RE: 3.3V signaling PCI

["Ingraham, Andrew" <Andrew.Ingraham@compaq.com>]

>The chip vendor
>has specified that even though the datasheets of the chip shows that PCI
>signals are not 5V compatible, they are actually 5V tolerant.
 
Perhaps not everyone means the same thing by "5V tolerant" ... but to me, it
means (in part) that you can apply at least +5.25V to its signal pins and no
significant current flows, only leakage current.  Also, the chip should not
suffer any damage or long term reliability problems due to the application
of +5.25V continuous plus higher on transients.

A chip that is designed according to the 3.3V signaling PCI specs, must sink
a very large current (on the order of 100mA) from the +5.25V and could not
meet the first criterion.  Chips can be designed to meet both 3.3V signaling
and 5V signaling PCI specs, but not both simultaneously.  They are two
different "modes".

>2. When a motherboard uses a PCI chipset which is 3.3V and 5V compatible,
>and has a 5V PCI slot, what will be the signaling environment? Is it 3.3V
>signaling with 5V tolerable inputs OR 5V signaling only?
 
By definition, if it has a 5V PCI slot, the signaling environment is 5V.

"3.3V signaling" does not have 5V tolerable inputs.

Now bear in mind that "5V signaling" does not necessarily mean that the
signal voltages switch between 0.0V and +5.0V.  Note that the minimum Output
High Voltage (Voh) is only 2.4V, like TTL.  The driven signals could come
from a 3.3V-powered chip, and could switch to only 3.3V, even less.

>3. 66 MHz PCI operates only in 3.3V signaling environment. Am I right?
 
Correct.

Andy