk) James E. Morris Dept of Electrical & Computer Engineering Portland State University 1 Lecture topics