5V System Environment / 3.3V System Environment in PCI slots

5v add in boards for pci slot

In the PCI Local Bus Technical Summary **broken link removed**
it is said, that :

"The PCI specification defines two types of connectors that may be implemented at the system board level: One for systems that implement 5 Volt signaling levels, and one for systems that implement 3.3 Volt signaling levels.
Three types of add-in boards may be implemented: "5 Volt add-in boards" include a key notch in pin positions 50 and 51 to allow them to be plugged only into 5 Volt system connectors. "3.3 Volt add-in boards" include a key notch in pin positions 12 and 13 to allow them to be plugged only into 3.3 Volt system connectors. "Universal add-in boards" include both key notches to allow them to be plugged into either 5 Volt or 3.3 Volt system connectors. Universal boards must be able to adapt to operation at either signaling level. "

I checked a PCI slot ( on a common P4 motherboard ) intended for "5 Volt add-in boards".
It really had +5V on appropriate pins as should have been in a 5V System Environment.

BUT ALL I/O PINS IN THAT SLOT HAD 3.3Volts LEVELS!!!

Questions:
1). Did anybody see PCI slot intended for "3.3 Volt add-in boards" on common P4 motherboards?
Why are they ever needed if even PCI slot intended for "5 Volt add-in boards" have all I/O pins at 3.3 Volts?
2). If I design a PCI controller with a non-5Volts tolerant FPGA, than shall I put level convertors from 5V->3.3V?

Re: 5V System Environment / 3.3V System Environment in PCI s

3.3V PCI is rare, because the market is well-established with 5V cards.

You can interconnect 3.3V FPGA using MOSFET switches. There are chips designed to act as level cutter. They cut the 5V of PCI to 3.3V. On the other side, the 3.3V sent by the FPGA will pass through and end-up as 3.3V on PCI. This is still usually enough to drive most 5V PCI systems.

The design won't be PCI-complient (as per the specs) but will work in most systems, mainly because PCI specs tell that there shall be no intermediate chips between the PCI connector and the target chip, but the MOS gates are designed to have very low impact on the signals.

Plus, using the FET switches, you can design universal PCI cards.

Look at Xilinx app. notes XAPP646. This apply to Virtex-II FPGA, but is general for most FPGA.

**broken link removed**

Re: 5V System Environment / 3.3V System Environment in PCI s

But if even on PCI slot intended for "5 Volt add-in boards" I/O pins can never be 5 Volts, than these extra 5V->3.3V convertors are never needed !?

Re: 5V System Environment / 3.3V System Environment in PCI s

If the IO voltage of PCI interface is 5V, then, you can not assume that every systems on it will use 3.3V.

You may get a system which use 5V, and if you connect this directly to FPGA, you may end-up with a free 'smoke generator IP core'.

You may probe the interface to see that the motherboard send only 0V/3.3V, but even there, you may someday insert another PCI card on the same bus which really does send 0V/5V.