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1,066 Views
Registered: ‎11-27-2017

KCU105 - Serie resistor on EMCCLK

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Hi,

 

my question is why the prototype board includes a 24.9 Ohms resistor in the EMMCLK net (clock of 90 MHz for SPI flash programming). The clock datasheet Si5335 specifies an output impedance of 50 Ohms for single-end LVCMOS 1.8V, and in the board gerbers, the microstrip width of this net is 0.095mm, corresponding with an impedance of 50 ohms. Why this resistor is included?

 

Anyway, where can I find the input impedance? I have found in the document ds892, in table 3, some values of input termination resistance, but I am not totally sure how they are applied, and if they are also valid for LVCMOS 1.8V input pin.

 

Thanks in advance!

Antonio

 

pd: I has also seen the following link, but it doesn't help me a lot because it says only general considerations:

 

https://forums.xilinx.com/xlnx/board/crawl_message?board.id=XLNXBRD&message.id=14765

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Scholar drjohnsmith
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1,271 Views
Registered: ‎07-09-2009

Re: KCU105 - Serie resistor on EMCCLK

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Sorry I was not trying to brag there

 

 

You are right, in this case, board works, so it seems to be doing nothing,

 

       

I can tell you my thoughts on why its there, 

   I can share with you thoughts on layout, 

 

To be honest, in this case, I'd be tempted to have used AC termination at the far end, ( and rc series network to ground ) , and probably no fit it,

 

its just its very hard with good layout engineers not to get 90 M working, even with as you have seen sub optimal termination, 

    what I would say, is make certain you do fit termination, 

 

 

its down to you how you do it.

 

 

 

8 Replies
Scholar drjohnsmith
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1,061 Views
Registered: ‎07-09-2009

Re: KCU105 - Serie resistor on EMCCLK

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why do you need the info ?

 

Id guess, and it is only that,  the resistor was put in because it is normally needed.

 

Things like this tend to get put on schematics by us conservative engineers on tight time products, as we know it can be needed, 

    

as to if its worth taking of / tweaking its value or what , depends on how the board testing goes.

 

get into habit of a series resistor of 20 odd ohms on these sort of lines, saves the day more than you would believe.

 

1,038 Views
Registered: ‎11-27-2017

Re: KCU105 - Serie resistor on EMCCLK

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Hi drjohnsmith,

 

I don't think so:

 

- 90MHz is considerable frequency for choosing a random value for a serie discharge resistor. In fact, this is a very important signal (clock for programming the SPI flash), and low frequency considerations are not valid at this clock frequency.

 

- Resistor is placed close to the clock generator, when serie discharge resistors are normally placed close to the receiver component (the goal is avoid a voltage peak due to the track capacity).

 

- In the link posted, the xilinx employer explains that the reason is because of the signal integrity.

 

Thanks!

Antonio

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Scholar drjohnsmith
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1,025 Views
Registered: ‎07-09-2009

Re: KCU105 - Serie resistor on EMCCLK

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yep the concept is to add series dc termination, for signal termination,

 

But you asked why its not 'perfect',

 

I asked why do you need t know,  cause then we can tailor our answer to you

 

I also gave you the most likely reason, 

   some one like me designed conservatively, and put a series resistor in, 

      in a place that I knew I could tune if I needed to, or allowed us to access the signal easily.

 

As for 90 M ,

    sorry that is almost DC now days , I'm routing 25 G normally, and just doing 100 G stuff,

 

Back in the 80s I'd have worried about 90 M, but now, most layout engineers standard is good enough for that.

 

 

 

 

 

 

Scholar drjohnsmith
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1,012 Views
Registered: ‎07-09-2009

Re: KCU105 - Serie resistor on EMCCLK

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If you want to go a bit further into termination theory, then glad to discuss:

 

http://www.analog.com/media/en/training-seminars/tutorials/MT-097.pdf

 

The series resistor option has multiple affects,

 

It limits the current flow, and forms a RC network with the trace capacitance, 

    which lowers the edge speed but adds a delta delay,

 

A slower edge, means a worse ( normally longer ) trace miss mach can be driven,

   The edge speed affects the reflection off the far end impedance miss match.

       

The resistor also is used to absorb the reflected "spike" that came due to the edge sent. 

     if its not absorbed, then we end up with another edge reflecting off the transmitter back to the receiver,

         causing a lovely ring in voltage.

