Arty: I/O, max switching speed of the signals?

[Korken]

Hi all,

We are looking to use the Arty board for first tests on a machine vision system we are developing, which runs a 72 MHz parallel interface.
For ease of testing I was planning to use the Arty board and its I/Os, but I see that it has 200 Ohm series resistors on each pin which will put a limit on the max allowed switching speed.

Are there any specifications on the pins when it comes to speed? With 200 Ohms about 5-8 pF is maximum allowed after the resistor for a 72 MHz signal.
Else, as I do not have the board yet, are the resistors easy to get to (silkscreen designators for correct identification) to replace the resistors manually with a lower value?

Thank you for your time!

[jpeyron]

Hi @Korken,

I talked with our design and layout engineers. Unfortunately, we can not guarantee 72 Mhz but we think that the high speed and standard pmod ports will work for you.  Using either JB or JC Ports you should be able to get the fastest speeds off of those pins. Those traces are routed as differential pairs but you can use then as single ended. Noise might be an issue with JB and JC due to being routed as differential pairs so you will need to look out for that. Here is a thread that talks about using the high speed pmod ports. Also see section 10.2 High-Speed Pmod here.  The I/O resistors are accessible/replaceable on the bottom of the board and are labeled. You can use the schematic as a reference here. We do also have the Zedboard, Nexys Video and Genesys 2 that have a high speed FMC here which might be a better solution to your design needs. 

cheers,

Jon

[Korken]

18 hours ago, jpeyron said:

Hi @Korken,

I talked with our design and layout engineers. Unfortunately, we can not guarantee 72 Mhz but we think that the high speed and standard pmod ports will work for you.  Using either JB or JC Ports you should be able to get the fastest speeds off of those pins. Those traces are routed as differential pairs but you can use then as single ended. Noise might be an issue with JB and JC due to being routed as differential pairs so you will need to look out for that. Here is a thread that talks about using the high speed pmod ports. Also see section 10.2 High-Speed Pmod here.  The I/O resistors are accessible/replaceable on the bottom of the board and are labeled. You can use the schematic as a reference here. We do also have the Zedboard, Nexys Video and Genesys 2 that have a high speed FMC here which might be a better solution to your design needs. 

cheers,

Jon

Thank you for taking the time to check and give a lot of information!

What would be the problem with the standard I/O headers on top?
72 MHz is not a high frequency, and taking standard rules of thumb for "when to think RF / impedance" (10x frequency & 1/10 of that wavelength in the medium) gives that any trace under a total length of 5 cm needs no high frequency considerations, this is why I was asking about the capacitance vs resistance.

But great that the resistors are easily accessible, then I can reduce them significantly for my purpose.
The Arty is in the perfect price/performance range for general prototyping, even for cameras.

Another question, is there a table of trace lengths FPGA -> I/O 1-40, or if it is possible to generate it? Just something rough.
Just to take the ones that have about the same length / to compensate on my adapter board.
It shouldn't really matter, but I like to play it safe.

Thanks for all the help!

[jpeyron]

Hi @Korken,

I have attached the trace lengths for the Arduino/chipkit header and here is a thread that has the Pmod trace lengths. Hope this helps!

cheers,

Jon

CK_Netlengths.txt

[Korken]

Hi again,

I have received the board and have tested using the standard 200 ohm series resistors and 33 ohm replacements on a 100 MHz signal.
What I did was simply to forward the input 100 MHz clock on the Arty to two output pins, one with the original series resistance on and one with my replacement.
This was measured with an 16:1 passive (resistive) probe which presents itself as an load of about 800 ohms (made from an 750 resistor + coax), with a 50 ohm input termination on the scope (the image show 1Meg, this is from another channel).

The results, as can be seen in the images, the 200 ohm pin still performs quite okey, though it was very sensitive to anything coming close or touching the board.
The 33 ohm signal was rock solid. So I will continue and replace all the I/O resistors to 33 ohm, something a little bigger could probably be used as well but I have a lot of 33 ohm resistors.

But is shows that the Arty has no problem, what so ever, with single ended signals up to 100 MHz so far.
Another 2 cm of signal path will be added when I add my board, but this should not have any large effect on signal integrity at these speeds.
Plus I have looked around the headers for GND points where I will solder extra GND paths, so there won't be a deficiency.

