Optimal Positions of Twists in Global On-Chip Differential Interconnects

Abstract

Abstract-Crosstalk limits the achievable data rate of global on-chip interconnects on large CMOS ICs. This is especially the case, if low-swing signaling is used to reduce power consumption. Differential interconnects provide a solution for most crosstalk and noise sources, but not for neighbor-to-neighbor crosstalk in a data bus. This neighbor-to-neighbor crosstalk can be reduced with twists in the differential interconnect pairs. To reduce via resistance and metal layer use, we use as few twists as possible by placing only one twist in every even interconnect pair and only two twists in every odd interconnect pair. Analysis shows that there are optimal positions for the twists, which depend on the termination impedances of the interconnects. Theory and measurements on a 10-mm-long bus in 0.13-m CMOS show that only one twist at 50% of the even interconnect pairs, two twists at 30% and 70% of the odd interconnect pairs, and both a low-ohmic source and a low-ohmic load impedance are very effective in mitigating the crosstalk.

Power Efficient Gigabit Communication Over Capacitively Driven RC-Limited On-Chip Interconnects

“…If wires cross orthogonally, the effective coupling capacitance is small, and the differential receiver can handle the resulting common-mode cross-talk. Crosstalk from neighboring interconnects belonging to the same bus is minimized by alternatingly placing one or two twists in the differential interconnects, as analyzed in [16]. Running full-swing lines in parallel to low-swing lines might cause problems, and some kind of shield at the edge of the bus might be needed, causing only a relatively low area overhead for wide busses.…”

“…In the frequency range of interest, a capacitance will appear to be a fair approximation of the ideal source impedance, while also reducing the dynamic power consumption (reduced voltage swing). In order to cope with the lower signal swing, it is important to mitigate cross-talk via twisting [16] and to design a receiver with sufficient sensitivity and speed [15]. We will discuss circuit techniques that achieve this goal at record low power consumption, especially a low offset dynamic sense amplifier and a low-power decision feedback equalizer exploiting analog feedback.…”

On-chip data communication : analysis, optimization and circuit design

“…The twists invert the sign of the crosstalk for successive wire segments, such that (most of) the crosstalk is cancelled at the end of the wire, provided that the twist positions are chosen correctly [81,82]. The (minimum) number of twists to be used and their optimal positions depend on the type of wire termination and bus configuration, as will be discussed in more detail in section 4.4.4 and section 4.4.7.…”

“…It is however also possible to solve the response with a multi-input excitation, which can be easier to solve analytically for certain well-chosen input signals. In [81] for example (and in [2,82]), Eisse Mensink used modal analysis to obtain solutions for the (crosstalk) transfer functions of twisted differential interconnects. The response was evaluated for two situations: In the first, the interconnect pairs were all excited with the same sign (even mode) and in the second with opposite sign (odd mode).…”

“…But with the above mentioned modal analysis, it can be shown that positioning of these twists is not trivial. The optimal positions and the effectiveness of twists depend for example on the type of termination (in [82], this is analyzed in detail for conventional and resistive termination). Figure 4.15 illustrates this for a single twist in the uneven channels, which is sufficient to reduce the crosstalk from a differential aggressor to the differential-mode signal of the neighboring victims.…”