Please consider that when analyzing the performance of a channel with
AMI model, the EDA tool has to make assumptions about the structure of
the signal. So, for example, the calculations for PAM4 are different
from the calculations for NRZ. For example, the EDA tool may assume that
the decision threshold for NRZ is 0V. It should be possible, however, to
shift the DC point for all signals so that the decision threshold
remains at 0V.
The key point is to make sure you know what assumptions both the model
and the EDA tool are based on.
FWIW,
Mike Steinberger
On 01/25/2016 02:43 PM, Bob Miller (Redacted sender bob.miller for
DMARC) wrote:
I think there is an annoying issue in single-ended IBIS-AMI simulations in that the impulse fed into the algorithmic model in AMI_Init has no DC information (it is functionally the derivative of the step response, which does have DC information.) In a purely differential receiver we implicitly assume that both the Tx differential reference and the Rx differential trip point is 0V and interpret the impulse in such a way that a step response or data stream generated from this impulse is symmetrical about 0.
However, in a "single-ended" system, the Tx and Rx references are not 0V (and may be different from each other) so the Rx cannot merely process the impulse through the Rx front end; it probably needs to perform the equivalent of integrating the impulse into a step/pulse/data waveform (which if you recall calculus101 requires the restoration of the DC "constant"). This DC constant probably has to come from the Tx.
In AMI_Getwave, the Tx can supply the "single-ended" output waveform with the DC component preserved; as long as the EDA platform properly preserves this through the channel convolution all is well (I hope!).
As in the IBIS Open Forum, I may be blathering nonsense; feel free to advise me of it (again), Walter...
Regards,
Bob
On Mon, Jan 25, 2016 at 12:44 PM, Walter Katz <wkatz@xxxxxxxxxx <mailto:wkatz@xxxxxxxxxx>> wrote:
John,
I verified that the IBIS 6.0 parser generate no warnings or errors
when an [Algorithmic Section] (interface to IBIS AMI model) is
added to a singled ended Input, Output or I/O buffer.
On page 171 IBIS 6.1 says:
“The [Algorithmic Model] always processes a single waveform
regardless whether the model is single-ended or differential. When
the model is differential, the waveform passed to the [Algorithmic
Model] must be a difference waveform.”
The bottom line is that this is nothing in the IBIS specification
that limits AMI modeling to Inputs, Outputs, I/O, single ended or
differential.
There is also no constraint in IBIS that the Impulse Response must
start at zero, and end at zero. A number of tools will generate an
impulse response by differentiating a step response. This will
lose any DC offset in the data, but this is a limitation of the
implementation of AMI on that tool.
IBIS does say that AMI modeling is used for SerDes designs, but
does not limit it as such. It certainly would not hurt to add some
language that AMI modeling can be used for other than SerDes.
Walter
Walter Katz
wkatz@xxxxxxxxxx <mailto:wkatz@xxxxxxxxxx>
Phone 303.449-2308 <tel:303.449-2308>
Mobile 303.335-6156 <tel:303.335-6156>
