Fixing GND in IBIS Walter Katz SiSoft IBIS-Packaging May

The Power Aware Schematic

What are Currents and Voltages when Tx is in High State? “Current” at Tx is same as current at Rx “Current” at Tx is function of (Tx-VDD_Tx) and Pullup IV “Current” at Rx is function of (Rx-VDD_Rx) and Power Clamp (Rx-Tx) is function of “Current” and values of R1, R2, R3. VDD_Rx, VSS_Rx, VDD_Tx, VSS_Tx can all be calculated relative to VSS_VRM and are a function of VDD_VRM, R4:11 and currents consumed by all of buffers in Rx chip and all of the buffers in Tx chip.

What are Currents and Voltages when Tx is in Low State? “Current” at Tx is same as current at Rx. “Current” at Tx is function of (Tx-VSS_Tx) and Pulldown IV. “Current” at Rx is function of (Rx-Vss_Rx) and Ground Clamp. (Rx-Tx) is function of “Current” and values of R1, R2, R3. VDD_Rx, VSS_Rx, VDD_Tx, VSS_Tx can all be calculated relative to VSS_VRM and are a function of VDD_VRM, R4:11 and currents consumed by all of buffers in Rx chip and all of the buffers in Tx chip.

Global Ground and IBIS Reference Voltages? There is no Global Ground node in this schematic. All references in IBIS that refer to GND or Global Ground need to be changed to say that these GND nodes refer to the buffer local ground rail. What is the meaning of a non-zero value of the Pulldown or Ground Clamp Reference in an IBIS file? This is the voltage applied to the buffer local ground (relative to the test fixture local ground) that was used to measure the IV curves The current generated by the IV curve in simulation is the voltage between A_signal and the voltage at the pulldown (or GND clamp).

Does are New Packaging Scheme Support this View? Mostly, as long as the interconnect models do not contain Global Ground. The Terminator Model is not fully supported in the new package scheme If the interconnect models contain Global Ground, the accuracy of simulations will be limited by the accuracy of the voltages at VSS_Tx and VSS_Rx since not all currents to nodes between VSS_Tx and VSS_Rx will be accounted for.

The Terminator Model

Alternative Terminator Model Replace C_comp with C_comp_power_clamp and C_comp_gnd_clamp Replace symbol with “Local_GND” Replace R-pkg, L_pkg, and C_pkg with new package model IBIS-ISS subckt with terminals: A_signal A_pcref A_gcref A_locref (New local reference node) Pin_A_signal Pin_signal_name (e.g. VDD connected to A_pcref) Pin_signal_name (e.g. VEE connected to A_gcref) Pin_signal_name (e.g. VSS connected to A_locref)

Change to [Buffer Rail Mapping] Add new column local_gnd [Buffer Rail Mapping] columns: Pin_name pulldown_ref pullup_ref gnd_clamp_ref power_clamp_ref ext_ref local_gnd

IBIS “GND” Changes in [Model]s The ground symbol and name “GND” within buffer models shall refer to a local ground. The signal name of this local ground must be a signal name that exists in the [Component] [Pin] section, and associated with the gnd_clamp_ref, pulldown_ref or local_gnd_ref in the [Buffer Rail Mapping] record for the Pin associated with the buffer instance. If a buffer has unique signal names gnd_clamp_ref, pulldown_ref or local_gnd_ref then pulldown_ref.

IBIS “GND” Changes in Legacy Package Models Power aware simulations with legacy package models will not correctly account for all currents to ground. IBIS 7.0 should simply state this and recommend that the new IBIS-ISS package modeling be used to correctly account for these currents.