5V PCI interface

Dr,

Spartan 2 will be around a long time. That we have demoted it from the limelight is a marketing issue (just so much shelf space for the new products to showcase).

As you may be aware, we still provide the 3100A series of FPGAs, which are still supporting designs done 15 years ago!

We discontinue devices once they are not able to be manufactured and sold economically. This means that there is little business, and the process used to make the chips has become obsolete at the fabrication facilities. We also may discontinue a particular part/package combination when that package is running at extremely low volumes or becomes difficult to procure.

Since we are still making almost all of our FPGA products, I don't think you have anything to worry about with Spartan II.

The original Virtex, and Spartan II are a lot like classic Coca-Cola -- they may never go away.

However, the cost/function of newer devices is so much better than the older devices, that you may want to consider designing with the latest devices (at some point).

The app notes we have published for 5V PCI details all of the tricks to make the latest 90nm devices work on the 5V PCI bus. (Xapp 646, 311)

I hope this helps,

Austin

Austin,

thanks a lot for your reply - that was most reassuring, and I got both questions answered!

That's kind of what I was hoping for :-)

I have overlooked said application note - shall download it at once and study it carefully. Anyway it's good to know that the Spartan-II will remain in production.

Regards,

DJ

Austin, Maybe you can give me more insight to a problem I have with xapp646. The note states that "Since the device is a set of series-connected NMOS transistors, any voltage larger than a few hundred millivolts below the VCC pin voltage will be cut off." From reading the IDT appnotes and what I'm seeing on a circuit board, the output will always be limited to less than VCC-1. With VCC at 3.3v as shown in xapp646, under light loading, the output voltage is about 2.3v, and with a 10k load, it's closer to 2v which means essentially no noise margin for TTL. Look at figure 4 of

or figure 5 of Do you think that I should be seeing around 2 to 2.3v output with the ckt shown in xapp646?

Dr, take a look at TI's sn74cb3t3384 or sn74cbtd3384c as well as some appnotes on their site.

gja

Will do. Thanks for the tip!

DJ

gja,

Basically, I am using the fact that the IDT device is just a simple NMOS transistor, and since I know how that works (physically) I am ignoring the data sheet (as it is misleading in this case).

I know that IDT does not support this from their data sheet specifications, and they actually called me to tell me that they would not support this.

Odd. It works fine. They are sand-bagging their specifications like crazy here, and their parts work far better than the data sheet implies (in this circuit).

Could be the loading (none), could be the voltages (less variation than what they spec), could be they don't want to support the application. Fine, call Xilinx. I'd much rather you call us than IDT. OK by me. We have built it, used it, tested it, and are still doing so. I know a lot of folks out there who have done likewise. Haven't heard a single complaint.

Officially, the PCI specification does not allow any devices to be placed in series with a PCI compatible part. That is fine as well.

Aust> Austin,

Hi Austin,

Hmmm... I thought that PCI-Compliant was "nothing in series", and PCI-Compatible was "whatever you need to do to make it work".

Apart from that, I second all your statements there. Plus, I would like to repeat here, TI seems to be more open to use for this type of application of their sn74cbtd3384c parts.

Best regards,

Ben

Austin, are you saying that you've actually seen the circuit actually pass

3v across the switch ? Or are you saying that the output SHOULD be clamped around 2.3v ?

Perhaps I should've been more clear, but I am currently testing a new ckt board built with the circuit in xapp646 and a virtex2 part. With the IDT vcc at 3.3v, the maximum voltage I am seeing with a scope is about 2.3v on the output side of the switch. IE., when the virtex2 part is driving (lvttl) one side of the switch at 3.3v, the other side of the switch is about 2.3v. When the 5v ttl part 74FCT645 is writing to the virtex2 part, I see 4v on that side of the switch and 2.3v on the virtex2 side. When I originally looked at xapp646, it stated that it would clamp to less than ~3v outputs, I didn't think it would be closer to 2.3v. If I had used the implementation in IDT's or TI's appnote where they drive the vcc pin at

4.3v using a diode from 5v, I wonder if that would have worked better.

gja

gja,

See below,

Aust> Austin, are you saying that you've actually seen the circuit actually pass

Yes, I have. Or are you saying that the output SHOULD be clamped

No, I am not.

Into what load? On a "real" PCI bus is where I made the measurements. Perhaps if you use the resistor "standard termionation" (which is anything but a standard) you will get some other result? When the 5v ttl part 74FCT645 is writing to the virtex2 part,

OK. I will admit that the TI part has some advantages, but it is also an active device, and adds delay, doesn't it? When I

Except that then you do not limit the voltages to less than what we require.

Hi Austin,

Yep. About 500ps worst-case. I tell my users to edit the constraint file so that because of this, all PCI signals should have 1ns subtracted from their timing constraints. This takes care of the combinatorial delays as well.

In Quartus, the difference is unnoticeable with a 33MHz bus and usually adds a little compilation time with a 66MHz bus. I'd hope the same situation would apply to Xilinx users.

Best regards,

Ben

Austin, thank you for your responses. My replies are below:

OK, I will have to investigate further now that I know that it should.

My application uses the switch to connect a Virtex2 to a slow (1us access times) 5v TTL databus, not PCI. The virtex2 pins are configured as lvcmos33 iobuf and is the only device on one side of the quickswitch, so I don't think it is a loading problem. When the 5v FCT chip is driving the virtex2 (thru the switch), I see 4v on the ttl side but only 2.3v on the virtex2 side.

No, the TI devices SN74CB3T3384 and SN74CBTD3384C are also FET switches with the same delay spec as the IDT part. The SN74CB3T3384 device uses a vcc of 3.3v while the SN74CBTD3384C uses 5v. Both devices claim to do 5v to

3.3 v level translation.

They do because the QS3861 clamps to VCC-1, so 4.3 - 1 = 3.3v

I investigated further and was able to get 3v across the switch by isolating the TTL side of the switch from the rest of the bus and driving the switch with a signal generator. And for some reason now, with everything as it was, I do see 3v on the virtex2 side of the switch, even though I know at one point I did not. So I will continue investigating based on the assumption that something on the TTL bus is loading the bus down when performing a read from the virtex2 device. Austin, thanks again for letting me know that what I was seeing was abnormal, and that I should relook at it.

gja

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gja,

No problem. Good luck with your troubleshooting.

Austin

Austin, I hate to bring this up again, but I've just been made aware that the circuit in XAPP646 would exceed the absolute worst case conditions for the IDTQS3861 part. With BE and GND pins biased around 0.7v (can be as high as

0.875v if 1.5v +5%), the IDT QS3861 datasheet states ABSOLUTE MAXIMUM RATINGS for the bus inputs as -0.5 to +7 volts. With GND at 0.875, this means 0.375v should be the lowest voltage at the bus pins. Usually TTL Vol is 0.4v but under light loading it's closer to 0.2v and this would violate the max ratings. I would like to hear your comments on this and your comments that this circuit has actually been used in production by others without problems.

Thanks, gja

gja,

I replied back to you personally.

Xilinx stands behin the app note. I am aware of, and have stated that, the quick switch folks do not support our application .... we do.

Fine with me, they told me that if anyone calls them about it, they refer the call to us (and take your order).

Aust> Austin,