[ibis-macro] Re: clock_times .... yet again
- From: "Dmitriev-Zdorov, Vladimir" <vladimir_dmitriev-zdorov@xxxxxxxxxx>
- To: <fangyi_rao@xxxxxxxxxxx>, <scott@xxxxxxxxxxxxx>, <ibis-macro@xxxxxxxxxxxxx>
- Date: Wed, 14 Apr 2010 09:48:17 -0600
Thank you Fangyi for touching this important issue: ideally the spec
should explain the intended usage of the clock times so that
implementation becomes unambiguous.
> For each tick, EDA tools will use waveform from tick_time to
(tick_time+UI) to fill the eye histogram. If tick_time < 0, ...
That makes practical sense. If anyone agrees with that, then why not put
this into the spec?
But here is a minor technical problem: if you do this way, and the clock
times are not equidistant, then some portions of the waveform in your
histogram are counted twice and some could be missing. This is another
Pandora box: what's a definition of an eye histogram built from waveform
and non-equidistant clock times?
Vladimir
-----Original Message-----
From: ibis-macro-bounce@xxxxxxxxxxxxx
[mailto:ibis-macro-bounce@xxxxxxxxxxxxx] On Behalf Of
fangyi_rao@xxxxxxxxxxx
Sent: Wednesday, April 14, 2010 9:28 AM
To: fangyi_rao@xxxxxxxxxxx; scott@xxxxxxxxxxxxx;
ibis-macro@xxxxxxxxxxxxx
Subject: [ibis-macro] Re: clock_times .... yet again
There was a typo in my previous reply.
For each tick, EDA tools will use waveform from (tick_time-UI/2) to
(tick_time+UI/2) to fill the eye histogram. If tick_time < UI/2, ...
I meant
For each tick, EDA tools will use waveform from tick_time to
(tick_time+UI) to fill the eye histogram. If tick_time < 0, ...
Sorry for the confusion.
Fangyi
From: RAO,FANGYI (A-USA,ex1)
Sent: Tuesday, April 13, 2010 10:34 PM
To: 'scott@xxxxxxxxxxxxx'; IBIS-ATM
Subject: RE: [ibis-macro] clock_times .... yet again
Hi, Scott;
For each tick, EDA tools will use waveform from (tick_time-UI/2) to
(tick_time+UI/2) to fill the eye histogram. If tick_time < UI/2, a
portion of the waveform does not even exist. Ignore_Bits can be used to
deal with the situation you described, or the model can choose to not
return the first few ticks (missing ticks).
Let's make the clock tick definition simple: clock tick is used for AMI
models to instruct EDA tools where to center the eye. This is all what
EDA tools care about.
Regards,
Fangyi
From: ibis-macro-bounce@xxxxxxxxxxxxx
[mailto:ibis-macro-bounce@xxxxxxxxxxxxx] On Behalf Of Scott McMorrow
Sent: Tuesday, April 13, 2010 5:08 PM
To: IBIS-ATM
Subject: [ibis-macro] clock_times .... yet again
Fellow AMI travelers
In the specification for clock_times is the statement:
"The clock times are referenced to the start of the simulation (the
first AMI_GetWave call). The time is always
greater or equal to zero."
According to the discussion today, it has been agreed that all known
implementations of AMI dlls calculate the correct sample time and then
calculate clock_times as sample_times minus 1/2 the nominal clock
period. If this is truly the case, then there is a problem.
During startup the CDR is free-running, with no defined phase
relationship to the received data. In order to model this, the period
of the CDR would be set to it's free-running period, and the phase of
the CDR would need to be randomly initialized at INIT. The location of
the receiver sample point is then random within the first UI data
window, as it is in reality. No matter what the Tx does, there will
generally be a free-running sample occuring with no phase relationship
to anything, until the first received differential transition occurs.
If the first value of clock_times is calculated as receiver sample time
minus 1/2 the nominal clock period, then clock_times can take any value
from -0.5 UI to +0.5 UI.
The specification should be clarified to state that:
The clock times are referenced to the start of the simulation (the first
AMI_GetWave call). The time is always
greater or equal to -0.5 nominal clock periods or symbol UI."
best regards,
Scott
--
Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
(401) 284-1827 Business
(401) 284-1840 Fax
http://www.teraspeed.com
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Teraspeed Consulting Group LLC
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