I think we need to be clear at all times whether it is sufficient to identify a
reference signal_name or if an exact pin must be identified. The first sentence
of this goes both ways. One question is whether the parasitics of the
interconnect shorting the pins associated with one signal together are small
enough to warrant claiming that it wouldn't matter which of those pins were
used as a measurement reference node. I'm not so sure, particularly if package
parasitics are in play.
A potential conflict might come up with BIRD 161.1 Supporting Incomplete and
Buffer-only [Component] Descriptions . It has not yet passed, but it proposes
allowing the first column of [Pin] to give pad names instead of pin names. At
least in that case we would know pin parasitics were absent. But it allows says
that a [Pin] section could have just one pin.
IBIS has always supported producing models from hardware measurement, but it is
not certain that IBIS requires models to be expressed as though hardware had
been measured when it wasn't. A quick search tells me I have 169 IBIS files on
hand that do not contain the word "GND".
----- Original Message -----
From: "Walter Katz" <wkatz@xxxxxxxxxx>
To: "IBIS-ATM" <ibis-macro@xxxxxxxxxxxxx>
Sent: Sunday, February 28, 2016 12:08:20 PM
Subject: [ibis-macro] Determining the reference signal_name of an I/O buffer
Continuing this thought, since IBIS is a component measurement based system,
and since measurement are made at the pin of a component, and since every
measurement at an I/O pin is made between that pin and some nearby reference
pin, then all we need to do is define a method of determining the reference
signal_name for every I/O pin. The following algorithm should work for all
known existing IBIS models:
1. [Pin Mapping] tells the bus_label on each of the buffer rail terminals.
2. These bus_labels define the signal_name on each of the rail terminals.
3. If just one signal_name is on a Pin with Model_name GND, and the values of
the Pullup Reference, Power Clamp Reference, Ground Clamp Reference, Pulldown
Reference assigned that has that signal_name has a value of 0.0V in the
[Model], then a pin of that signal_name near the I/O pin is the reference for
the measurements at the I/O pin. Similarly, the I/O buffer rail terminal with
the reference signal_name at the I/O buffer is the reference node for
measurements at the I/O buffer. Similarly, the supply pads at the die/package
boundary near the I/O die pad are the reference for measurements at the I/O die
If this algorithm does not work for an I/O buffer (and I claim such a case does
not exist) then we can enhance IBIS by adding an option [I/O Reference] section
that has two columns. The first column is the Pin_name of an I/O buffer, and
the second column is the signal_name of the reference for all measurements at
the I/O buffer.
From: Walter Katz [mailto:wkatz@xxxxxxxxxx]
Sent: Saturday, February 27, 2016 8:58 PM
To: IBIS-ATM <ibis-macro@xxxxxxxxxxxxx>
Subject: One of the rail voltages of every IBIS buffer is a GND signal_name and
a reference node.
I have looked at the data sheets for parts with RS232, ECL, PECL and MECL
buffers, and every one of them has one of the rail voltages connected to a
Ground (GND) data book name (signal_name).
Although the IBIS standard does allow the user to associate all of the rail
voltages with bus_labels on POWER pins, such a buffer is an unnatural act.
If we state that it is a given that every buffer has a Ground rail connection,
then we can state that every buffer has a well-defined reference node for every
other terminal measurement at the buffer.
I challenge anyone on this committee to find a part with an I/O buffer that has
no rail terminals that are Ground.