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DDR2 Vdd and signal levels...

F

Fred Bartoli

Jan 1, 1970
0
Ok, I thought assembling a PC from parts was a nobrainer. Pick what you
want, assemble and it should work.
Standards are there to that purpose.

Well, no quite. I just got caught with DDR2 rams.
Picked the mobo, proc,... and that fast DDR2 ram.
No Boot, beep, beep, beep....

A few tries... Swapping the ram from another PC, and bingo all run nicely.

A bit of investigation later, arghhh, the mobo supports ram voltage up to
1.95V and is defaulted by the bios to 1.8V while the fast ram wants 2.2V

Now the PC runs with the new ram at 1.95V but of course with degraded
timings.
Of course all those voltage limitations aren't in the mobos manuals (you
have to discover this once all is installed) so it'll be a pain to RMA it
and look for another one that fits.

OTOH it's easy to modify the ram vdd controller to get the required
voltage, but... the question is how are handled the signal levels WRT to
the other elements on buses?
I've noticed on some datasheets there's a reference voltage input on the
DDR2 ram chips, but that doesn't tell all the story, so...
Is there more to change the ram supply voltage than simply changing it or
can I go safely with this?

Any enlightened hint?
 
P

PeteS

Jan 1, 1970
0
Fred said:
Ok, I thought assembling a PC from parts was a nobrainer. Pick what you
want, assemble and it should work.
Standards are there to that purpose.

Well, no quite. I just got caught with DDR2 rams.
Picked the mobo, proc,... and that fast DDR2 ram.
No Boot, beep, beep, beep....

A few tries... Swapping the ram from another PC, and bingo all run nicely.

A bit of investigation later, arghhh, the mobo supports ram voltage up to
1.95V and is defaulted by the bios to 1.8V while the fast ram wants 2.2V

Now the PC runs with the new ram at 1.95V but of course with degraded
timings.
Of course all those voltage limitations aren't in the mobos manuals (you
have to discover this once all is installed) so it'll be a pain to RMA it
and look for another one that fits.

OTOH it's easy to modify the ram vdd controller to get the required
voltage, but... the question is how are handled the signal levels WRT to
the other elements on buses?
I've noticed on some datasheets there's a reference voltage input on the
DDR2 ram chips, but that doesn't tell all the story, so...
Is there more to change the ram supply voltage than simply changing it or
can I go safely with this?

Any enlightened hint?

All DDR (1,2,3 etc) memory uses Vref = Vmem/2 for a number of reasons,
not least as it sets the effective clock crossing. Vref *should* be
generated directly from the Vmem supply. If it is (which it is supposed
to be) then simply increasing the Vmem supply should be ok.

As to other things on the bus - DDR busses can't easily be shared, so
the only things on those busses should be the DDR.

Cheers

PeteS
 
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