Technical Q&A QA1191

Faster DDR DRAM in the 867MHz Power Mac G4 (Mirrored Drive Doors)

Q: Apple recently released 3 new dual processor, PoweMac G4 (Mirrored Drive Doors) models. The 1.25GHz and the 1 GHz models have a faster, 167MHz SDRAM bus and support DDR333 SDRAM, but the 867MHz Power Mac G4 model has a 133MHz SDRAM bus that uses DDR266 SDRAM. Can the original DDR266 SDRAM modules in the 867MHz dual processor, PowerMac G4 be replaced by DDR333 only, SDRAM?

A: The 867MHz model Power Mac G4 (Mirrored Drive Doors) has a RAM bus speed of 133MHz. One of the new features of this model PowerMac is that it now uses DDR (Double Data Rate) SDRAM.

The minimum speed DDR for this model PowerMac would be 2x133 or DDR 266 (A.K.A PC2100).

The machine is capable of using faster DDR SDRAM, such as DDR 333 (A.K.A. PC2700), but since the RAM bus is still 133MHz there is no advantage over the slower DDR 266.

There should also be no problem mixing and matching DDR 333 and DDR 266 DIMMs, but the bus on the 867MHz G4 is still 133MHz and the DDR 333 DDR SDRAM will simply act like a DDR 266.

In either case, developers should take care to follow the JEDEC Standard No. 21 - C (Appendix D). In particular, make sure to program the Serial Presence Detect (SPD) EEPROM correctly. Byte 2 of the SPD should be a 07 (hex) to indicate DDR (for PC2700 / DDR333) and byte 9 should indicate 6ns or smaller clock cycle (1/6nS = 167MHz approx.).

The only other thing to watch out for with DDR is to remember to specifically state ALL CAS Latencies and half CAS Latencies you support (in byte 19), since we can now have CAS Latencies like 2.5. Failure to follow the specifications and to add the supported CAS Latency would prevent the DIMM from being recognized.

[Aug 19 2002]