[coreboot] More details about ram issues

[Nico Huber]

On 12.11.2016 05:00, Charlotte Plusplus wrote:
> Hello
> 
> On Fri, Nov 11, 2016 at 5:37 PM, Nico Huber <nico.h at gmx.de> wrote:
> 
>>> The W520 does only have 1.5V DDR voltage. If it's stable with vendor
>>> bios, it's not a DDR voltage problem at all.
>>
> 
> Based on my reading of the block diagram and crossing that with a cpu
> pinout and the cpu specs, I disagree.  The W520 indeed only support 1.5V,
> if you mean 1.5V vs 1.3 "low voltage" DDR3L.
> 
> But SA_DIMM_VREFDQ is in direct control of the DDR3 voltage: "The step size
> is 7.7 mV". So it supports 1.5V +- k*0.007V, with k being given by the XMP
> profile.
> 
> In case this is not clear, on
> http://www.intel.com/content/dam/www/public/us/en/documents/datasheets/3rd-gen-core-family-mobile-vol-1-datasheet.pdf
> :
> Page 30 : "The processor memory controller has the capability of generating
> the DDR3 Reference  Voltage (VREF) internally for both read (RDVREF) and
> write (VREFDQ) operations. The generated VREF can be changed in small
> steps, and an optimum VREF value is  determined for both during a cold boot
> through advanced DDR3 training procedures in  order to provide the best
> voltage and signal margins."
> 
> That seems to be a lot of evidence in the voltage not being an absolutely
> fixed 1.500V. It is something more flexible!!!

You're confusing the memory's operating voltage (what XMP and DDR3 vs.
DDR3L is about) with i/o voltages. VrefCA is the reference for Command/
Address lines and VrefDQ is the reference for Data lines from what I
understand.

For the operating voltage have a look at page 86 in the Kendo-3 schema-
tics. The TPS51916 provides the voltage VCC1R5A which is VDD for the
DIMMs. You can see from its datasheet that it's controlled by REFIN
which is generated by dividing VTTREF (1.8V) by the resistors R1056
(10K) and R843 (48K7). If those resistors were configurable in any
way, you could control the operating voltage. But they are just plain
resistors.

So even if the processor had dedicated pins to control the voltage, they
are not used in this machine.

Nico