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
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