The spec of interest for signal integrity purposes will be the V/I curves
for 5V signaling if your 3V device is operating in a 5V signaling environment.
The spec for 3V devices is for those devices in a 3V signaling environment.
A 3.3 volt component in a PCI bus designed for a 5V signaling environment must
meet the 5V requirements. The 2.4 V, 0 A, operating point is only the minimum
allowed by the spec. Only a bipolar device operating at 3.3 volts would ever
approach this minimum. I have never seen a bipolar PCI device. However,
a 3.3 volt cmos device operating on a 5V bus, will, of course have an Iol=0 at
3.3 volts. This causes problems with mixed 5V/3.3V devices on a bus.
The 5 Volt PCI spec does not require clamps on the high rail. For a 5V only
system, this does not usually pose a problem, because, even with high ringback,
there is ample margin around the nominal 1.5V switching threshold.
However, when a 3.3 V device drives in a 5V signaling environment, ringback will
often occur within the input threshold region. Generally, this is where we
signal integrity engineers make our money on the PCI bus. The original
implementers of the spec left out a few details in how to make these mixed
systems work over worst case. Often they don't work in the typical case.
McCormick, Robert wrote:
> The worst case high state vi curve in the PCI spec shows Voh=2.4v for
> I=0 to 2mA (Figure 4-3, VI curve for 5V signaling). Section 4.1.2 of
> the PCI spec says that "PCI is a CMOS bus", which would mean that Voh
> should be close to the rail (note that Ioh &IiL ~ 70uA per the PCI
> spec.) Granted, 3.3v devices are allowed to drive in a 5V environment,
> and specs for 3.3v parts do say that Voh_min=2.4v but isn't this a
> holdover from the TTL / Bipolar days?
> The difficulty arises in meeting the PCI spec of Tpd_max = 10nsec (due
> to ringback).
> Does anyone know of PCI devices that actually put out 2.4v at I=0 amps
> (high temperature, low supply voltage) or is the spec just too worst
> case?Note that I also question Vol=0.55v at I=0amps for the low state vi
> curve as well.
> Bob McCormick
> Stratus Computer
> Marlboro, MA
> TEL: 508-460-2485