======================================================================== BIRD ID#: 30.1 ISSUE TITLE: Programmable buffers in IBIS models REQUESTOR: Arpad Muranyi DATE SUBMITTED: 10-03-95 DATE ACCEPTED BY IBIS OPEN FORUM: Pending ****************************************************************************** ****************************************************************************** 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. ****************************************************************************** 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 first entry in the list is considered to be the default [Model]. To help the user of the simulator tool to make an intelligent choice, a single word description must appear on the right of each of the model names in this list. ============================================================================== | 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: None. | 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 section under the [Model selector] keyword must have two | columns. The two columns must be separated by at least one | space or tab character. However, the use of tab characters is | not recommended in general. The first column lists the | [Model] names. The second column contains a single word | description of the [Model] in the first column. The contents | and format of this description is not standardized, however it | shall be limited in length so that none of the descriptions | exceed the 80-character length of the line that it started on. | The purpose of the descriptions is to aid the user of the | simulator tool in making intelligent buffer mode selections | and it can be used by the simulator tool in a user interface | dialog box as the bases of an interactive buffer selection | mechanism. | | The first entry under the [Model selector] keyword shall be | considered the default by the simulator tool for all those | pins which call 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 2_mA_buffer__without_slew_rate_control ABCD0123456789ABCDE1 4_mA_buffer__without_slew_rate_control ABCD0123456789ABCDE2 6_mA_buffer__without_slew_rate_control ABCD0123456789ABCDE3 4_mA_buffer__with_slew_rate_control ABCD0123456789ABCDE4 6_mA_buffer__with_slew_rate_control | [Model selector] Progbuffer2 | ABCD0123456789ABCDE0 2_mA_buffer__without_slew_rate_control ABCD0123456789ABCDE3 16_mA_buffer__without_slew_rate_control ABCD0123456789ABCDE4 6_mA_buffer__with_slew_rate_control ABCD0123456789ABCDE5 8_mA_buffer__with_slew_rate_control ABCD0123456789ABCDE6 10_mA_buffer__with_slew_rate_control ****************************************************************************** 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 character for the user. The main problem with this method 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 preceded 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 choices 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 until 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 characterized. 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. ****************************************************************************** ANY OTHER BACKGROUND INFORMATION: Based on the IBIS Open Forum discussions, two slight modifications were made to the original BIRD issued as BIRD30. One of the changes was the removal of the sub-parameter Default and making the first entry in the [Model] list the default. The other change was the addition of a second column which contains a description for each [Model] to be used in the user interface of the simulation tools. The original BIRD30 became BIRD30.1 with these changes. ******************************************************************************