@jpeyron Thanks for the signal lengths and the assistance!

 

200 Ohm:

 

33 Ohm:

 

Probe:

[Fields]

Impressive!

I am also very interested in the maximun switching speed of the Arty board signals, and this post
is exactly what I was looking for.

@Korken I see you have succesfully forwarded a 100 MHz clock to the output pins. I wonder what would
be the limit of the Arty board.

Do you think it would be possible to forward a 500 MHz clock to the output pins?

[Korken]

@Fields With the measured rising/falling edges of about 1 ns I'd say 500 MHz is going to be a challenge.
But it could also be that my scope can't measure that fast (as it is only 500 MHz).

[darren]

On 3/8/2017 at 12:57 PM, jpeyron said:

Hi @Korken,

I talked with our design and layout engineers. Unfortunately, we can not guarantee 72 Mhz but we think that the high speed and standard pmod ports will work for you.  Using either JB or JC Ports you should be able to get the fastest speeds off of those pins. Those traces are routed as differential pairs but you can use then as single ended. Noise might be an issue with JB and JC due to being routed as differential pairs so you will need to look out for that. Here is a thread that talks about using the high speed pmod ports. Also see section 10.2 High-Speed Pmod here.  The I/O resistors are accessible/replaceable on the bottom of the board and are labeled. You can use the schematic as a reference here. We do also have the Zedboard, Nexys Video and Genesys 2 that have a high speed FMC here which might be a better solution to your design needs. 

cheers,

Jon

@jpeyron/Jon,

 

I bought a couple of Cora boards for my project, and now I wonder if I have trouble coming my way.  The Cora has 2 high speed PMOD connectors.  I want to digitize at 1 Gbps.  That's for an ADS4142 from TI, 65 MSps, 14 bits.  I would be running the digital lines from the A/D over the PMOD into the Cora.  Is this going to work for me or do I need to go to the FMC type connector and scrap using the Cora?  The plots from above show 100 MHz, which should be plenty for me.  But will that work over the High Speed PMOD on the Cora?

 

Thanks,

Darren

[jpeyron]

Hi @darren,

I would also look at this forum thread as well. We can not guarantee 100 Mhz out of the high speed Pmod Port as you can see from the forum thread you quoted. You might be able to accomplish your data rate needs using the high speed Pmod ports but we can not guarantee it. You will be able to accomplish your data rate using an FMC. 

thank you,

Jon

[Ahmed Alfadhel]

On 3/20/2017 at 8:00 PM, Korken said:

Hi again,

I have received the board and have tested using the standard 200 ohm series resistors and 33 ohm replacements on a 100 MHz signal.
What I did was simply to forward the input 100 MHz clock on the Arty to two output pins, one with the original series resistance on and one with my replacement.
This was measured with an 16:1 passive (resistive) probe which presents itself as an load of about 800 ohms (made from an 750 resistor + coax), with a 50 ohm input termination on the scope (the image show 1Meg, this is from another channel).

The results, as can be seen in the images, the 200 ohm pin still performs quite okey, though it was very sensitive to anything coming close or touching the board.
The 33 ohm signal was rock solid. So I will continue and replace all the I/O resistors to 33 ohm, something a little bigger could probably be used as well but I have a lot of 33 ohm resistors.

But is shows that the Arty has no problem, what so ever, with single ended signals up to 100 MHz so far.
Another 2 cm of signal path will be added when I add my board, but this should not have any large effect on signal integrity at these speeds.
Plus I have looked around the headers for GND points where I will solder extra GND paths, so there won't be a deficiency.

@jpeyron Thanks for the signal lengths and the assistance!

 

200 Ohm:

 

33 Ohm:

 

Probe:

Hi @Korken , @jpeyron

 Are you using G pin as Ground pin ?

Thanks in advance. 

[jpeyron]

Hi @Ahmed Alfadhel,

The schematic for the Arty-A7 here on the top of page 3 show that the Arduino styled header pin named G is a ground pin. I have attached a screen shot of this as well.

thank you,

Jon