The GDDR7 SGRAM specification supports a maximum data transfer rate of 48 GT/s, but the first generation GDDR7 memory devices can operate with up to 32 GT/s data transfer rate. But it looks like GDDR7 will evolve pretty quickly as at the upcoming International Solid-State Circuits Conference (ISSCC), both Samsung and SK hynix plan to discuss their GDDR7 solutions with a 42 GT/s data transfer rate, according to the advanced program of the event.
Samsung plans to describe its 24Gb (3GB) GDDR7 memory device with various enhancements that can support data transfer rates of up to 42.5 GT/s, which means a peak bandwidth of up to 170 GB/s. This device is set to offer tangible improvements in capacity and performance compared to existing GDDR7 products. Currently, the company produces 16Gb (2GB) GDDR7 memory chips that can operate at up to 32 GT/s, thus providing a peak memory bandwidth of 128 GB/s.
In practical applications, a 24Gb GDDR7 memory chip with a 42.5 GT/s data transfer rate can enable graphics board makers to build graphics cards with 48GB of memory on a 512-bit interface and a whopping 2.7 TB/s peak bandwidth.
To achieve such a high data transfer rate for its upcoming 24Gb GDDR7 SGRAM devices with predictable power consumption and reliability, Samsung had to implement features such as a low-power Write Clock (WCK) distribution for energy efficiency, a resistance/capacitance-optimized dual-emphasis transmitter for high signal integrity, and advanced voltage and time margin enhancements to maximize performance reliability and stability under high loads.
Unlike Samsung, which describes its 24Gb GDDR7 chip in detail, SK hynix plans to focus on its 42 GT/s-capable single-ended PAM-3 receiver for GDDR7 memory interfaces. The key feature of the receiver is its single-ended hybrid decision feedback equalization (DFE), which ensures that signals traveling over the memory interface maintain their integrity, even in the presence of high noise levels, crosstalk, and inter-symbol interference (ISI), which are expected at 42 GT/s.
This single-ended hybrid DFE is actually a key feature required to reliably achieve high data transfer rates, so let's discuss it in more detail.
Feed-forward equalizers (FFEs) typically help reduce signal distortions caused by inter-symbol interference, which occurs when past transmitted symbols interfere with the current symbol. While FFE is effective for addressing linear ISI (caused by frequency-dependent losses), it is less effective for non-linear ISI (caused by reflections, noise, or crosstalk). Non-linear ISI often requires additional techniques, such as DFE. Decision feedback equalizers (DFEs) use feedback from previously decoded bits (decisions) to cancel out interference from previously decoded signals.
Now, hybrid DFE combines FFE and DFE to remove both linear ISI and non-linear ISI, thus ensuring proper signal integrity and predictable performance at high data transfer rates.
Neither Samsung nor SK hynix has announced when they plan to make GDDR7 memory with a data transfer rate of up to 42 GT/s. Still, the fact that both companies plan to discuss it at the upcoming ISSCC event indicates that they have a vision of achieving this performance level in the foreseeable future.
