Differential signals as single ended connection

Hello,

can I use a single differential I/O signal as two single ended I/Os?

I am using Spartan 6 Bank 2 (LVDS25).

What about the signal integrity? Performance?

What should I keep in mind to apply such configurations? Any hardware changes required?

Regards,
AW

AWahab said:

Hello,
can I use a single differential I/O signal as two single ended I/Os?

I am using Spartan 6 Bank 2 (LVDS25).

What about the signal integrity? Performance?

What should I keep in mind to apply such configurations? Any hardware changes required?

Regards,
AW

Click to expand...

If it's configured as a differential I/O you can't use the two signals as single ended I/O. Ignoring issues like voltage levels and such...the pins are looked at as a difference between them.

As it is unclear what you are trying to do (always explain things in excruciating detail...forum members are not psychic). Maybe what you are suggesting is instead to change what was a differential pair of lines into two single ended lines. If so that will probably fail to function very well. Differential routing is design to tightly couple the traces together. If you then change the design to send a single ended signal down one of the traces you will run into signal integrity problems with cross coupling of the aggressor onto the victim trace. I wouldn't recommend heading down that path.

Regards

AWahab said:

can I use a single differential I/O signal as two single ended I/Os?

Click to expand...

As two single ended outputs 'yes', as an input or I/O 'no'

What about the signal integrity?

Click to expand...

What about it? I'm all for it.

Performance?

Click to expand...

Read the datasheet

What should I keep in mind to apply such configurations?

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Use flip flop outputs only and make sure that those flops are in the device's I/O block and not just some flip flop buried inside the device. That will minimize time skew between the two outputs.

Any hardware changes required?

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You should ask that of somebody that knows what hardware you have.

Kevin Jennings

I have to make a daughter board with the FMC LPC connector. According to the FMC standard and design of the hardware of motherboard, it gives 34 differential I/O pairs.

According to the requirements, I need more than 34 I/Os (60) in the daughter board. I can't change the hardware of motherboard. That is the reason why I wanted to use differential signal as a two single ended connection. Any solutions?

Regards,
AW

Oh, so this is merely a connector issue. Just use the FMC connections pairs as non-differential signals. You'll probably see some small amount of discontinuity in the impedance of the signals as they pass through the connector, but unless you're running the single ended signals at very high speed you will probably not notice any signal integrity issues.

Just make sure the board and the daughter card weren't designed with differential pairs connecting to the FMC. If they are then you'll run into a lot of signal integrity issues due to the previously mentioned cross coupling.

Your motherboard has 34 differential pairs, and they are configured as 34 diff pairs, and you cannot change that. Correct? Excellent.

Then no, you cannot take those 34 diff pairs and magically make 64 single ended pairs out of it for use on your daughterboard. (*)

However you can of course convert those 34 diff pairs to 34 single ended signals on your daughterboard if you want for some reason.

And if those 34 diff pairs can be configured as single ended on the motherboard (requiring config on the motherboard), then you can use them as 64 single ended. Because then they ARE 64 single ended on the motherboard. Obviously. And that what ads-ee said regarding pcb layout of those diff signals. ;-)

(*) Well actually you can convert 34 diff pairs to 64 single ended signals. As long as you don't mind the fact that one half of those single ended signals is going to be the inverse of the other half. Always. No, don't take this last comment seriously. 34 is what you get.

I have schematics of the motherboard available. The designer has used Bank 2 of Spartan 6 with 2.5V LVDS connection and designed it as differential signals as well.

For the reference of motherboard: https://www.ohwr.org/projects/spexi/files

Regards,
AW

You expect me to download a bunch of big files and who knows what format they are in...

can't you just do a screen capture of the motherboard layout near the connector to the Spartan 6 so we can see if the traces were routed as differential pairs? (i.e. the traces will be routed parallel to each other with a gap that is the same as the trace width, i.e center to center trace separation of 2x the trace width.

If the signal pairs were routed to the FMC from the Spartan 6 as differential traces expect signal integrity issues with those signals if used as single ended connections.

I strongly recommend going back to the drawing board and develop a solution which takes advantage of the LVDS and transfers data at 2x the bandwidth of the 60 single ended signals by time multiplexing a 30 differential pair bus (you can add a signal to determine which phase is for which part of the 60-bit data). Depending on the bandwidth required you might even be able to get away with a few serial lines that transfer at 600-700 MHz or more (not sure what is the upper limit for Spartan 6, Kintex 7 is >1Gbps over LVDS).

Regards

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As a side benefit with an LVDS solution you'll see less cross talk and noise from the motherboard/daughter card combination as the tightly coupled differential lines will radiate significantly less.

ads-ee said:

You expect me to download a bunch of big files and who knows what format they are in...

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Didn't you get the memo? Posting minimal information, and expecting other people to do the work is the new forum standard.

Thinking three entire seconds about presenting the information in a usable format for the reader? Pfffffrt, that is crazy talk!

But yes, close up photo of the actual board traces should do the trick. Although you can probably already predict what it's going to be. It's going to be all squiggly pair wise diff traces. In which case your advice to use it as such (differential that is) would be a good idea.

I also don't see the point of abusing diff lines as single ended. Only reason I would do that is for a quick and dirty prototype or something, where I could get away with sloooow signals.

But who knows why the OP is asking. He didn't mention. :???:

I am sorry about the above link.

Here are the main features of the board:

https://www.ohwr.org/projects/spexi/wiki

If you the FMC connector. Its clearly mentioned that it has 34 differential pairs as I mentioned above.

Till now I have the feeling that I can't use the differential pair as two single ended connection. Ryt?

Regards,

AW

AWahab said:

Till now I have the feeling that I can't use the differential pair as two single ended connection.

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Let's just go with the short version. You are right, you cannot use the differential pair as two single ended connection.

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Incidentally, are you working on a TDC daughterboard?

@mrflibble, I don't have any idea about TDC :/

Regards,

AW

Ah okay. It was just a guess based on that motherboard. Oh what the hell, while we are at it... what kind of function is your daughterboard going to be doing?

As far as I see provides the LPC (low pin count) FMC standard 34 differential pairs or 68 single ended lines. This means that the connector has sufficient ground pins for single ended signals. So one prerequisite for using the IOs single ended should be fulfilled The other is board layout. The differential pairs are probably routed as differential pairs, this still doesn't tell exactly about their coupling.

I expect that slow or medium speed signals which can use single ended I/O standards without too much problems should work over this connector.

I can always count on someone giving the long version of the answer. Damn engineering personality types. ;-)

@FvM

Thanks.

Can you please put some light on the difference b/w differential pair routing and coupling as you mentioned in your comment?

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@mrfliblle

the connections to be used for RS422 communication.

AWahab said:

Can you please put some light on the difference b/w differential pair routing and coupling as you mentioned in your comment?

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He's likely referring to the fact that there will be significantly less cross coupling of the pair if they are routed over or between ground planes. In this case the primary coupling of the signals is with the ground plane and not between the pairs.

You should read the like I posted in #8 under See here.

Regards