BIRD 30: Programmable Buffers

Text item: Text_1

BIRD ID#: 30
ISSUE TITLE: Programmable buffers in IBIS models
REQUESTOR: Arpad Muranyi

DATE SUBMITTED: 8-10-95
DATE ACCEPTED BY IBIS OPEN FORUM: Pending

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STATEMENT OF THE ISSUE:

The current IBIS specification (2.1) does not provide a selection mechanism for
buffers which can change characteristics based on configuration options in a
component (programmable buffers). This BIRD is being written to provide a
solution to this problem. Using this feature of the IBIS models simulator tools
can implement a mode selection user interface for programmable buffers.

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STATEMENT OF THE RESOLVED SPECIFICATIONS:

A new keyword shall be introduced in the IBIS specification to provide a user
friendly buffer mode selection mechanism for components which use programmable
buffers. The proposed keyword [Model selector] shall have a unique model
selector name on its right with the same syntax rules which apply to the model
name used with the [Model] keyword.

The model selector name is used in the [Pin] list in place of the buffer name
for pins which have programmable buffers. Under the [Model selector] keyword a
list of those model names should appear which can be used by the pin referencing
this model selector name. The default buffer can be defined by a sub-parameter,
such as Default.

==============================================================================
| Keyword: [Model selector]
| Required: No.
| Description: Used to pick a [Model] from a list of [Model]s for a pin which
| uses a programmable buffer.
| Sub-Params: Default (?required if keyword used?)
| Usage Rules: A programmable buffer must have an individual [Model] section
| for each one of its modes used in the .ibs file. The names of
| these [Models] must be unique and can be listed under the
| [Model selector] keyword and/or pin list. The name of the
| [Model selector] keyword must match the corresponding model
| name listed under the [Pin] keyword and must not contain more
| than 20 characters. A .ibs file must contain enough [Model
| selector] keywords to cover all of the model selector names
| specified under the [Pin] keyword.
|
| The sub-parameter Default is used to point to a default [Model]
| to be used on the pin(s) calling this [Model selector].
|
| The operation of this selection mechanism implies that a group
| of pins which use the same programmable buffer (i.e. model
| selector name) will be switched together from one [Model] to
| another. Therefore, if two groups of pins, for example an
| address bus and a data bus, use the same programmable buffer,
| the user must have the capability to configure them
| independently. One can use two [Model selector] keywords
| with unique names and the same list of [Model] keywords;
| however, the usage of the [Model selector] is not limited to
| these examples. Many other combinations are possible.
|
|
[Pin] signal_name model_name R_pin L_pin C_pin
|
  1 RAS0# Progbuffer1 200.0m 5.0nH 2.0pF
  2 EN1# Input1 NA 6.3nH NA
  3 A0 3-state
  4 D0 Progbuffer2
  5 D1 Progbuffer2 320.0m 3.1nH 2.2pF
  6 D2 Progbuffer2
  7 RD# Input2 310.0m 3.0nH 2.0pF
| .
| .
| .
  18 Vcc3 POWER
|
|
[Model selector] Progbuffer1
                ABCD0123456789ABCDE0
                ABCD0123456789ABCDE1
Default ABCD0123456789ABCDE2
                ABCD0123456789ABCDE3
                ABCD0123456789ABCDE4
|
[Model selector] Progbuffer2
                ABCD0123456789ABCDE0
Default ABCD0123456789ABCDE3
                ABCD0123456789ABCDE4
                ABCD0123456789ABCDE5
                ABCD0123456789ABCDE6
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ANALYSIS PATH/DATA THAT LED TO SPECIFICATION:

Problem statement

Some components have programmable I/O buffers which can be configured for
different operating modes. Their strength or slew rate (and maybe other
characteristics) can be changed during operation by some mechanism. Currently,
the IBIS buffer modeling syntax provides keywords to describe a buffer model in
only one configuration or mode. Even though there are ways to work around this
problem in the present form of IBIS, there is a definite need to provide an
easier and more user friendly selection mechanism for the various configuration
modes of programmable buffers.

Possible solutions

(1) Currently it is NOT illegal to have more [Model] keywords in the IBIS
models than what the pin list section calls for. Therefore, the writer of an
IBIS model can make as many [Model] sections as many are needed to describe the
various modes of the programmable buffers. The user of the IBIS model can
change the model names of the programmable buffers in the [Pin] list section
with a text editor to select the desired buffer mode. Instructions about these
edits can be placed in the Notes section and/or behind the comment charater for
the user. The main problem with this metod is that it requires tedious manual
editing of the IBIS file.

(2) A slightly different solution would use the same amount of [Model] sections
as described above, but repeat each pin description in the [Pin] list as many
times as many configurations a buffer associated with that pin has. This method
is not allowed in the current IBIS specification, but could be made to work if
the repeated pin descriptions are preceeded with a comment character. This way
the user could uncomment the line(s) which contain the model name with the
desired buffer mode. This editing is much less tedious and error prone than the
one described in (1) above, but it can be cumbersome. If the repeated pin
description was legal in IBIS without the use of the comment characters, the
simulation tool could pick up the various model choises from the [Pin] list
section and provide a dialog box showing a list of the available buffer modes
for that pin. The main problem with this method is that the pin list can grow
very long due to the repetitions.

(3) Use #Define syntax to compile the IBIS model beforehand with the proper
buffers based on a model with multiple choices. There are various C tools that
can do this. One issue is if models can be changed dynamically such as might be
done to optimize a driver for a certain net. This method has the advantage that
it does not require a new keyword, therefore it can be used immediately.
However, it requires an extra compilation step before the IBIS model can be
used. This contradicts one of the main goals of the IBIS specification which
was to provide simulation tools with models which can be used directly. This
method could be used as an interim solution untill the new keywords are defined
in the next IBIS revision.

(4) A similar method to the one described in the STATEMENT OF THE RESOLVED
SPECIFICATIONS section would introduce a new keyword or sub parameter to be used
under the [Model] keyword. This means that a [Model] could have many complete
sets of I-V and V-t curves which describe the various operating modes of the
buffer. For example, one set of keywords describing a buffer in one mode could
come after the sub-parameter called [mode1], the next set of keywords describing
the buffer in another mode could reside under the sub-parameter called [mode2],
and so on, till all of the modes of the buffer are dharacterised. The default
buffer could be defined with another sub-parameter, such as Default. The only
difference between this method and the suggested one in the main section of this
document is that the selection mechanism is inside the [Model] keyword and not
outside. However, this minor difference imposes some limitations and might make
an IBIS file unnecessarily larger.

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ANY OTHER BACKGROUND INFORMATION:

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Received on Thu Aug 10 18:03:46 1995