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What is the pci bus voltage supported by modern PCs?

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Member level 2
Dec 14, 2002
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build your own pci board

Hello, I'm planning to build a simple pci (32bit / 33Mhz) card using a Spartan-II, a pci core and some custom vhdl code..
I'm trying to make all points clear but I think I don't have clear a thing, even if I read specs, forums etc :

If I want the pci card to work correctly on current PCs what is the voltage that my card must support on the pci bus ?
5 Volts or 3.3 V?

In other words: I don't understand what is the pci bus voltage currently usually supported by modern PCs !

Please help.
Thank you,


almost all PC's with PCI 33/32 support only 5V signalling standard on the PCI bus.

For PCI chip power circuit it's better to use +5V voltage, because some old computers does not have +3,3V supply.

An engineer from PLX told me that in most cases it is necessary to use the 5V from the PCI bus to power the 3.3V circuit with a regulator (5V to 3.3V) because it is possible that the 3.3V from the bus are below the desired voltage. This could result in an error of your 3.3V logic.


> In other words: I don't understand what is the pci bus voltage currently usually supported by modern PCs !

I think nobody knows. You receive both powers. In case of duty, it`s very simple: take an oscilloscope and look at the pci pins !!

You will have 5V for at least some years. But you can`t always design thinking in the almost the past.

Another solution is making a universal card (take a look at the pci specification).

In any case, be sure that although working at 3v, your FPGA have 5v inputs tolerants.

Please read spec, it is pretty clear. If you want to design single voltage card or universal card. Best way is universal card (see page 148-150 of PCI 2.2 spec), use dual voltage bus transceiver such as available from TI or Fairchild & connect the 'bus side' buffer supply to Vio pins. The Vio pins are either 3.3v OR 5v depending on PCI slot.

Remember to take into account delay through buffers & net length constraints & matching in layout and not be tempted to just buffer some pins (see page 151 onwards) Also you must also indicate to the PCI controller on the MoBo what you will need power wise, max 25W, power up must be < 5W really using PRSNT #1, #2 pins (page 146).

The PCI spec is like any other spec, you cannot speed read it. Best to 'look up' your needs while doing SCH and do a text search within spec on each pin as you connect it. It is not well written, but all infos are there


Thank you very much to all of you !

I think I'll build a 5V only board at first.
And then I will try to switch to a universal version.

Does anybody have a [ sp*rtan-II -->PCI ] PCB or SCH ?? :oops:

By the way, what do you think of this application note from X*linx
on how to connect 3.3V I/O pins of a V*rtex-II (but it could
be applied to any 3.3V I/O fpga) to a 3.3/5 V PCI bus ???
**broken link removed**
They suggest the usage of a Q*ickSwitch device.

Also I'm wondering if, being the Sp*rtan-II 5V tolerant, wouldn't be sufficient
to connect the spartan pins directly to the pci bus without caring of voltage ?


In the mean time I've found something
very interesting and informative:
The schematic and bom of i\n\s\i\g\h\t sp@rtan-II
devel board:

I've found answers to some of my questions there...
But still I'd like to know what you think about
the two questions in my previous post, thanks.



the mentioned board supports both voltages (spartan II has built in buffers that are compatible to the 3.3V as well as the 5V standard).
If you plan to build your own PCI board with some fpga, spartan II seems to be a good choice although I don't know anything about "the next generation", called spartan III. Maybe someone else can help you with some more information about "is spartan III compatible to both standards?"...
Please note that the virtex family is only 3.3V compatible!!!



- Virtex and Spartan II series are fully PCI 5V compatible, Virtex E, Spartan IIE, and later - are not, so all new ships with core voltage 1,8V and less (including Spartan III – probably clone for Virtex II) will only be PCI 3 V compatible;

- there is no Xilinx chip on the market, that supports both voltages simultaneously, without reprogramming (except old XC4000T series), or "glue" logic.

When you layout the board for PCI, be advise that PCI work as reflective wave. That means you won't get full voltage swing until the signal is reflected back. Otherwise you have only half the signal. It is all in the PCI spec.

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