 

If you really want to tune that one line to get a perfect edge, then you need something like HyperLynx, 

this site is also of interest

http://www.signalintegrity.com/

 

 

But at 90 M,  with a short line like this, in a dev board with great ground / power planes , I bet it just worked.

      

 

BTW: I've not heard the term serial discharge resistors, you have any references to that, I'd love to read, 

    

   

 

 

1,011 Views
Registered: ‎11-27-2017

Re: KCU105 - Serie resistor on EMCCLK

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Hi drjohnsmith,

 

I am designing a board based on XCKU040 FPGA, and I am reviewing the KCU105 board for getting some ideas. I understand that serie resistor are used when the output impedance is low, and it is necessary to increase it until match with the transmission line impedance. And in my opinion, this resistor does not make this function, and in fact, it is disturbing, because both input and output impedance of clock and FPGA, respectivally, are 50 ohms, as the designed track. This is why I am asking for an explanation: why this value? Does a 100G stuff designer think that may be more useful and logic to introduce a 0 ohms resistor instead of?.

 

Thanks,

Antonio.

 

pd: I understand the conservative considerations.

 

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Scholar drjohnsmith
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1,272 Views
Registered: ‎07-09-2009

Re: KCU105 - Serie resistor on EMCCLK

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Sorry I was not trying to brag there

 

 

You are right, in this case, board works, so it seems to be doing nothing,

 

       

I can tell you my thoughts on why its there, 

   I can share with you thoughts on layout, 

 

To be honest, in this case, I'd be tempted to have used AC termination at the far end, ( and rc series network to ground ) , and probably no fit it,

 

its just its very hard with good layout engineers not to get 90 M working, even with as you have seen sub optimal termination, 

    what I would say, is make certain you do fit termination, 

 

 

its down to you how you do it.

 

 

 

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924 Views
Registered: ‎11-27-2017

Re: KCU105 - Serie resistor on EMCCLK

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Hi drjohnsmith,

 

I really appreciate all your answers, they are useful for me.

 

My question is also related with, searching an answer to explain this resistor, I have not found in ultrascale documentation a specific I/O impedance for LVCMOS18 type, for in this case, the EMCCLK clock. The related data found is:

 

- DS892, pag. 5,6: impedance of HP / HR ports, for calibrated and uncalibrated termination --> selection of RTT_xx where xx is the termination impedance. However, on UG571, pag. 66: "The ODT attribute supports split or single termination on the inputs of the HSTL, SSTL, POD, and HSUL standards" --> As I understand, then LVCMOS can not configure its input impedance

 

https://www.xilinx.com/support/documentation/data_sheets/ds892-kintex-ultrascale-data-sheet.pdf

https://www.xilinx.com/support/documentation/user_guides/ug571-ultrascale-selectio.pdf

 

- Also in UG571, pag. 65: In some forums, I have read that the Output Drive Strength for outputs (not inputs) is related to output impedance, then it can be used for impedance matching. However, in this case, I have not found a specific relation between drive strength and output impedance, as well as it is applicable for LVCMOS18 standard, this is not applicable for inputs. And after that, a specific parameter OUTPUT_IMPEDANCE can be configured, also only for HSTL, SSTL, HSUL, LVDCI, HSLVDCI, and POD drivers

 

 

At last, I have found this thread, which says something in the same line we are discussing in this thread: if you need to match impedance, you should use DCI drivers and its corresponding signals. In other case, the input impedance is unknown, so you should design without the receiver input impedance, as best as you can.

 

https://forums.xilinx.com/t5/7-Series-FPGAs/Output-impedance-of-HP-bank-IO-signals/td-p/550919

 

Thanks!

Antonio.

 

pd: With discharge resistor I mean, the resistor placed as close as possible to the transistor gate, in order to limit the current through the gate and protecting the IC during switching. 

 

 

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Scholar drjohnsmith
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911 Views
Registered: ‎07-09-2009

Re: KCU105 - Serie resistor on EMCCLK

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Regarding 'IO inpedances' 

  

now days people would say you need to use the IBIS model of the IO to see what impedance you need,

     as its far from linear or constant,

 

but thats another level of detail , which in this case you just don't need