ATI Radeon X800 XT (R420): Extreme force
GDDR3 Memory
Radeon X800 cards offer 256 Mb of the GDDR3 SDRAM memory. This is an entirely new architecture of graphic memory suggested by engineers at ATI and approved by JEDEC as an open standard.
| JEDEC - Joint Electronic Devices Engineering Council. The Council was established in 1960 by EIA (Electronic Industries Alliance) and NEMA (National Electrical Manufacturers Association) in order to perform joint development of standards for electronic devices and reduce costs to do with creation of new devices due to reciprocal data exchange. Today, most manufacturers and developers of electronic components worldwide are members of JEDEC. |
It's just Joe Macri, ATI's chief technical executive in Santa Clara), who headed the definition of GDDR3 industry standards to be further approved by JEDEC and is currently the chairman of committee engaged in definition of the next standard GDDR4 at JEDEC.
 Joe Macri (ATI's chief technical executive)The tasks which were posed in development of GDDR3 are predictable enough - to raise the memory efficiency while providing compatibility to the original DDR architecture:
GDDR2 vs GDDR3:
| Criterion |
GDDR2 |
GDDR3 |
| Improve on DDR’s imperfections and limitations |
+ |
++ |
| Remain backwards compatible with original DDR |
++ |
++ |
| Architecture that initially supports UHE desktop and workstation markets |
++ |
++ |
| To be also applicable in mobile and mainstream desktop (within 6--9 months) |
+ |
++ |
| Simplify DRAM Design as well as System Design or DRAM Use where possible |
+ |
++ |
| Architecture to invest in for next few years |
+ |
++ |
| Frequency support greater than 600MHz |
- |
++ |
| Industry consensus i.e. everyone building to the same specification |
- |
++ |
++ task fully completed
+ task completed with some reservations
- failed to complete
As you can see from this table, this time they successfully avoided the major error of the previous architecture - the variant reading of the standard.
The GDDR3 was first demonstrated at the last autumn's IDF. But the first company who by the quirk of fate was the first to implement GDDR3 in a mainstream commercial product was NVIDIA with its FX5700Ultra DDR3 video card. The experiment proved successful - the card consumed less power, was cooler even despite the small performance boost (for test results, see the VGA Roundup).
What is GDDR3?
| Feature |
DDR |
GDDR2 |
GDDR3 |
| I/O |
SSTL-2 |
SSTL-18
with ODT |
POD-18 |
| Clocking interface: |
DQS |
Differential DQS or DQS |
Unidirectional DQS |
| Frequency |
166-450Mhz |
400-500Mhz |
500-800Mhz |
To avoid variant reading, we left the table as is. Specialist find it more appropriate to read it in the original, and common people would be absolutely not interested in that :). But it makes sense to explain what POD and Unidirectional DQS are.
POD (Pseudo Open Drain)
:
- It is voltage based open drain vs. current based drain area (reduced to implement driver);
- reference voltage Vref = 1.26V vs. 0.9V on DDR2;
- Allows controller that can support DDR, GDDR2 and GDDR3;
- Simplified DRAM design (allows use of Nmos transistors to build DRAM receiver);
- Reduced power and simplified MC/IO design;
- Idle state of strobe signals = VDDQ for simplified clocking.
Unidirectional DQS
GDDR3 offers transmission of unidirectional DQS strobes, which is advantageous over transmission of bi-directional differential DQS strobes in DDR2 for P2P applications:
Differential strobes:
- 4 strobes per byte
- Requires 8 pins
- No benefit on architectural side
- Minimal improvement to system timings
- Improved ISI and SSO (tDS/DH)
|
Unidirectional strobes
- 4 Read and 4 Write strobes per byte
- Requires 8 pins
- Benefits on architectural side
- Eases system timings
- Preamble pulse for improved ISI
- Strobes always valid for simplified clocking
|
Other areas remain compatible with DDR and DDR2
- Data as in DDR (Centered for writes; Edge aligned for reads);
- CLK pulses as in DDR
I don't like the idea of turning the material into a textbook on electronics, so simply put, all excessive resistors have been removed, and the voltage stabilizer unit on the card has turned twice simpler, which is well seen on the photos:
GDDR3's low power consumption results in less heat to the chips while providing essential boost in clock speeds. The chips are simply cold, and on Radeon X800 cards there aren't even radiators on the memory chips. This should appeal to the overclocker very much :). Nothing prevents them from fitting it on their own. But remember - 105 C is the critical point after which the data starts leaking away. The GDDR3 I/O are constantly calibrated to provide consistent characteristics across temperature and voltage.
And the last nice improvement - simplified clocking. As a result, there is more natural headroom available in the I/O system, with the critical paths minimized.
Whoosh... That's it!
It's a rare occasion when we are loaded with too much of technical stuff, so I'd better unload all the fresh info upon the readers. Our next lecture, already on GDDR4 will be held in 2005, because its standardization completion is scheduled for the end of this year.
GDDR3 memory on X800 series cards
Nowadays, the GDDR3 memory is being produced by all the leading players on the memory market - Samsung, Hynix, Micron, Infinion and Elpida.
The Radeon X800 card of the XT make offers 256 Mb GDDR3 SDRAM made up of eight Samsung K4J55323QF-GC16 chips of maximum frequency 600Mhz (resultant 1.2 GHz). In the BIOS of the video card, the memory frequency is set to 560(1120MHz), i.e. with some margin. The chips are set in four pieces on each of the sides and do not require any cooling. The supply voltage is 2.0V (compare it to 2.8-2.5V in the popular series of K4D263238E-GC chips).
In its own GDDR3 line, Samsung offers even more speedy chips - GC14 and GC12 (700 and 800MHz, respectively), so it's quite probable that more advanced and expensive video cards built on R420 chip may appear from manufacturers who prefer doing VGA developments on their own, e.g. ASUS. But in this case the memory chips would have to be cooled, which requires revamping of the whole cooling system, which is not going to be much trouble for some manufacturers.
 GDDR3 Samsung K4J55323QF-GC16 (for Radeon X800XT video cards)In the Radeon X800 Pro version, used was the low-end chip of the same series - Samsung K4J55323QF-GC20 but at the maximum clock speed 500MHz(1GHz). In the BIOS of the video card, the memory frequency is set to 450(900 MHz), i.e. again with some overclocking margin. But for now there aren't any tools to overclock with.
 GDDR3 Samsung K4J55323QF-GC20 (for Radeon X800Pro video cards)
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