[SI-LIST] Re: Minimum eye opening of 8b10b encoded signals
- From: Dave Instone <dave.instone@xxxxxxxxxx>
- To: "T.K. Jeon" <tkjeon@xxxxxxxx>
- Date: Wed, 23 Mar 2005 09:01:37 +0000
Hi TK,
according to the SATA II spec Table 4, for Gen1i and Gen1m there is
no Tx minimum Voltage measurement interval specified, in fact there are
many holes in that table for 1i and 1m and my interpretation of 6.1.1
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
it states ' The EYE is more of a qualitative measurement than a
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,
> SATA II specifies 'min/max amplitude', but not the eye opening.
> Therefore, there is no eye mask specified in the standard. I'm saying
> SATA II because I guess it's the latest spec.
>
> On SATA II, the min amplitude measurement is based on statistical
> distributions from n samples collected between 0.45UI and 0.55UI using
> three different patterns, which are HFTP(high frequency pattern),
> 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
>> also I suspect in SATA as a lot of the work of the Fibre Channel
>> jitter methodology appears in the SATA spec. As yet no single answer
>> has been decided. Essentially it depends on how the clock recovery
>> 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
>> mean, this will be correct for mean following CRCs. However if the
>> jitter distribution is skewed this can result in a lot of margin in
>> one direction, and much less in the other, there are also
>> oversampling type SERDES which may center themselves differently and
>> there are also SERDES which used undisclosed methods of
>> 'compensating' for skewed distributions. To be safe you should use
>> the mean of the crossing to position the mask.
>> Regarding your particular violations you don't say if you are
>> 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
>> 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
>>> 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.=20
>>> 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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>
>
>
>
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