2

The sales support of (the consumer/prosumer sales subsidiary of) a DRAM manufacturer answered a pre-sales question I made with (translated from an email in French):

1.35V Memory DIMMs are dual voltage, and can work at both 1.35V et 1.5V; your computer system will adjust the voltage automatically as required by the motherboard and other factors like pre-existing DIMMs kept in the computer.

I was initially skeptical, but indeed the data sheet of even some relatively old DDR3L SDRAM chip states their are

Backward-compatible to VDD = VDDQ = 1.5V ±0.075V

and

Refer to the DDR3 (1.5V) SDRAM data sheet specifications when running in 1.5V compatible mode.


Is this the case for all DDR3L (1.35V) DIMMS on the market? If not, it is because they are unspecified, or without proper SPD memory content, or otherwise inoperative, or perhaps plain destroyed, when operated at 1.5V?

Note: This is an engineering question; I'm looking for technical details explaining incompatibility, if any. Like that pure speculation: for cost reason, some DDR3L chips are not tested at 1.5V (or happened to fail that test), and are thus sold on DDR3L 1.35V-only DIMMs not indicated as 1.5V-compatible in the SPD memory.

6

Micron have an interesting Tech note where they say the following:

Micron 1.35V DDR3L and DDR3L-RS devices use the same die as 1.5V DDR3 devices, but have been separated during the test screen and marking process. The Micron 1.35V test screen incorporates testing to ensure backward compatibility to 1.5V operation. Therefore, all parts marked as DDR3L or DDR3L-RS are backward compatible to partsmarked as DDR3, and meet the JEDEC 1.5V voltage level operation specifications. This is in compliance with both Micron and JEDEC specifications.

Highlight mine; Micron is implying with this statement that the JEDEC standard requires such backward ccompatibility to 1.5V systems for DDR3L.

Note that those statements may or may not apply to DIMMs although I see no reason it should not.

I have used a fair amount of DDR3L (always ICs, never DIMMs) and all of them have had the 1.5V compatibility statement which did come in useful on one particular occasion.

I will check the JEDEC Specification on Monday.

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