ECE 497 JS Lecture - 21 Noise in Digital Circuits Spring 2004 Jose E. Schutt-Aine Electrical & Computer Engineering University of Illinois jose@emlab.uiuc.edu 1
Announcements - NL05 program available - Projects due Friday May 14 th - HW No. 4 due Friday May 14 th 2
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
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
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
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
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
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 140.8 n 141 pins When rise time is 3 ns L I n V t n 46.9 At least 46 pins 8
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) 0.03 9
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
Telegraphers Equations for Coupled Transmission Lines Maxwellian Form V = L I + L I z t t 1 1 2 11 12 V = L I + L I z t t 2 1 2 21 22 I = C V + C V z t t 1 1 2 11 12 I = C V + C V z t t 2 1 2 21 22 11
Crosstalk noise depends on termination 50 Ω line 1 line 2 50 Ω 50 Ω line 1 line 2 line 1 line 2 line 1 line 2 12
Crosstalk depends on signal rise time 50 Ω line 1 line 2 50 Ω t r = 1 ns t r = 7 ns line 1 line 2 line 1 line 2 13
Crosstalk depends on signal rise time 50 Ω line 1 line 2 t r = 1 ns t r = 7 ns line 1 line 2 line 1 line 2 14
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
Near- and Far-End Crosstalks Driver Driver Current Line 1 (driver) Mutual Capacitance (Cm or C12) I(Lm) I near (Lm) I far (Cm) Line 2 (victim) Crosstalk current due to mutual capacitance will split into 2 parts and flow toward both ends of victim line Crosstalk current due to mutual inductance will flow from the far end toward the near end of victim line 16
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) 17
Driver Near End Crosstalk Driver Current Line 1 (driver) Mutual Capacitance (Cm or C12) I(Lm) I near (Lm) I far (Cm) Line 2 (victim) Crosstalk seen on the victim line at the end closest to the driver 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 I = I( L ) + I ( C ) near m near m 18
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 lx 19
Driver Far End Crosstalk Driver Current Line 1 (driver) Mutual Capacitance (Cm or C12) I(Lm) I near (Lm) I far (Cm) Line 2 (victim) Crosstalk seen on the victim line at the end farthest away from the driver Assumes that load is terminated with characteristic impedance of single isolated line I = I ( C ) I( L ) far far m m 20
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 lx 21
Digital Crosstalk - Case 1 V(input) Length=X V R=Zo V(near) R=Zo Zo Zo R=Zo R=Zo V(far) V(input) 0 Tr Input Waveform V(near) A V(far) Tr 0 2 X LC Tr 0 X LC B Tr V ( input ) LM CM A= + 4 L C V ( input ) X LC LM C B= 2Tr L C M 22
Digital Crosstalk - Case 2 V(input) Length=X V R=Zo Zo R=Zo V(input) Input Waveform V(near) R=Zo Zo R=Zo V(far) 0 Tr V(near) V(far) A Tr 0 X LC 0 2 X LC Tr D Tr B V ( input ) LM C A= + 4 L C M D V ( input ) X LC L M C = T r L C M 23
Digital Crosstalk - Case 3 V(input) V R=Zo V(near) Length=X Zo R=Zo V(far) V(input) Tr Input Waveform R=Zo Zo R=Zo 0 V(near) V(far) A D Tr Tr X LC Tr 0 2 X LC 0 B 3 X LC Tr V ( input ) LM CM A= + V ( input ) X LC L M C M V B = M 2 L C L C 2T D = r L C 4 L C M 24
Crosstalk Facts If the rise or fall time is short compared to the delay of the line, the near-end crosstalk noise is independent of the rise time. If the rise or fall time is long compared to the delay of the line, the near-end crosstalk noise is dependent on the rise time The far-end crosstalk is always dependent on the rise or fall time 25
Crosstalk Facts Assume that the transmission lines are terminated The near-end crosstalk will begin at t=0 and have a duration of 2 t D. The far-end crosstalk will occur at time t=t D and have a duration approximately equal to the signal rise or fall time t = X LC D X is the length of the lines 26
Example - Determine Near- and Far-End Crosstalk 1.05 0.85 Input waveform 0.65 V(input) Length= 2 in. V Volts 0.45 V(1) 70 Ω Zo=70 Ω 70 Ω Zo=70 Ω R1=70 Ω R2=70 Ω V(2) 0.25 9.869 2.103 L( nh / in.) = 2.103 9.869 0.05 V(1) - Near end crosstalk 2.051 0.239 C( pf/ in.) = 0.239 2.051 V(2) - Far end crosstalk -0.15 0.00 0.20 0.60 0.80 1.00 1.20 1.40 1.60 Time (ns) 27
Solution V ( input) L12 C12 1 2.103 nh 0.239 pf V (1) = 0.082 V 4 + L11 C = 11 4 + = 9.869 nh 2.051 pf ( ) 12 12 V ( input) X LC L C V(2)=- 2T r L C 11 11 1 2 (9.869 nh)(2.051 pf) 2.103 nh 0.239 pf V(2)=- 0.137 V 2(100 ps) = 9.869 nh 2.051 pf 28
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 29
Intersymbol Interference (ISI) Signal launched on a transmission line can be affected by previous signals as result of reflections ISI can be a major concern especially if the signal delay is smaller than twice the time of flight ISI can have devastating effects Noise must be allowed to settled before next signal is sent 30
Intersymbol Interference Volts Ideal waveform beginning transistion from low to high with no noise on the bus Timing difference due to ISI Receiver switching threshold Time Different starting point due to ISI Waveform beginning transition from low to high with unsettled noise on the bus 31
Intersymbol Interference and Signal Integrity V 30 ohms Zo = 65 ohms Probe point 4 3 200 MHz switching on above bus 400 MHz switching on above bus 2 1 0-1 -2 Ideal 400 MHz waveform Time 32
Minimizing ISI Minimize reflections on the bus by avoiding impedance discontinuities Minimize stub lengths and large parasitics from package sockets or connectors Keep interconnects as short as possible (minimize delay) Minimize crosstalk effects 33
ISI Project Full swing line (e.g. 3.3 V) Low swing line (e.g. 300 mv) - The full-swing line will induce crosstalk noise on the low-swing line. Study the effects of crosstalk on ISI and determine some design constraints that could minimize timing errors due to ISI Parameters to be considered include: - Length of lines - Termination impedances - Signal rise time 34