Hi all, I'm designing a system in which a 4-bit + clock LVDS point-to-point bus has to connect two FPGAs. The two FPGAs are on two different boards--one is on a mainboard and the other is on a plug-in board. What kind of board-to-board connector is recommended for high-speed (~400 Mbps) LVDS signals? Connector parameters to look for? Signal integrity issues? Board layout with regard to the connectors? Rules of thumb? Thanks, -- Georgi
LVDS through connectors
Started by ●January 19, 2005
Reply by ●January 19, 20052005-01-19
take a look at samtec they have good ones! But be careful: They are pretty hard to solder, because of the fine pitch. QSH series regards, thomas
Reply by ●January 19, 20052005-01-19
Mouser has a small selection of cables and connectors for LVDS mostly, I think, to support the Camera Link systems. The wires are matched pairs. That may be a relatively inexpensive way to go. 3M has some cables but watch out, some are non-stock items. Give me a call if your PCB software doesn't support dual and match length traces. Brad Smallridge 415-661-8068 www.aivision.com
Reply by ●January 19, 20052005-01-19
Georgi Beloev wrote:> Hi all, > > I'm designing a system in which a 4-bit + clock LVDS point-to-pointbus> has to connect two FPGAs. The two FPGAs are on two differentboards--one> is on a mainboard and the other is on a plug-in board. > > What kind of board-to-board connector is recommended for high-speed > (~400 Mbps) LVDS signals? Connector parameters to look for? Signal > integrity issues? Board layout with regard to the connectors? Rulesof> thumb?Howdy Georgi, I hate to say that it doesn't matter, but in the grand scheme of things, the type or style of the connector is not of huge importance at that speed, as long as one pin isn't massively longer than another (which occurs with some types of right angle connectors). We run many times that speed using the worst connector you can imagine. Much more important is the stuff that Brad mentioned: keep _p/_n pair trace length the same and routed as a diff pair into and out of the connector - and routed against a ground plane if possible. Give yourself a ground pin next to each pair within the connector. Have fun, Marc
Reply by ●January 19, 20052005-01-19
On Wed, 2005-01-19 at 17:29 -0800, Marc Randolph wrote:> I hate to say that it doesn't matter, but in the grand scheme of > things, the type or style of the connector is not of huge importance at > that speed, as long as one pin isn't massively longer than another > (which occurs with some types of right angle connectors). We run many > times that speed using the worst connector you can imagine. > > Much more important is the stuff that Brad mentioned: keep _p/_n pair > trace length the same and routed as a diff pair into and out of the > connector - and routed against a ground plane if possible. Give > yourself a ground pin next to each pair within the connector.Not true. Its not the frequency that matters but the edge rate. Given the 4 bits and clock it sounds an awful lot like the TigerSHARC link port and it has a very fast edge rate. The data sheet indicates a MAX of 200ps (that's pico-seconds). Anyways, at that edge rate any change in impedance in the trace will cause reflections. If the path of the differential pair through the connector has a different impedance than the rest of the trace you will get a reflection. In fact you'll get two--one at each side of the connector. That is the difference in high-speed connectors--they pay attention to this. Now it is true that the trace length through the connector needs to be matched as well. Otherwise you'll get the + and - signals getting out of phase. You'll always have slight mismatches in impedance and trace length. But with a 200ps edge rate you have a lot less margin for error than at more reasonable rates. Hope that helps.
