BIRD ID#: 15 ISSUE TITLE: Clarification on the usage of the V/I tables. REQUESTER: Arpad Muranyi, Intel Corporation DATE SUBMITTED: May 10, 1994 DATE ACCEPTED BY IBIS OPEN FORUM: May 20, 1995 (with revisions in Standard) ************************************************************************* ************************************************************************* STATEMENT OF THE ISSUE: There is room for different interpretations regarding the usage of the various V/I tables in the current IBIS specification because it does not clearly explain all of the initial assumptions. If models are generated with one assumption and a simulation tool is based on another, inaccurate results may occur. ************************************************************************* STATEMENT OF THE RESOLVED SPECIFICATIONS: A new section must be added to the IBIS specification, in which the assumptions governing model generation and usage is clearly spelled out. This section should be placed at the end of the "Usage Rules" of the [Pulldown], [Pullup], [GND_clamp], [POWER_clamp] keyword section in the IBIS specification and should say the following: | Other Notes: ... | | Assumptions: It is assumed that the simulator sums the clamp | curves together with the appropriate pullup or | pulldown curve when a buffer is driving high or low, | respectively. From this assumption and the nature | of 3-statable buffers it follows that the data in | the clamping curve sections are handled as | constantly present curves and the pullup and | pulldown curves are used only when needed in the | simulation. | | The clamp curves of an input or I/O buffer can be | measured directly with a curve tracer, the I/O | buffer being 3-stated. However, sweeping enabled | buffers results in curves that are the sum of the | clamping curves and the output structures. Based on | the assumption outlined above, the pullup and | pulldown curves of an IBIS model must represent the | difference of the 3-stated and the enabled buffer's | curves. (Note that the resulting difference curve | may demonstrate a non-monotonic shape). This allows | the simulator to sum the curves, without the danger | of double counting, and arrive at an accurate model | in both the 3-stated and enabled conditions. | | Since in the case of a non 3-statable buffer this | difference curve cannot be generated through lab | measurements (because the clamping curves can not be | measured alone), the pullup and pulldown curves of | an IBIS model may contain the sum of the clamping | characteristics and the output structure. In this | case the clamping curves must contain all zeroes, or | the keywords must be omitted. | ************************************************************************* EXAMPLES: N/A ************************************************************************* ANALYSIS PATH/DATA THAT LED TO SPECIFICATION: Confusion raised in the IBIS open forum meetings during April 1994 led to the generation of this clarification for the IBIS specification. ************************************************************************* ANY OTHER BACKGROUND INFORMATION: N/A *************************************************************************