Graphics memory: Samsung's GDDR7 uses PAM-3 for 36 Gbit/s

At the IEDM 2022, Samsung revealed further details about the upcoming graphics memory generation GDDR7 with 36 Gbit/s. Accordingly, the signal transmission is switched to the three-stage pulse amplitude modulation (PAM-3). This should increase energy efficiency.

Samsung revealed this at the IEEE International Electron Devices Meeting (IEDM 2022). A slide shared via Twitter is said to indicate a 25 percent improved energy efficiency of PAM3 compared to the previously used NRZ (Non-Return-to-Zero) technology. However, the graphic for this is misleading, because a minus of 25 percent in “power efficiency” would mean a deterioration. There must be an error here. Elsewhere there is talk of a 25 percent improvement.

I'm not attending this one, but here's a fascinating slide. Samsung confirms that GDDR7 uses PAM3 signalling.

NRZ is 0 or 1: 1 bit/cycle.
PAM3 is -1, 0, 1: 3 bit/2 cyc
PAM4 is 0, 1, 2, 3. 2 bit/1 cyc

80G Thunderbolt will also use PAM3. https://t.co/DJ8GvIDCJi pic.twitter.com/9Yw5zdUzDU

— 𝐷𝑟. 𝐼𝑎𝑛 𝐶𝑢𝑡𝑟𝑒𝑠𝑠 (@IanCutress) December 4, 2022

The change to PAM3 is an important detail, after all the previous GDDR generations up to GDDR6 used the binary NRZ signaling technology. PAM-4 has only been used for GDDR6X.

With NRZ, only one bit (1 or 0) is transmitted per clock cycle. With PAM-3 it is 3 bits with 2 clocks and with PAM-4 again 2 bits per clock, explains Ian Cutress in his comment on Twitter. The next generation of the Thunderbolt interface with 80 Gbit/s should also use PAM-3.

GDDR7 should reach 36 Gbit/s

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Samsung had already announced GDDR7 memory with 36 Gbit/s on its own Tech Day at the beginning of October, while last year there was still talk of 32 Gbit/s. With a graphics card with a 256-bit wide memory interface, a memory bandwidth of 1,152 GB/s would be possible with 36 Gbps chips. With 384 bit it would be 1,728 GB/s or around 1.7 TB/s.

GDDR6 will soon also be stacked

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Samsung recently announced a completely different variant of graphics memory: the stacked GDDR6W. This should double the memory capacity and memory bandwidth compared to GDDR6.

Graphics memory: Samsung's GDDR7 uses PAM-3 for 36 Gbit/s

GDDR6 will soon also be stacked h2> Samsung recently announced a completely different variant of graphics memory: the stacked GDDR6W. This should double the memory capacity and memory bandwidth compared to GDDR6.

GDDR6 will soon also be stacked

h2>

Samsung recently announced a completely different variant of graphics memory: the stacked GDDR6W. This should double the memory capacity and memory bandwidth compared to GDDR6.