[SI-LIST] Re: Minimum eye opening of 8b10b encoded signals

Hi Dave, Hi TK,

we are referring to SATA 1.0 and do measurements at the receiver. In
this case our aim is to get a feeling about the quality of our link. In
this case the eye opening is a good indicator.
=20
Now I understand that it is the safe way to center the double trapezoid
with respect to the zero crossings of the measured signal. I did not
expected this but it sounds reasonable with your explanations.

We were able to tweak the direction from board to disk of the link with
adjusting drive strength and/or preemphasis of the SATA controller
buffers on the board. Unfortunately up to now we do not see a
possibility for tweaking the same parameters for the SATA controller on
the disk.

Kind regards
Michael

-----Original Message-----
From: Dave Instone [mailto:dave.instone@xxxxxxxxxx]=20
Sent: Wednesday, March 23, 2005 10:02 AM
To: T.K. Jeon
Cc: Kurten, Michael; si-list@xxxxxxxxxxxxx
Subject: Re: [SI-LIST] Re: Minimum eye opening of 8b10b encoded signals

Hi TK,
    according to the SATA II spec Table 4, for Gen1i and Gen1m there is=20
no Tx minimum Voltage measurement interval specified, in fact there are=20
many holes in that table for 1i and 1m and  my interpretation of 6.1.1=20
of SATA II is that it states, somewhat unclearly, that the existing SATA

1 spec applies to these variants.  SATA  1.0a does show an eye although=20
it states ' The EYE is more of a qualitative measurement than a=20
spec.........Nonetheless this method is useful and easy to set up in the

lab.'

Regards
Dave Instone
T.K. Jeon wrote:

> Hello Michael,
>
> I'm not sure if you make measurements on SATA 1.0 or SATA II. But,=20
> SATA II specifies 'min/max amplitude', but not the eye opening.=20
> Therefore, there is no eye mask specified in the standard. I'm saying=20
> SATA II because I guess it's the latest spec.
>
> On SATA II, the min amplitude measurement is based on statistical=20
> distributions from n samples collected between 0.45UI and 0.55UI using

> three different patterns, which are HFTP(high frequency pattern),=20
> MFTP(middle freq. pattern) and LBP(lone bit pattern).
>
> Regards,
> TK
>
> Dave Instone wrote:
>
>> Dear Michael,
>>     The question of where to position the eye has been the subject of

>> many many discussions in Fibre Channel (which also uses 8b10b)and=20
>> also I suspect in SATA as a lot of the work of the Fibre Channel=20
>> jitter methodology appears in the SATA spec.  As yet no single answer

>> has been decided.  Essentially it depends on how the clock recovery=20
>> circuit centers itself.   The majority view is that you determine the

>> mean of the crossing and place the mask such that 0 UI is on the=20
>> mean, this will be correct for mean following CRCs.  However if the=20
>> jitter distribution is skewed this can result in a lot of margin in=20
>> one direction, and much less in the other, there are also=20
>> oversampling type SERDES which may center themselves differently and=20
>> there are also SERDES which used undisclosed methods of=20
>> 'compensating' for skewed distributions.   To be safe you should use=20
>> the mean of the crossing to position the mask.
>>    Regarding your particular violations you don't say if  you are=20
>> looking at the Rx end or the Tx end.  If at the Rx end it seems to me

>> that your link has too much HF loss, alternatively the TX  risetime=20
>> could be too slow and/or its amplitude too low.
>> Regards
>> Dave Instone
>>
>> Michael.Kurten@xxxxxxxxxxxxxxxxxxx wrote:
>>
>>
>>> Hello all,
>>>
>>> I have a question with regard to the minimum eye opening. I'm
examining
>>> an SATA link which has 8b10b encoding. The measured eye opening does

>>> not
>>> exhibit a lot of jitter. Therefore the double trapezoid, describing
the
>>> minimum eye opening, can be shifted along the time axis. The eye
height
>>> instead violates massively the minimum eye opening if the minimum=20
>>> eye is
>>> centered or placed at the left corner of the measured eye. Shifting
the
>>> minimum eye opening to the right corner results in an only slightly
>>> violated minimum eye opening.=3D20
>>> To date my understanding is that if the minimum eye opening can be
>>> placed anywhere along the time axis and not violating the eye
opening
>>> the link will work fine. Otherwise the minimum eye opening would not
be
>>> the minimum eye opening. But now I am looking for a more hardware
>>> oriented explanation of this matter.
>>> Any help is highly appreciated.
>>>
>>> Kind regards
>>> Michael Kurten
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>
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