GDDR5 will initially appear on 512 Mbit and Gbit chips supporting data transfers at up to 4 Gbits/s per pin. It can stretch to data rates as fast as 7 Gbits/s/pin to deliver throughput of 12-28 Gbytes/second per chip.
The interface retains the single-ended structure of the previous generation, but uses a new clocking technique and new low power modes to consume an average of 2.5 watts at 5 Gbits/second running at 1.5 volts. Macri estimates the new interface reduces power by about 30 percent compared to today's mainstream GDDR3.
The interface is backward compatible with previous graphics and systems memory interconnects from Jedec. "It's possible to build a controller that handles everything from DDR2 up to GDDR5 running from 400 MHz to 5 Gbits/s which is pretty amazing," Macri said.
Reductions in pins and other streamlining steps aim to keep die size, and thus cost, as low as possible. "Our GDDR5 physical layer block is not a whole lot bigger than our competitor's GDDR3 PHY," said Macri.
GDDR5 extracts clock information from the data stream in a way that allows it be flexible across different operating conditions, a fact that will help optimize performance when PC gamers over clock the chips. "This is much more flexible than any DRAM we have ever worked with in terms of going up and down in frequency and power," said Macri.
The spec supports error detection in read and write directions and can do real-time error detection and repair. The clocking scheme for the spec enables simpler routing on pc boards.
The spec itself is in a final stage at Jedec. "There's a lot of clean up, but after a September ballot, it should be ready for publication," said Macri.
Macri said advanced signaling technologies from Rambus will not be competitive, in part because they use a differential (two-wire) approach rather than the single-wire technique in GDDR5. The extra wires typically require more pins and power, Macri said.
"We don't think a differential solution make sense until you get to speeds of 8-10 Gbits/s and the Rambus XDR technology is not at these speeds yet," Macri said.
The Rambus technology is used as an interconnect for main memory in the Sony PlayStation3. However all three major video game consoles today use GDDR3 as their graphics memory interconnect, he said.
"XDR doesn't have a footprint in any console for graphics today," said Macri. "I believe GDDR5 will be a nice fit for the console space."