Reply by ●January 20, 20052005-01-20
James Morrison wrote: (snip)> Not true. Its not the frequency that matters but the edge rate. Given > the 4 bits and clock it sounds an awful lot like the TigerSHARC link > port and it has a very fast edge rate. The data sheet indicates a MAX > of 200ps (that's pico-seconds).If you believe in Fourier you know that edge rate is frequency.> Anyways, at that edge rate any change in impedance in the trace will > cause reflections. If the path of the differential pair through the > connector has a different impedance than the rest of the trace you will > get a reflection. In fact you'll get two--one at each side of the > connector. That is the difference in high-speed connectors--they pay > attention to this.Well, the two reflections will partially cancel in many cases. That is why you don't notice them at lower frequencies. Even at 200ps the wavelength is still significantly longer than most connectors.> Now it is true that the trace length through the connector needs to be > matched as well. Otherwise you'll get the + and - signals getting out > of phase.Out of phase relative to what? There are coaxial cables where the center conductor is helical to increase its inductance. Signal velocity depends on the inductance per unit length of the shield and center conductor, but one doesn't arrive at the other end ahead of the other. -- glen
Reply by ●January 20, 20052005-01-20
Hi Georgi, For Higher speeds, I would recommend the AMP Mictor connectors. This document is a useful design guide: http://www.latticesemi.com/lit/docs/technotes/tn1033.pdf Regards Mark Smith Strategic Marketing Lattice Semiconductor
Reply by ●January 20, 20052005-01-20
James Morrison wrote:> On Wed, 2005-01-19 at 17:29 -0800, Marc Randolph wrote: > > I hate to say that it doesn't matter, but in the grand scheme of > > things, the type or style of the connector is not of hugeimportance at> > that speed, as long as one pin isn't massively longer than another > > (which occurs with some types of right angle connectors). We runmany> > times that speed using the worst connector you can imagine. > > > > Much more important is the stuff that Brad mentioned: keep _p/_npair> > trace length the same and routed as a diff pair into and out of the > > connector - and routed against a ground plane if possible. Give > > yourself a ground pin next to each pair within the connector. > > Not true. Its not the frequency that matters but the edge rate.Howdy James, I was figuring that someone would disagree with what I said - I just didn't figure it would be within a half hour! I was actually referring to the whole solution when I was referring to the speed: signalling type (LVDS, which has slower edges than PECL and CML), termination location (hopefully on-chip), voltage swing, etc.> Given > the 4 bits and clock it sounds an awful lot like the TigerSHARC link > port and it has a very fast edge rate. The data sheet indicates aMAX> of 200ps (that's pico-seconds).No arguing that 200ps is fast. But it is fast enough to cause problems on a properly terminated and routed differential pair? Our extensive experience on our telecom boards have shown it isn't even close to causing problems. In fact, I was referring to both CML *and* LVDS when I was mentioning our past experience. CML has edge rates closer to 100ps.> Anyways, at that edge rate any change in impedance in the trace will > cause reflections. If the path of the differential pair through the > connector has a different impedance than the rest of the trace youwill> get a reflection. In fact you'll get two--one at each side of the > connector. That is the difference in high-speed connectors--they pay > attention to this.And the the termination that differential signals have do an excellent job of minimizing those reflections. Go check out the people running LVDS buses with 20 drops!> Now it is true that the trace length through the connector needs tobe> matched as well. Otherwise you'll get the + and - signals gettingout> of phase. > > You'll always have slight mismatches in impedance and trace length.But> with a 200ps edge rate you have a lot less margin for error than atmore> reasonable rates.The beauty of terminated differential signalling is that you have such an improved signal infrastructure, I think you actually have more margin than you had with signal ended, unterminated signals that swing all the way from 3.3v, even if their edge rates were "only" 1.5ns. I think that's where people go wrong... while the concepts between unterminated single-ended and terminated differential are close to the same, the rules of thumb for one don't translate well to the other. I stand by my original post. Let the flames begin. Marc
Reply by ●January 20, 20052005-01-20
On Thu, 2005-01-20 at 05:00 -0800, Marc Randolph wrote:> I was figuring that someone would disagree with what I said - I just > didn't figure it would be within a half hour!Life is all about timing!> I was actually referring to the whole solution when I was referring to > the speed: signalling type (LVDS, which has slower edges than PECL and > CML), termination location (hopefully on-chip), voltage swing, etc.I don't disagree with what you've said and appreciate the insight from your past experience. I guess what I was reacting to is the concept that the connector doesn't matter. It does matter, but as I said, it all depends on how much margin you have in your design. In a particular design it may or may not matter based on a hundred other factors. As you mentioned, in a lot of ways differential signalling is a whole lot easier to deal with. Of course there is no such thing as a single-ended signal. Ground is typically the other end that happens to be common among all signals. James.
Reply by ●January 20, 20052005-01-20
"James Morrison" <spam1@emorrison.ca> wrote in message news:1106192534.5033.8.camel@oxygen.emorrison.ca...> > Not true. Its not the frequency that matters but the edge rate. Given > the 4 bits and clock it sounds an awful lot like the TigerSHARC link > port and it has a very fast edge rate. The data sheet indicates a MAX > of 200ps (that's pico-seconds). >Hi James, Not true either! You've brought out the pedant in me! It's not edge rate, but rise time. For example, what's the edge rate in a 400kV power line? Still only 50 or 60 Hz though! Also, In the context of this thread, I assume you mean bit-rate when you say frequency. Marc didn't mention frequency anywhere. (And as Glen meant, rise time and frequency are equivalent anyway.) If that's the case, here's a counter-example where bit rate does matter. The bus is required to get data from one end to the other. If the bit-rate was 1Mbps instead of 400Mbps, a 200ps edge rate would give you overshoot/undershoot problems through a shoddy connector plus lots of EMI, but you'd still have a big enough eye to recover the data. So, bit-rate is a part of the design exercise. That said, I understand what you're saying. I'm just being pedantic.... Cheers, Syms.