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too-fast RAM will not work?



 
 
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  #1  
Old December 29th 18, 04:58 AM posted to alt.comp.hardware.pc-homebuilt
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Default too-fast RAM will not work?

I received a feeble PC - AMD E2 CPU with a jogging 1866 speed memory
controller. Hoping that I could improve it somehow, I stuck in a second
DDR4 DIMM. The spare I had was 2666 speed, but didn't work.
My understanding is that DIMMs offer 2 speeds below the rated speed, so
it would have offered 2133 and 2400.
I went to local PC shop and bought the cheapest 4 GB 2400 DIMM they had,
and that worked.
  #2  
Old December 29th 18, 06:35 AM posted to alt.comp.hardware.pc-homebuilt
Paul[_28_]
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Posts: 1,467
Default too-fast RAM will not work?

wrote:
I received a feeble PC - AMD E2 CPU with a jogging 1866 speed memory
controller. Hoping that I could improve it somehow, I stuck in a second
DDR4 DIMM. The spare I had was 2666 speed, but didn't work.
My understanding is that DIMMs offer 2 speeds below the rated speed, so
it would have offered 2133 and 2400.
I went to local PC shop and bought the cheapest 4 GB 2400 DIMM they had,
and that worked.


http://www.cpu-world.com/Cores/Stoney_Ridge.html

"Stoney Ridge processors have only 2 CPU cores, 1 MB L2 cache,
up to 3 GPU compute units (192 shaders), and support
single-channel DDR4 memory with maximum data rate of 2133 MHz.
"
The two DIMMs sit on a common channel, and the
BIOS has to select settings below the normal value
for either DIMM by itself. It can relax Tcas or Trcd
or even attempt to drop the clock rate below normal.
This is an AMD tradition (dropping clock when two
DIMMs sit on a channel). In DDR400 days, it was
typical to drop to DDR333 setting when two DIMMs
were present on a channel.

You can drop the clock a lot, before the PLL packs it in.
The BIOS can compute acceptable settings, other
than the data points in the SPD table. For example,
it's possible a DDR400 would run at DDR160 or so, just
to give some idea how far it could stretch. (You can find
this information in a Micron or Samsung DRAM chip datasheet.)

If the design uses a DLL for controlling phase, if you
drop the frequency too much, it runs out of taps to select.
So there is a method to their madness. There are hardware
resource limits which are generous, but not infinite.

For kicks, test the 2666 DIMM by itself and see
if the BIOS can configure it. There's a remote possibility
the 2666 will come up by itself. And that's how I would
have carried out my first test case, is the 2666 by itself.

You can always dump the SPD tables and admire the parameters
yourself, and seek an answer to why it did that. You'll
need a "JEDEC decoder" PDF to help you. Some JEDEC info is
members only. Some info is "public" but you need an account
to do a download (which is intended to limit their bandwidth
exposure, if they opened it up for fully public download).
It's really a shame they couldn't just move the useful
stuff to a separate server, so the handful of decoders
would be there for public consumption.

Paul
 




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