I/O CIRCUITS AND MODELS - Advanced Signal Integrity for High-Speed Digital Design - page 481

11.10.2 Generating Model Data

Current Versus Voltage (i - v) Curves In constructing the i - v curves for a
transmitter circuit, the output is connected to an independent voltage source,
as shown in Figure 11-36. The current flow into the pad is then measured as the
source is swept across a range from

− V DD to 2 V DD . IBIS uses the convention
that current flow into the transmitter is positive. Separate curves are required
for pull-up and pull-down devices. For example, the input to an inverter-based
push-pull transmitter would be set to ground to obtain the curve for the pull-up
device.
The circuit setup for characterizing a receiver or ESD clamp is identical except
that the independent voltage source is connected to the input node. As mentioned
earlier, incorporating on-chip termination in an IBIS model is accomplished by
including it in the i - v curves for the clamp circuits. This can be accomplished
by including it with the clamp circuit during the data extraction process or by
extracting the curves separately and adding them together.
The data tables should use enough data points around sharply curved areas
of the i - v characteristics to describe the curvature accurately. IBIS does not
require equally spaced points in the models, so in linear regions there is no need
to include unnecessary data points. Also, as Figure 11-37 demonstrates, the i - v
curves for pull-ups and power clamps are V DD referenced, while pull-downs and

ground clamps are referenced to ground.

Voltage Versus Time Curves Transmitter output waveforms are described with
tables that specify the output voltage versus time ( v - t ). These v - t tables are
created when the transmitter is driving into a test load, which is usually a 50-
resistor that is connected to the appropriate supply, as described below. The actual
v - t data are generated by simulating rising and falling edges, with the output of
the transmitter to each rail through the test load, as shown in Figure 11-38. For a

i i i

Sweep from
−

Sweep from
−

Sweep from
−

Pull-up Pull-down Clamp

V DD to 2 V DD V DD to 2 V DD V DD to 2 V DD

i i

v

v

v

i

Figure 11-36 i - v curve extraction process.