On-chip surfing interconnect

Abstract

With growing chip sizes and operating frequencies, on-chip global interconnect has become a critical bottleneck for CMOS technology. With processes scaling into deep submicron scales, the gap between gate delay and globalinterconnect delay increases with each technology generation. Bandwidth is also important for on-chip interconnect and is limited by skew and jitter. Due to temperature variation, crosstalk noise, power supply variation and parameter variation, timing variation increases with the length of global interconnect lines. Jitter and skew in the transmitter and receiver’s clocks add timing variation to on-chip interconnect communication. Repeaters in a buffering technique amplify clock jitter and drop pulses due to intersymbol interference. Latches can be inserted in place of some of the buffers to control the timing variation. However, these latches increase latency and power consumption. In 2002, a novel circuit technique called “surfing” was proposed to bound the timing uncertainty in wave pipelines [57]. This thesis extends the application of surfing to on-chip interconnects and introduces surfing RC interconnect and surfing LC interconnect techniques. For RC interconnects, we present a jitter attenuating buffer. This buffer uses inverters with variable output strength to implement a simple, lowgain DLL. Chains of these surfing buffers attenuate jitter making them well suited for source-synchronous interconnect. Furthermore, our chains can be used to reliably transmit handshaking signals and support sliding-window protocols to improve the throughput of asynchronous communication. We use distributed varactors to dynamically vary the latency of LC interconnects and thus effect surfing. Different from RC signaling, signals on LC interconnect propagate at nearly the speed-of-light. The varactors not only modulate the line latency, but also sharpen the edges of signals. We present both a full-swing and a low-swing LC interconnect designs. In both interconnects, the jitter and skew are attenuated along the line due to the surfing effect. In the low swing interconnect, the surfing effect also helps to reshape the pulses to increase the eye height. To demonstrate these techniques in real silicon, we designed, fabricated and tested a chip. The testing