LSI recently launched a new era in disk-read-channel technology with the RC9500, a mixed-signal read-channel-IP (intellectual-property)-core cluster. LSI intends the block for integration with a drive vendor’s IP to create a single-chip drive-electronics subsystem. The IP is LSI’s—and, possibly, the industry’s—first venture into 40-nm process technology, and it appears to be the first application of an LDPC (low-density-parity-check) algorithm in read-channel products. LSI intends the IP for the coming generation of 500-Gbyte/disk, 2.5-in. and 1-Tbyte/disk, 3.5-in. drives.
The 40-nm part allows the read channel to reach 4 Gbps within the power budget customers specify. At this speed, even minimal parasitics and small variations can sneak up on designers. Compounding the design problem for analog designers is the fact that the whole game in read channels is about noise. The head and media designers will push areal density until every hint of noise margin is gone, so there’s nothing left over for the low-noise-amplifier designers. But low-noise design at 40nm—especially in IP, in which you can’t control the charge other circuits might be pumping into the substrate next door—is an art. It would be easy for circuit designers to lose the whole advantage of the new LDPC algorithm in the analog section.
The 40-nm process also presents issues to LSI’s customers. LSI’s interface to customers using this IP will be its familiar ASIC model. But everything will go better if customers bring to the table a design team and IP that are ready for the rigors of a 40-nm design. The other milestone is LDPC, a signal-recovery technique that has for years found use in such areas as satellite communications, in which SNRs (signal-to-noise ratios) are small and no one minds that the receiver fills a rack and runs slower than real time. Getting the algorithm into a manageable gate count and power consumption and getting it to keep up with a 64-Gbps bit stream are challenges. But LSI officials claim that LDPC provides at least 1 dB of improvement in SNR—a major increment in disk drives.
The algorithm required some new architectural thinking compared with previous read-channel DSP designs. LSI designers also worked on adapting the algorithm to the signal characteristics they expected from the next generation of head and media designs, which are themselves still evolving.
LSI Corp.
