Design and Implementation of VLSI Systems (EN1600) Lecture 16: Interconnects & Wire Engineering. Prof. Sherief Reda Division of Engineering, Brown University Spring 2008. [sources: Weste/Addison Wesley – Rabaey/Pearson]. A capacitor does not like to change its voltage instantaneously.
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Lecture 16: Interconnects & Wire Engineering
Prof. Sherief Reda
Division of Engineering, Brown University
[sources: Weste/Addison Wesley – Rabaey/Pearson]
A capacitor does not like to change its voltage instantaneously.
A wire has high capacitance to its neighbor.
When the neighbor switches from 1→ 0 or 0 →1, the wire tends to switch too.
Called capacitive coupling or crosstalk.
Crosstalk has two harmful effects:
Increased delay on switching wires
Noise on nonswitching wiresInterconnects introduce cross talk
Assume layers above and below on average are quiet instantaneously.
Second terminal of capacitor can be ignored
Model as Cgnd = Ctop + Cbot
Effective Cadj depends on behavior of neighbors
Miller effect1. Crosstalk impacts delay
Crosstalk causes noise on nonswitching wires instantaneously.
If victim is floating:
model as capacitive voltage divider2.Crosstalk also creates noise
Usually victim is driven by a gate that fights noise instantaneously.
Noise depends on relative resistances
Victim driver is in linear region, agg. in saturation
If sizes are same, aggressor = 2-4 x RvictimCrosstalk noise effects
(time constant depends on the ratio of time constant of aggressor and victim)
Goal: achieve delay, area, power goals with acceptable noise instantaneously.
1. Width, Spacing, Layer
3. Repeater insertion
4. Wire staggering and differential signalingWire Engineering
R and C are proportional to instantaneously.l
RC delay is proportional to l2
Unacceptably great for long wires
Break long wires into N shorter segments
Drive each one with a repeater or buffer
buffer/repeater3. Repeater insertion
Minimum delay is attained when
Makes sense to add a buffer only if D0-D1 > 0
If Rbuf = Rsnk and Csnk = Cbuf
then the optimal location for the buffer is at distance L/2
If there are N buffers then minimum delay will occur when they are equally spaced, i.e., separation distance is L/(N+1)