I/O CIRCUITS AND MODELS - Advanced Signal Integrity for High-Speed Digital Design - page 451

In general, simulation tools create multiple i - v curves to account properly
for the transient nature of the input signal to the model. Stated another way,
a time-varying input signal, v in , to the circuit in Figure 11-1 causes the
device gate-to-source potential, v gs , to also be transient, so that the output
current-voltage relationship varies with time as well. This effect is shown in
Figure 11-10 and Table 11-2 for an inverting transmitter. The rising-edge input
signal starts at a value of 0.0 V, which corresponds to v gs = 0 . 0 V for the

NMOS device and to v gs =− 2 . 5 V for the PMOS device. As the input edge

rises, the v gs values for each device change. For example, when v in is equal to
1.0 V, v gs is 1.0 V for the NMOS device and 1 . 5 V for the PMOS device. We

see that for the case of a rising-edge input signal, as the PMOS device moves
from the

−

v gs =− 2 . 5 V curve to the v gs = 0 V curve, the NMOS device is

transitioning from the curve for v gs = 2 . 5V.

As a final note, we point out that the i - v data in the model should extend well
beyond the expected operating range for the device to ensure proper operation
in the event of significant signal overshoot. For example, if the signal swing
expected ranges from 0 to V DD , the IBIS specification expects that the i - v data
span a range from

0 V to that for v gs =

− V DD

to 2 V DD .

11.2.4 Advanced Design Considerations

In this section and in the counterpart sections for other transceiver types, we
touch briefly on several of the issues that face circuit designers when developing
transmitter circuits. For more extensive discussions of the issues and techniques,
we refer the reader to topics by Dabral and Maloney [1998] and Dally and Poulton
[1998].

Overlap current control When the transmitter circuit of Figure 11-1 makes a
rising or falling transition, the PMOS and NMOS devices will conduct current
simultaneously for a brief period of time. The magnitude of this overlap current
(also known as “crowbar” current) is sufficiently high that designers usually
design the transmitter such that the initially conducting device turns off before
the other device turns on. This is typically implemented with “pre-driver” control
logic.

Tristate function Systems in which multiple devices can drive a common sig-
nal, such as a multiprocessor bus, require that the transmitter be placed in a
high-impedance state when it is not actively driving the system. This is also
accomplished with pre-driver logic, as shown in Figure 11-11. The circuit uses
the enable signals, en/en , to control whether the transmitter is connected and
can actively drive the bus, or is disconnected and presents a high impedance, as
described by Table 11-3.

Process and environmental compensation When fabricating large numbers
of components, the physical features (e.g., gate length, oxide thickness) and