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IBIS Voltage Time VT Data Overview

IBIS Voltage Time VT Data Overview

By Timothy Coyle

Overview of the Voltage-Time (VT) Data

So far we have focused on the IV data in the IBIS model but remember that we still need to have the voltage-time data in order to know when to turn the pulldown and pullup IV curves on and off.

VT Data Generation

The above image shows the setup to generate the various VT waveforms. The IBIS Specification does not put a requirement on the maximum number of VT tables needed in an IBIS model but at a minimum there should be four VT waveforms. With four VT waveforms for a standard CMOS buffer each piece of the buffer can be properly modeled: the pullup turning on and off and the pulldown turning on and off.

Pullup On

The Pullup On VT waveform represents the buffer with the pullup on in the form of a rising waveform. An input rising waveform with an appropriate device input edge rate is applied to the input and a reference load is applied to the output with a V_fixture voltage of 0 Volts and user specified R_fixture value.

Pullup Off

The Pullup Off VT waveform represents the buffer with the pullup off in the form of a falling waveform. An input falling waveform with an appropriate device input edge rate is applied to the input and a reference load is applied to the output with a V_fixture voltage of 0 Volts and user specified R_fixture value.

Pulldown Off

The Pulldown Off VT waveform represents the buffer with the pulldown off in the form of a rising waveform. An input rising waveform with an appropriate device input edge rate is applied to the input and a reference load is applied to the output with a V_fixture voltage of Vcc and user specified R_fixture value.

Pulldown On

The Pulldown On VT waveform represents the buffer with the pulldown on in the form of a falling waveform. An input falling waveform with an appropriate device input edge rate is applied to the input and a reference load is applied to the output with a V_fixture voltage of Vcc and user specified R_fixture value.

Selecting R_fixture

The R_fixture value should always be representative of the actual system load so a value of 50 Ohms is often used since most PCB designs use 50 Ohm trace impedance. Depending on the IO buffer technology and required loading a different value may be used.

VT Transition Time

The time length of the voltage-time table should be long enough in order for the signal to reach steady state. If the signal does not reach steady state than the VT tables will not have sufficient data to be used in conjunction with the IV table data. A common mistake is to use a large value for R_fixture in order to see the signal reach steady state when in fact the signal time should be increased to get the proper steady state time.




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