Chapter 1 Introduction
6.2 Wire Performance Metrics
6.2.2 Crosstalk
Coupling noise can greatly affect signal integrity in on-chip interconnects, as both mutual capacitance and inductance terms for wires can be large. Capacitive noise coupling usually has a larger effect, therefore we will investigate it first. The large aspect ratios of modern wires cause a significant coupling capacitance between neighboring wires. In particular, for minimum pitch spaced wires, the sideways cap can exceed 70% of the total wire capacitance.
Many recent papers have modeled this noise carefully, and have shown that the noise voltage depends on the coupling capacitance to total capacitance ratio as well as on the ratio of the strengths of the gates driving the two wires [162–164].
Noise from inductive coupling can also present problems for VLSI wires. The current flowing in the aggressor wire generates a magnetic field which causes a return current to flow in the victim wire. Inductive coupling pushes the victim in the opposite direction to capacitive coupling: a rising edge on the aggressor wire drives the the victim up through capacitive coupling, while the same edge causes a negative glitch on the victim through inductive coupling. Capacitive coupling usually affects the nearest neighbor, while inductive coupling has a much larger range. Inductive noise becomes a problem only when a large
Figure 6.7: Simple model for evaluating capacitive coupling.
Figure 6.8: Capacitive coupling model in a wire with considerable resistive loss.
number of wires switch at the same time in bus-like situations [165, 166]. In the worst case multiple wires are switching, with near neighbors switching in one direction and far neighbors switching in the opposite direction, which results in constructive capacitive and inductive coupling noise from near and far neighbors, respectively. Therefore, the capacitive and inductive noises add, and the accumulated noise can be enough to cause signal detection failures.
There are different ways to reduce capacitive coupling. The simplest way is to increase the spacing between wires and hence reduce coupling capacitance. As will be explained later in this chapter, usually designers employ buffers (also known as repeaters) along the wire to improve its performance. Moving the repeaters in a bus such that each bit’s repeaters
Figure 6.9: Techniques to combat capacitive coupling in on-chip wires.
are staggered from its neighbors forces capacitive noise to cancel itself. The structure of such a bus is illustrated in Figure 6.9(a). Because half of the injected noise must propagate down the RC wire to negate the other half, this cancellation is not perfect, but still effective [155]. Another technique to cancel capacitive coupling is charge compensation, in which a physical capacitor is introduced between the coupled lines to inject reverse noise to counteract parasitic coupling [167]. This requires extra area and power but can minimize noise as well as reduce data-dependent delay variation. Moreover, choosing the proper size for the capacitor requires careful modeling and simulation and could be susceptible to process and temperature variation. This technique is illustrated in Figure 6.9(b).
One of the most effective methods of reducing capacitive coupling noise is to employ differential signaling. In a differential system the difference is sensed at the receiver and hence any noise that affects the two signals in a same way appears as a common-mode interference and gets canceled. In order to effectively remove capacitive coupling in a differential scheme, the wires should be also twisted periodically, as shown in Figure 6.10. As a result, injected noise affects both wires equally and hence the differential voltage is unchanged. In addition, these systems have minimal inductive coupling to the rest of the system, because each wire
Figure 6.10: Differential signaling along with wire twisting to remove crosstalk.
acts as the return path for the other, creating the smallest possible current loops. The main problem associated with this technique is the extra dynamic power. As the driver has to drive twice the capacitance as in a single-ended version, the total dynamic power is increased by at least a factor of two. In addition, driving the wires differentially causes a Miller factor of two which further increases the power consumption. The twisting scheme also requires jumping to other metal layers which imposes area over-head. Nevertheless, with differential and twisted bits, wires can easily reject noise even if the coupling ratio approaches 90 to 100% [155].
Another effective technique in reducing the capacitive coupling is to insert ground shields between the data wires [168], as shown in Figure 6.11. The advantage of this technique compared with the differential signaling is the lower dynamic power, which is associated with the lower total driving capacitance (this includes a lower Miller factor of 1). It also offers a better area efficiency as no twisting is required. The need for a custom differential receiver is rectified as a simple inverter can perform the data resolution. With proper shielding a maximum coupling noise of less than 5% can be achieved for a minimum-pitch set of wires [169].
Designers cope with inductive coupling by adding power planes or densely gridded power supplies to reduce the number of wires that can couple into a victim. Power planes, or dense
Figure 6.11: Ground shield insertion to avoid croostalk.
power grids, effectively reduce both self and mutual inductances for wires in the direction of the grid, because they provide very nice return paths within a few microns of the wire itself and thus limit the extent of the magnetic coupling [170]. Most companies have design rules for buses, such as requiring every fifth wire to be a power supply wire, which makes inductive noise much less than capacitive noise and under 5% of the power supply.