Arpad,
"connected legally" means the circuit works. It is a function of the total DC
load and AC load. A board drawing 100 amps of current from a 2V VDD supply is
not going work legally (i.e. pass the Rx and Tx buffer rail voltage
requirements if there is a 100 Ohm series resistor between the VDD connector
pins and the VDD designator pins. A 100 Ohm series resistor might be OK if the
board was drawing 1 mA.
Bottom line is IBIS assume a connection between Tx and Rx models, and this
defines "connected"
In the layout world this could be giving the Tx and Rx pins the same CAD Net.
Or there could be a series resistor between two different CAD nets. Now it is
the choice of how the model maker constructs his interconnect circuits. The
series resistor can be an element in the ISS interconnect model between the Tx
and Rx which are "connected". Or he can have the series resistor be an IBIS
component, and then there would be two CAD nets, one connection from the Tx to
the series resistor and another from the series resistor to the Rx. All EDA
tools have the ability to sniff this out and generate Extended Nets. So the
"connection" between the Tx and the Rx can be implement as a daisy chain of
connected sections.
Walter
From: ibis-macro-bounce@xxxxxxxxxxxxx <ibis-macro-bounce@xxxxxxxxxxxxx> On
Behalf Of Muranyi, Arpad
Sent: Friday, December 13, 2019 1:37 PM
To: IBIS-ATM <ibis-macro@xxxxxxxxxxxxx>
Subject: [ibis-macro] Re: What is a connection?
Walter,
Thanks for the thoughts. I have a little hard time with the "DC impedance"
terminology here.
If you look at the Wiki article:
https://en.wikipedia.org/wiki/Electrical_impedance<https://en.wikipedia.org/wiki/Electrical_impedance>
you will see that they say:
"In addition to resistance as seen in DC circuits, impedance in AC circuits
includes the..."
So I feel that at DC we should be talking about resistance, not impedance...
The other thing that still bothers me is your statement about Rail
connectivity. What does
"connected legally" mean?
If I were to summarize what you are saying, than the only difference between
PDN and signal
connectivity is that in PDN we require a low DC resistance, but in signal
connections we can
also consider a low AC impedance (at Nyquist) in addition to a low DC
resistance as a "good
connection".
Thanks,
Arpad
=========================================================================
From: ibis-macro-bounce@xxxxxxxxxxxxx<mailto:ibis-macro-bounce@xxxxxxxxxxxxx>
[mailto:ibis-macro-bounce@xxxxxxxxxxxxx] On Behalf Of Walter Katz
Sent: Friday, December 13, 2019 9:08 AM
To: IBIS-ATM <ibis-macro@xxxxxxxxxxxxx<mailto:ibis-macro@xxxxxxxxxxxxx>>
Subject: [ibis-macro] What is a connection?
All,
I/O Connectivity
The IBIS specification defines the electrical IV and VT characteristics of an
I/O buffer. It is fundamentally based on the fact that there is an electrical
path that can be defined between a Driving Output buffer and one or more Input
(and other Output) buffers. It is expected that the transitions of the Driving
Output Buffer are reflecting in transitions at the Receiving Input buffers
between Vinl and Vinh. One way of describing these electrical connections
between the I/O buffers are with IBIS ISS (and Touchstone File) models. A
connections is a "good" connection if the transitions at the Driver generate
transitions at the Receiver that satisfy the receiver "rules" in the IBIS file.
A good connection could have a low DC impedance, or in some cases a low
impedance at a Nyquist frequency.
Rail Connectivity
If two nodes in a rail distribution system are "connected" legally then there
is a "low" DC impedance between the two nodes. Again, "low" is dependent on the
current t flow and proper operation of the I/O buffers.
Walter
Walter Katz
wkatz@xxxxxxxxxx<mailto:wkatz@xxxxxxxxxx>
Office 978.461-0449 x 133
Mobile 720.417-3762
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