Questions about SAS coefficients' configuration

Harvey Newman harvey.newman at avagotech.com
Fri Mar 20 07:52:50 PDT 2015


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Hi Pengfei,
The figure you are asking about was the subject of a lot of discussion
during the specification development. It has been the subject of additional
discussion following the release of this document as well. Let me try and
provide some insights that hopefully provides you and everyone else a
consistent interpretation.
I start with the title of the figure "Minimum and maximum coefficient
ranges at maximum peak to peak voltage". When writing the specification we
realized the range the coefficients are capable of adjusting to on the
transmitter impacted the amount of equalization available to the over all
budget. In order to assure compatibility we needed to have some way to
require all SAS-3 compliant transmitters to support a minimum specified
range of the coefficients. Due to the interdependence between coefficient
range and amplitude we attempted to specify the range of the coefficients
when the amplitude (C2) was at maximum.
Maximum amplitude is limited by the 1200 mV differential peak to peak
number covered elsewhere in the specification. This limit is written into
the specification to protect receivers both now and in the future from
being over driven. Because it is a shall not exceed number all transmitters
are required to stay below this value when the coefficient C2 responds with
Maximum to an Increment command. The allowed voltage range for a
transmitter with C2 at maximum is 1200 mV to 850 mV. See SPL3r07 section
5.14 for details on the phy layer transmitter training coefficient
adjustments.
With C2 at maximum "...at maximum peak to peak voltage" the figure places
limits on the C1 and C3 "Minimum and maximum coefficient ranges". There was
concern that as amplitudes reduce we did not want the non-transition bit
(i.e., VMA) amplitude to fall below 80 mV differential peak to peak. The
dashed purple line is intended to communicate that the coefficient range
might be limited by the VMA requirement for lower amplitude signals. Keep
in mind that this figure is defined with C2 at maximum. Therefore we do not
need to add a constraint “C2-|C1|-|C3|>=Fixed Value” because C2 is not a
variable for the purpose of these limit lines.
Now to your questions.
Question 1:
Whether the value of C1 and C3 is the total value surrounding by the purple
solid line when Vpp_noeq = 1200mv?
Answer 1a:
The purple solid line represents the maximum range of C1 and C3 when C2 is
at maximum.
Question 1b:
If so, does it mean the value of C1 and C3 in purple solid line satisfied
equation “VMA>=80mv” when Vpp_noeq=1200mv?
Answer 1b:
If the amplitude of the signal is very close to the 1200 mV amplitude then
the VMA limit should be reached along the lower left diagonal limit line.
If the amplitude of the signal is lower, then the VMA limit will be the
limiting factor causing the maximum range to move in the direction
indicated by the dashed purple line.
Question 2:
Do we need a constraint like “C2-|C1|-|C3|>=Fixed Value” to ensure
“VMA>=80mv” when Vpp_noeq is variable between 850mv and 1200mv?
Answer 2a:
Since the figure applies when C2 is maximum we do not need to add the
constraint because for the purpose of this figure C2 is not a variable. As
soon as C2 is adjusted this figure does not apply. The VMA >= 80 mV always
applies and provides the primary maximum bound for C1 and C3 once C2 has
been reduced.
Question 2b:
If yes, when we constraint “C3>=-0.3” and “C1>=-0.15”(the range
surrounding
by the red dotted line shown in Figure 131, which is the minimum range we
should support), we do not need the constraint “C2-|C1|-|C3|>=Fixed
Value”
or any other constraint of C2 to ensure the minimum value of VMA, right?
Answer 2b:
Since my answer to 2a was no we need to address question 2b that starts "If
yes,". We wanted to provide the receiver the ability to adjust C2 and lower
the amplitude. We have placed the responsibility of maintaining the VMA >=
80 mV on the transmitter. We believe this provides the required constraints.
Please let me know if there are additional questions.
Regards,
Harvey Newman
Avago Technologies
On Mon, Mar 16, 2015 at 7:50 PM, Yupengfei (B) <yu.yupengfei at hisilicon.com>
wrote:
>  Hi All,
>
>
>
> I have questions needing your help.
>
>
>
> In SAS-3R06’s Figure 131 -- “Minimum and maximum coefficient ranges at
> maximum peak to peak voltage”,
>
> it defines the minimum range and maximum range of C1 and C3 with different
> Vpp_noeq.
>
>
>
> Question 1:
>
> Whether the value of C1 and C3 is the total value surrounding by the
> purple solid line when Vpp_noeq = 1200mv?
>
> If so, does it mean the value of C1 and C3 in purple solid line satisfied
> equation “VMA>=80mv” when Vpp_noeq=1200mv?
>
>
>
> Question 2:
>
> Do we need a constraint like “C2-|C1|-|C3|>=Fixed Value” to ensure
> “VMA>=80mv” when Vpp_noeq is variable between 850mv and 1200mv?
>
> If yes, when we constraint “C3>=-0.3” and “C1>=-0.15”(the range
> surrounding by the red dotted line shown in Figure 131, which is the
> minimum range we should support), we do not need the constraint
> “C2-|C1|-|C3|>=Fixed Value” or any other constraint of C2 to ensure the
> minimum value of VMA, right?
>
>
>
> Best Regards,
>
> Pengfei Yu
>
>
>
>
>  ------------------------------
>
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Harvey Newman
Avago Technologies
(408) 433-4028



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