Integrated Placement and Skew Optimization for Rotary Clocking A paper by: Ganesh Venkataraman, Student Member, IEEE, Jiang Hu, Member, IEEE, and Frank (Ying) Liu, Member, IEEE Presented By: Eric Tramel

Global Distribution Scheme  Each rotary section consists of a pair of cross connected differential transmission line circles which can be linked together to cover the entirety of the die.  This travelling wave clock approach allows us to physically predict at what point on the transmission line the clock will be at a particular phase  Closed loop line helps save on power for a global clock grid. All we need is a single or sporadic clock injection into the system and the clock will recirculate throughout the system.

Advantages of a Rotary Clock Scheme...  Low Power Dissipation: Because of the lack of a single clock source driving the entire chip, we can avoid the power dissipation caused by the charging/discharging over the chip's capacitive load. This is achieved by the closed loop geometry as well as inverters placed between phase-opposite locations within individual rings (also helps reduce skew variation). These features have been shown to reduce power consumption of the clock network by up to 70%.  Low Skew Variation: The consistent geometry of the rotary clock mesh allows us to accurately predict the phase of the clock signal on the transmission lines at any point. Test chip evaluation has shown skew variation as low as 5.5ps at a clock speed of 950 Mhz (0.52%)!

...and then Downsides.  Not efficient for zero skew designs: Because the zero skew points occur at exactly one position on each ring, it would be entirely inefficient to use only this single portion of the ring. In order to use the rotary clock scheme to its full potential, calculations must be made in order to tap the clock ring at different phase locations and buffers added in order to ensure proper edge arrival.  Requires a more complicated CAD flow: “Unlike the intentional skew design in the conventional clocking technology where no restrictions are imposed on the flip-flop locations, the skew at each flip-flop has to be matched with a specific location at the rotary clock ring. This requirement forms a difficult chicken-and-egg problem: the flip-flop placement depends on skew optimization while it is well known that skew optimization depends on flip-flop locations.”

Methodology for Overcoming the Design Hurdles