cheap way of interfacing 5V PCI to Spartan 6

[ali_asadzadeh]

Hi
I have used  330ohm series resistors in the PCI signals and connect it directly to Spartan 6, Does it a viable option? I do not wanna use level shifter chips to save cost.
Do you have any other idea?

[legacy]

330ohm series resistors in the PCI signals and connect it directly to Spartan 6

I say - No!

[asgard20032]

A resistance only limit current, you need a voltage divider if you want to limit the voltage.

Also, high speed digital (everything above 20 MHz) don't like any such thing, such as voltage divider and other low cost / easy thinking way of voltage conversion. Those usually limit the transition speed of the signal, because of capacitance and inductance effect. Almost everything is a Mosfet... if you put a resistance, you also limit the gate charge and discharge time. There must be a better way to connect 5V PCI to Spartan 6... Doesn't PCI use LVDS interface ? There exist transceiver for that, and in fact, I never heard anything connected to PCI or PCIe without a transceiver. Some FPGA even got the transceiver, so no need for an external one.

EDIT: PCI doesn't use LVDS. It use 3.3 V signaling. Just use a voltage converter.

EDIT EIDT: PCI can either be 3.3V or 5V. Check your system to be sure. Also, you will need some bidirectional converter for some of the line.

[legacy]

Some FPGA even got the transceiver

which ones?

[ali_asadzadeh]

I'm designing a universal card, So I might assume some motherboards have 5V signaling, the series resistors with the internal clamp diodes server as 5V down conversion to 3.3V and I think the outputs with 3.3V high state would be enough for driving the inputs of the motherboard bridge. and 33MHz is not a fast signal, any one has done it this way before?

[poorchava]

You need to remember thatmaximum voltage level is not all. I bet 3.3V and 5V variants of the bus have different thresholds for logic high and low. For CMOS to guarantee reading input as high it typically needs to be >0.7*VDD or  >3.5V which is obviously not possible to achieve with a 3.3V-powered FPGA.

You will either have to use an FPGA that supports PCI natively (and perhaps allows you to switch voltage powering the IO buffers at runtime) or use some dual-supply buffers which work at both 5V and 3.3V.

[nctnico]

Hi
I have used  330ohm series resistors in the PCI signals and connect it directly to Spartan 6, Does it a viable option? I do not wanna use level shifter chips to save cost.
Do you have any other idea?

IIRC Xilinx had some aplication notes on this issue and their solution might be using series resistors to limit the current going into the clamping diodes in the FPGA.

[ali_asadzadeh]

You need to remember thatmaximum voltage level is not all. I bet 3.3V and 5V variants of the bus have different thresholds for logic high and low. For CMOS to guarantee reading input as high it typically needs to be >0.7*VDD or  >3.5V which is obviously not possible to achieve with a 3.3V-powered FPGA.

You will either have to use an FPGA that supports PCI natively (and perhaps allows you to switch voltage powering the IO buffers at runtime) or use some dual-supply buffers which work at both 5V and 3.3V.

Then powering the FPGA with 3.6V would solve this issue

IIRC Xilinx had some aplication notes on this issue and their solution might be using series resistors to limit the current going into the clamping diodes in the FPGA.

Do you happen to know the App-note number?

[nctnico]

I think the Spartan3 application notes for their PCI core may have some info on how to deal with this. I don't know the numbers so better use Google or Xilinx' search engine on their website. It will take some digging/reading though to get the answer.

[Scrts]

I'm designing a universal card, So I might assume some motherboards have 5V signaling, the series resistors with the internal clamp diodes server as 5V down conversion to 3.3V and I think the outputs with 3.3V high state would be enough for driving the inputs of the motherboard bridge. and 33MHz is not a fast signal, any one has done it this way before?

PCI is not fast, but the timing is really critical there. You have to be sure to meet setup and hold requirements.