Vaibhav
First I would recommend the load resistance for the VT waveforms be 50 Ohms,
not the external high value resistance used on the board. The part is
really driving a circuit board trace on the board and most board trace Zo is
closer to 50 Ohms than 2-10k Ohm external pullup resistors. Let the SI
simulator handle the external resistive load; don't put it in the model.
Second, you are not clear about the internal weak pullup. Is this load a
switched active device or is this an internal pullup resistor or micro-amp
current source?
If this weak pullup is an active device that is on when the buffer is high
and off when the buffer is low then model as a push-pull driver. And again,
use a 50 Ohm load terminated to GND/VDD for the 4 VT tables.
If the weak pullup is an internal resistance or a weak non-switched constant
current source then that characteristic needs to be in the powerclamp table.
If it is not switching then it is always showing its effect and needs to be
in the non-switching powerclamp table. And to reiterate, use a 50 Ohm load
terminated to Vdd to generate the VT data.
The purpose of the VT tables in an IBIS model is to show the switching
characteristics of the active devices under typical loads they will see on a
PCB. When an open sink pulldown device turns off the risetime is determined
by both how the device shuts off plus how the pullup resistance/C_comp RC
time constant allows the output to rise. If you use a large resistance
relative to the PCB Zo then the output risetime for an open sink buffer will
be dominated by the RC time constant and not the pulldown device's turn off
characteristics. The model should characterize the turn off and let the
simulator handle the RC effects.
Tom Dagostino
971-279-5325
tom@xxxxxxxxxxxxxxxxx
Teraspeed Labs
10367 NW Alpenglow Way
Portland, OR 97229
From: ibis-users-bounce@xxxxxxxxxxxxx
[mailto:ibis-users-bounce@xxxxxxxxxxxxx] On Behalf Of Vaibhav R Katkar
Sent: Thursday, July 09, 2020 6:09 AM
To: ibis-users@xxxxxxxxxxxxx; 'ibis@xxxxxxxxxxxxx'
Subject: [ibis-users] Query regarding IBIS modeling of open drain buffer
Hi All,
We are working on IBIS modeling of open drain buffer which can be used with
either external pullup or internal weak pullup.
We are able to model configuration with external pullup, which is straight
forward where we are modeling the buffer as OD buffer and generating VT data
with R_fixture value equal to the given external termination resistor.
For configuration with internal pullup, we are not sure about the correct
approach to be followed.
Below are the approaches we thought of:
1. Modeling the buffer as a OD buffer where VT will be generated with
R_fixture = 50Ohms and keeping internal pullup information using [Submodel]
keyword.
2. Modeling the buffer as push-pull buffer where VT will be generated
with some R_fixture value greater than the internal weak pullup value. Here
internal weak pullup information will be captured under [Pullup] keyword.
Please let us know if these approaches look reasonable or any different
approach need to be followed.
Thanks and Regards,
Vaibhav
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