1. Field
The present invention relates to techniques for communicating between integrated circuits. More specifically, the present invention relates to a method and an apparatus for using capacitively coupled communication techniques to communicate between stacked assemblies of laminated integrated circuit (IC) chips.
2. Related Art
Proximity Communication is a technology that promotes improved input/output (I/O) and allows face-to-face chips to communicate without wires. Drost et al. describe the basic technology in “Proximity Communication,” IEEE Journal of Solid-State Circuits, vol. 39, pp. 1529-1535 (September 2004). Although it promises much higher I/O density and lower power than conventional I/O techniques, it requires accurate mechanical alignment between the communicating chips. The lamination may be accomplished by bonding together the two juxtaposed chips or fixing them together with a mechanical jig. Mechanical misalignment degrades performance by reducing signal and augmenting crosstalk noise; if the chips are sufficiently misaligned, noise may dominate the desired signal, and communication may fail.
In one implementation of Proximity Communication, mechanical misalignment is mitigated using electronic alignment correction through data steering. In this technique, as illustrated in FIG. 1, an array of large receive (Rx) pads 10 incorporated in an Rx chip overlies a larger array of small transmit (Tx) micropads 12 in a Tx chip. One Rx pad 10 is intended to overlap a two-dimensional array of smaller Tx micropads 12. However, when the chips are assembled into a laminated structure, the Rx pads 10 may not be not completely aligned with the Tx micropads 12. Each Tx pad 10 consists of a small array of the smaller micropads 12 in approximate correspondence to a respective Rx pad 10, and all of the micropads 12 in the small array couple the same signal to one Rx 10 pad on the Rx chip. Data steering adjusts the spatial placement of data on the sending Tx chip, depending on the relative position of the receiving Rx chip and its Rx pads 10. A switching fabric in the transmit array steers data to a set of selected Tx micropads 12′ (here illustrated as sixteen in number) that provides optimal overlap with the respective receiving pad 12. Only those selected micropads 12′ are powered. The Tx micropads 12 that are determined not to be closely associated with a receiving pad 10 are not powered. The data steering compensates for actual mechanical misalignment of the chips and allows dynamic alignment correction during operation.
However, the data steering circuitry is complex and costly in power. For a partitioning of one Tx macropad into N×N micropads, the cost in gates and power scales as N2. For a 10×10 array (N=10), with 3×3 μm micropads implemented in a 180 nm technology, the cost in transmit power is about 60× that required to drive signals without electronic alignment correction but with similar speed performance. One can use a much more coarsely partitioned 4×4 array (N=4) to reduce power, but in this case crosstalk noise can remove up to ⅓ of the signal, and the cost in transmit power is still 11×. Power consumed in the Tx array is typically about 75% of the total power in the proximity communication interface. Therefore, eliminating the need for electronic alignment correction can provide significant power savings. Further, the technology requires alignment between the two chips to be less than the size of the smaller Tx micropads.
Thus, an alternate means is desirable for mitigating misalignment which consumes lower power and significantly reduces crosstalk noise compared to electronic alignment correction through data-steering.