Re: Approaches for PCI Overvoltage design using 3.3volt technology

[Andy Ingraham <ingraham@wrksys.ENET.dec.com>]

> The PCI specification requires an input to be able to handle a 11 volt   
> overvoltage waveform in a 5 v signaling environment.  I've noticed that a   
> number of manufacturers have developed PCI chips based on 3.3 volt   
> technology that claim to have 5 volt tolerant inputs, while at the same   
> time give absolute maximum ratings much less than 11 volts.

Read the Spec carefully.  It doesn't require I/O pins to withstand
11 volts directly on their pins.  There is a 55 ohm series resistor.

If the device has no clamps and nothing breaks down, then it will see
the full 11 volts, and it must withstand that to be PCI compliant.  If
anything conducts or breaks down before 11 volts is reached, then the
maximum voltage would be less than 11 volts.  Again, the device must
withstand this condition, both voltage and current, to be PCI compliant.

> I was wondering how designers out there are handling this problem in 3.3   
> volt technology designs.  Are you designing only for a 3.3 volt signaling   
> environment?  Are you designing special I/O buffers which are actually 11   
> volt tolerant?  Are you ignoring this part of the spec altogether?

Almost nobody has designed only for the 3.3 volt signaling
environment.

The pair of test waveforms, with their series resistors, were meant to
approximate the actual overshoot and undershoot conditions one might
experience on a lightly loaded PCI bus.

Many devices have clamp or ESD protection diodes which will limit the
internal pin voltage.  The open-circuit voltage without such clamping
might have been as high as 11 volts.

Overshoot clamps, though not required in PCI's 5 V signaling
environment, are highly desirable.  The PCI Spec even points out that
they are probably necessary in practice when using 3.3 V components on
the 5 V bus.  You do need to be careful about how you implement them,
though; you can't just clamp to the 3.3 V rail.

Regards,
Andy Ingraham
²¤”