Notes on IBIS Waveform Properties

The IBIS buffer component Waveform properties allow for fine-tuning the power scaling and waveform rise and fall. The following table summarizes the property settings. Detailed notes on some of these properties follow the table.

**IBIS Buffer Waveform Properties**
Property	Description	Units	Default
INTERPOL	Interpolation: 1=linear, 2=spline	None	1
NOWARN	When present, turns off parser warnings	None	Warnings enabled
XV_PU	Name of node for pullup scaling factor output	None	None
XV_PD	Name of node for pulldown scaling factor output	None	None
RAMP_FWF	Falling waveform ramp specification: 0=ramp only, 1=single waveform in IBIS file, 2=double waveform in IBIS file	None	2
RAMP_RWF	Rising waveform ramp specification: 0=ramp only, 1=single waveform in IBIS file, 2=double waveform in IBIS file	None	2
FWF_TUNE	Falling waveform adjustment. Used only when RAMP_FWF=0 or 1. Waveform assumes device transitions from on to off in the fraction of the total transition time given by FWF_TUNE.	None	0.1
RWF_TUNE	Rising waveform adjustment. Used only when RAMP_RWF=0 or 1. Waveform assumes device transitions from off to on in the fraction of the total transition time given by RWF_TUNE.	None	0.1
C_COM_PC	Fraction of capacitance on power clamp vs. GC, PU, and PD, (value between 0 and 1). Capacitance fractions must sum to 1.0.	None	0
C_COM_GC	Fraction of capacitance on ground clamp vs. PC, PU, and PD (value between 0 and 1). Capacitance fractions must sum to 1.0.	None	1
C_COM_PU	Fraction of capacitance on pull-up vs. GC, PC, and PD (value between 0 and 1). Capacitance fractions must sum to 1.0.	None	0
C_COM_PD	Fraction of capacitance on pulldown vs. PC, GC, and PU (value between 0 and 1). Capacitance fractions must sum to 1.0.	None	1
PC_SCAL	Scale factor for power clamp current drive strength applied to IBIS table values	None	1.0
GC_SCAL	Scale factor for ground clamp current drive strength applied to IBIS table values	None	1.0
PU_SCAL	Scale factor for pull-up current drive strength applied to IBIS table values	None	1.0
PD_SCAL	Scale factor for pulldown current drive strength applied to IBIS table values	None	1.0
FWF_SCAL	Scale factor applied to falling ramp transition time	None	1.0
RWF_SCAL	Scale factor applied to rising ramp transition time	None	1.0
SPD_SCAL	Scale factor for pulldown current drive strength applied to IBIS table values Used only when POWER=OFF and the voltage difference between the pull-up and pulldown nodes is not equal to the [Voltage Range] value in the IBIS file.	None	1.0
SPU_SCAL	Scale factor for pull-up current drive strength applied to IBIS table values Used only when POWER=OFF and the voltage difference between the pull-up and pulldown nodes is not equal to the [Voltage Range] value in the IBIS file.	None	1.0
RM_DLY_RWF	Amount of time to trim off the front of the rising waveform, relative to time=0. Negative value invokes automatic removal of common delays on the fronts of rising and falling waveforms [see Automatic Delay Removal].	>=0: Sec <0: None	0.0
RM_DLY_FWF	Amount of time to trim off the front of the falling waveform, relative to time=0 Negative value invokes automatic removal of common delays on the fronts of rising and falling waveforms [see Automatic Delay Removal].	>=0: Sec <0: None	0.0
RM_TAIL_RWF	Amount of time to trim off the end of the rising waveform, relative to the time of the last point in the waveform	Sec	0.0
RM_TAIL_FWF	Amount of time to trim off the end of the falling waveform, relative to the time of the last point in the waveform	Sec	0.0

[1] If the values of RM_DLY_RWF and RM_DLY_FWF are not equal, Nexxim generates a warning, since the unequal values alter the duty cycle of the output waveform relative to what is specified by the IBIS file itself. The simulation runs.

[2] If the value of RM_DLY_RWF, RM_DLY_FWF, RM_TAIL_RWF, or RM_TAIL_FWF is such that more than 10 percent of the original voltage transition is trimmed off, Nexxim generates a warning, but the simulation runs.

[3] If the values of RM_DLY_RWF and RM_TAIL_RWF are such that fewer than three (3) data points are left in the resulting waveform, Nexxim ignores the settings and issues a warning. With fewer than three data points, ramp transitions may give better results.

[4] If the values of RM_DLY_FWF and RM_TAIL_FWF are such that fewer than three (3) data points are left in the resulting waveform, Nexxim ignores the settings and issues a warning. With fewer than three data points, ramp transitions may give better results.

[5] If the sum of RM_DLY_RWF and RM_TAIL_RWF is such the entire rising waveform is removed, Nexxim halts with an error.

[6] If the sum of RM_DLY_FWF and RM_TAIL_FWF is such the entire falling waveform is removed, Nexxim halts with an error.

Automatic delay removal. Some IBIS models represent the throughput delay of the device by providing model waveforms with long initial plateaus, after which the device is switched at a much shorter period corresponding to the actual slew rate. However, the slew rate observed in Nexxim can be too high in such cases because the initial delay is not recognized as such, and the device appears to transition directly from steady-state low to steady-state high (and vice versa) when it is switched at the shorter period. Using RM_DLY_RWF and RM_DLY_FWF to manually trim the initial delay can be cumbersome when the model's waveforms have widely differing initial delays according to corner case (typ, min, max, fast, slow). The overall timing of the simulation is also shifted by trimming the waveforms.

Setting either RM_DLY_RWF or RM_DLY_FWF to a negative value (<0) invokes automatic delay removal for both rising and falling waveforms. The initial delay plateau is computed for each waveform in a given device's IBIS model. The common delay is the minimum of the values computed for all waveforms for the selected corner case (typ, min, max, fast, slow). The common delay time is removed on the beginning of each waveform. Then, an ideal delay element is added into the simulation flow to adjust for the time trimmed off the front of the waveforms.