GoKawiilJEDEC has released its new specification that aims to push down the pricing of the ultra-expensive HBM that powers the fastest AI processors. While the new standard will not help relieve the DRAM shortage as it uses large HBM4 DRAM devices, it can make high-bandwidth memory a bit cheaper as it enables attaching SPHBM4 memory stacks without advanced packaging and using inexpensive organic substrates.
The standard's body published the specification of SPHBM4, Standard Package High Bandwidth Memory (JESD330-4), that combines HBM4 DRAM ICs with standard packaging and a fast 'narrow' 512-bit interface. Here are the details.
HBM4 performance with a 512-bit wide interface
Although 1024-bit and 2048-bit interfaces used by HBM3 and HBM4 memory deliver unbeatable performance, their wide interfaces consume significant silicon area inside processors, they require expensive interposers, and advanced packaging technologies with limited capacity, such as TSMC’s CoWoS, for integration with host processors. The upcoming SPHBM4 memory continues to use the same HBM4 DRAM stacks as JESD270-4, but swaps the conventional HBM base die for a new SPHBM4 PHY/buffer die featuring a narrower 512-bit interface that enables mounting on standard organic substrates without using sophisticated packaging methods for integration. To offset the effect of the narrower interface, SPHBM4 supports considerably higher data transfer rates ranging from 22.4 GT/s to 46.0 GT/s.
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Instead of connecting to the host processor using a 2048-bit memory interface like HBM4, SPHBM4 uses 32 independent 16-bit DDR channels organized into eight Quad Channels. Since 'Quad Channel' is a new term, let us explain how things work. Internally, an HBM4 stack contains 32 memory channels, each 64 bits wide, for a total external interface width of 2048 bits. SPHBM4 needs to 'convert' the 2048-bit internal I/O onto a 512-bit external interface, which is why it groups every four HBM4 channels into a Quad Channel. As a result, externally, a Quad Channel exposes 64 data pins (4 × 16 bits), which replace the 256 data pins (4 × 64 bits) that those four HBM4 channels would normally require. To preserve bandwidth, these 64 pins operate at four times the data rate of the original HBM4 interface.
While SPHBM4 dramatically increases I/O bandwidth, it does not make the DRAM array itself faster. The HBM4 memory core retains the same fundamental architecture and timings, including core frequency, row activation, precharge, and refresh operations, though the additional PHY is expected to introduce some latency. For example, the DRAM core runs at only one-quarter of the external interface frequency, which means 2 GHz in the case of SPHBM4 with a 32 GT/s speed bin.
The major change is the new base die, which implements a high-speed SerDes-like PHY that maps each 16-bit external channel to four conventional 64-bit HBM4 channels. As a result, SPHBM4 introduces equalization, lane training, BER requirements, and other high-speed signaling features that are unnecessary in HBM4’s slower, wide parallel interface. To support transfer rates of up to 46.0 GT/s/s per pin, each Quad Channel uses a shared command/address interface protected by forward error correction (FEC), while data transfers rely on dedicated differential write (WCK) and read (RCK) clocks, as well as ECC and error-reporting signals.
When it comes to capacity, SPHBM4 can use stacks containing 4, 8, 12, or 16 DRAM dies featuring 24 Gb or 32 Gb densities, so the largest standardized SPHBM4 configuration is a 64 GB memory stack built from sixteen 32 Gb DRAM dies, identical to the maximum capacity supported by HBM4E.
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