Technical Q&A QA1271

DRAM DIMM power pin connections for Macintosh Computers

Q: Can the power pins for a DIMMs Memory and Serial Presence Detect (SPD) be tied directly together if they require the same Voltage levels?

A: No.

Apple design follows the JESD21-C specifications for power for DIMMs. Sec 4.5.10 (the 184 pin unbuffered DDR DIMM spec.) makes it clear that the Memory and Serial Presence Detect (SPD) on a DIMM are separate circuits and connecting them in any way would violate the specification.

VDD (and / or VDDQ) for main memory and VDDSPD for the SPD of a DIMM are also specifically designed to be separate circuits and are treated as such on Apple Macintosh Computers. Connecting these two power pins together may inadvertently link separate circuits on a Macintosh computer.

[Jul 04, 2003]