[SI-LIST] Re: Minimum eye opening of 8b10b encoded signals
- From: <Michael.Kurten@xxxxxxxxxxxxxxxxxxx>
- To: <dave.instone@xxxxxxxxxx>, <tkjeon@xxxxxxxx>
- Date: Wed, 23 Mar 2005 11:21:19 +0100
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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