Crosstalk and Crosstalk delay effects CMOS by VLSI Universe - April 23, 2020July 21, 20210 In the SI of Physical design, the design will be verified for crosstalk, crosstalk noise, and delays. In the situation when one of the wire switches, the wire will tend to change or affect its neighbor through capacitive coupling. This effect is called Crosstalk. The digital design functionality and its effective performance can be limited by noise. This noise occurs mainly due to the crosstalk with other signals. And it also may occur due to noise on primary inputs or the power supply Vdd. Crosstalk delay effects In the situation when the wire and its neighbor wire are switching simultaneously, the direction in which both are switching will affect the amount of capacitance that must be delivered to the destination and also the delay of the switching. Refer to the following figure to understand the dependence of effective capacitance on the switching time period. Switching of Wire B Change in Voltage ΔV The effective capacitance of Wire A (Ceff) Miller Coupling Factor MCF Constant VDD Cgnd + Cadj 1 In the same direction as wire A 0 Cgnd 0 In the opposite direction as A 2*VDD Cgnd + 2*Cadj 2 A better design technology will assume the neighbor wires are switching while computing the propagation delay tp and contamination delays tcd. This kind of procedure will lead to a wide variation in the delay of the wires. Tracking the timing window when each of the signals is switching is a more aggressive and better methodology. And if the potential switching window gets overlaps at that time the switching neighbors must be accounted for or considered. Cross talk noise effects Let us consider a situation when wire A switches while neighbor wire B is supposed to remain stable or constant. This kind of change introduces the noise in the circuit as B partially switches due to the switching effect of wire A. So here wire A becomes the aggressor and B becomes a victim in this situation. If the victim wire B is floating the circuit can be modeled to compute the noise effect on the victim, as a capacitive voltage divider as shown in the below figure. The voltage change in the victim (ΔVvictim) equation can be written as, ΔVvictim = Cadj / (Cadj + Cgnd_v) * ΔVaggressor 1. When the wire B victim is floating, there will be an indefinite amount of noise and when the wire B victim is driven, the restoration of the victim will happen by the drivers of the circuit. The percentage of the noise will be very less if there are larger drivers. The larger drivers oppose the coupling effect very fast and result in a low noise effect. 2. The noise effect will be very high almost twice if both aggressor and victim are switching. If many lines or wire are switching ups ans down, for a long line there will be no much contribution to the crosstalk delay or crosstalk noise. 3. And for a shorter line for example, over a 64-bit bus in that, if all the 64 bits are switching from 0 to 1 or 1 to 0, the line will be very strongly influenced by the crosstalk delay or crosstalk noise effects.