A couple of weeks ago, MOSAID and NOVACHIPS announced plans to jointly develop an SSD controller based on the MOSAID high-speed HLNAND interface specification. If you’re not familiar with the MOSAID HLNAND high-speed serial interface, join the club. Most NAND Flash devices employ either the ONFi or Toggle NAND interfaces but MOSAID decided a while back that another interface was needed and so it developed HLNAND a few years back. It’s worthwhile exploring the reasons and I downloaded the MOSAID White Paper “Enabling Ultra-High Scalable SSDs with HLNAND” dated August, 2010 to get the explanation.
Briefly, the MOSAID White Paper notes that the ONFi 2.0 interface is limited to 166Mbytes/sec per device while a SATA 6Gbps storage interface can transfer 600Mbytes/sec. So there’s a 3.6:1 bandwidth gap to bridge. SSD controllers generally bridge this gap by implementing multiple NAND Flash channels to bridge the bandwidth gap and to achieve the desired SSD capacity. As the MOSAID White Paper explains:
“Developing high capacity SSDs without dramatically increasing the number of memory channels and, hence, the complexity, requires a memory interface that is more scalable. Likewise, as system interconnect throughput continues to grow, designers require Flash devices with an interface that can accommodate correspondingly higher speed operation without loading induced roll-off. Both characteristics are essential to producing high performance, high capacity SSDs.”
So MOSAID developed a proprietary serial, daisy-chain interconnect with a ring topology for NAND Flash devices and named it “HyperLink.” As many as 255 Hyperlink NAND (HLNAND) devices can be connected in a single HyperLink ring. Because each device in the ring connects to only its two adjacent neighbors, ring expansion doesn’t increase device loading so the ring’s link speed doesn’t slow as the ring grows. The HyperLink definition creates an 8-bit, synchronous DDR data bus that runs at 133MHz using a parallel-distributed clock. Here’s a block diagram:
The peak effective throughput for this configuration is 266Mbytes/sec. There’s also an HLNAND2 configuration that boosts the ring transfer clock to 400MHz resulting in a peak transfer rate of 800Mbytes/sec. The HLNAND2 achieves the higher clock speed by employing a source-synchronous clocking scheme where each NAND device in the ring supplies a regenerated clock to the next device in the daisy chain. This new MOSAID NAND Flash interface scheme may not seem like an absolute requirement for current SATA interface speeds, but the White Paper points out that:
“Conventional Flash, operating at about 40MB/s, is rapidly becoming obsolete and is already virtually unworkable with interconnects like SATA 3, PCIe 1.x and PCIe 2.x, at mid-level lane counts. As the industry considers PCIe lanes and moves to PCIe 3, we see that conventional 40MB/s Flash requires more than 100 channels to saturate the interconnect. Even ONFi 2.0 Flash, operating at 166MB/s, requires several 10’s of channels to saturate PCIe 2.x and PCIe 3 storage systems.
…HLNAND, operating at 266MB/s, brings the channel count back down to a workable number of 24. HLNAND2, operating at 533MB/s to 800MB/s, offers a solution that brings the memory channel count back down into the comfortable range of 8 to 12. HLNAND and HLNAND2 are well suited to deliver high performance to the enterprise segment, which many observers believe will be the first to take advantage of the high throughputs offered by PCIe 2.x and PCIe 3.”
Which brings us back to the MOSAID/NOVACHIPS announcement. The goal: an SSD controller that “will support up to 8TB (terabytes) capacity with a SATA3 host interface.” According to the announcement, the HLNAND controller is scheduled for availability in mid-2013.
However, an SSD controller alone doesn’t do much; you also need NAND Flash devices for the controller to, er, control. MOSAID announced sampling of 512Gbit HLNAND Flash memory devices in April, 2012. According to The Memory Guy, Jim Handy, this announcement was for a packaged device consisting of 16 stacked memory die atop a specialized interface die. Handy likened the announced HLNAND device to a NAND Flash version of the Hybrid Memory Cube because both devices stack multiple memory die atop two logic die that supply the 2-channel HLNAND system interface.Here’s an X-ray cross section of the 512Gbit wire-bonded 3D device:
MOSAID is an IP company rather than a NAND Flash vendor, so commercial availability of HLNAND Flash devices to go with the announced controller remains uncertain until the semiconductor memory vendors decide to weigh in.
Note: MOSAID will be presenting a paper about this technology on Tuesday, August 21 at the Flash Memory Summit in Santa Clara, California.