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Explorer
Explorer
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Registered: ‎11-23-2017

Comparison of I/O standards and recommended uses

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Hi,

Is there a document that compares the specs for the different allowable i/o standards for implementation?

Also, would be useful if it was explained which standard is recommended for different use cases.

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Advisor
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Registered: ‎04-26-2015

Re: Comparison of I/O standards and recommended uses

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UG471 has the specs on the various standards. In terms of what they should be used for, it's pretty simple for most interfaces...

 

LVDS - is used for communicating with devices that use a LVDS interface

HSTL - is used for communicating with devices that use a HSTL interface

SSTL - is used for communicating with devices that use a SSTL interface

HSUL - is used for communicating with devices that use a HSUL interface

 

... you can see where this is going. The only significantly different one is LVCMOS, which is the "general-purpose low-speed" interface (used for things like SPI and UART, but also for LED/transistor driving etc).

 

The only time you'd be using a high-speed interface and actually have a choice is when you're talking between two FPGAs; in that case I'd generally use either LVCMOS (low-speed data), LVDS (high-ish speed data), or the MGTs (very high-speed data).

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Adventurer
Adventurer
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Registered: ‎04-04-2018

Re: Comparison of I/O standards and recommended uses

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Hi @simchask, if you open up the DC and AC Switching characteristics for the device series you are using, there is a section "DC input and Output levels.

Here is the Virtex7 for example:

https://www.xilinx.com/support/documentation/data_sheets/ds183_Virtex_7_Data_Sheet.pdf

 

Also refer to the IO users guide:

https://www.xilinx.com/support/documentation/user_guides/ug471_7Series_SelectIO.pdf

 

Additionally, the topic was discussed here:

https://forums.xilinx.com/t5/Welcome-Join/IO-STANDARD/td-p/655080

Steve Markgraf - Distinguished FPGA Design & Support Engineer E5-E
www.designlinxhs.com
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Advisor
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Registered: ‎04-26-2015

Re: Comparison of I/O standards and recommended uses

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UG471 has the specs on the various standards. In terms of what they should be used for, it's pretty simple for most interfaces...

 

LVDS - is used for communicating with devices that use a LVDS interface

HSTL - is used for communicating with devices that use a HSTL interface

SSTL - is used for communicating with devices that use a SSTL interface

HSUL - is used for communicating with devices that use a HSUL interface

 

... you can see where this is going. The only significantly different one is LVCMOS, which is the "general-purpose low-speed" interface (used for things like SPI and UART, but also for LED/transistor driving etc).

 

The only time you'd be using a high-speed interface and actually have a choice is when you're talking between two FPGAs; in that case I'd generally use either LVCMOS (low-speed data), LVDS (high-ish speed data), or the MGTs (very high-speed data).

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Guide
Guide
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Registered: ‎01-23-2009

Re: Comparison of I/O standards and recommended uses

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In terms of what they should be used for, it's pretty simple for most interfaces...

 

Basically this is a decision of speed vs. power and board complexity.

 

LVCMOS is the simplest I/O standard - it requires no termination, and consumes no static power. However, it is pretty slow - you probably shouldn't use it for system synchronous interfaces faster than 66Mbps and source synchronous interfaces faster than 100MHz.

 

SSTL/HSTL are in between. They still are single ended so only require one trace per signal. But they have aggressive termination and hence consume lots of power. But they are faster - you can easily do source synchronous interfaces at or above 250Mbps. They also need reference voltages (which take pins on the FPGA).

 

LVDS is faster yet. It has significant cost in that you now need two pins and two traces per signal. It uses a fair amount of power as well. But it is fast - you can do source synchronous interfaces above 600Mbps. It also has some advantages in terms of electromagnetic interference (since it is differential).

 

But as @u4223374 - mostly it is determined by what you need to interface to...

 

Avrum

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Explorer
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Registered: ‎11-23-2017

Re: Comparison of I/O standards and recommended uses

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Sorry, @u4223374 this isn't helpful for me.

I am driving a 50Ohm load with a hopefully narrow pulse.

I would like to achieve the fastest, cleanest, and highest voltage possible into a single ended 50Ohm load.

If I have to sacrifice voltage then I will add a microwave amplifier.

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Explorer
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Registered: ‎11-23-2017

Re: Comparison of I/O standards and recommended uses

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@avrumw

Thank you!

If I am using the AC701 development board, are those reference voltages already setup or do I need to set them somehow?

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Guide
Guide
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Registered: ‎01-23-2009

Re: Comparison of I/O standards and recommended uses

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If you are using the AC701, what signal are you planning to use? One of the FMC expansion pins? One of the PMOD pins?

 

To answer that question you have to look at the AC701 documentation and/or the AC701 schematic. For example, the PMOD pins are driven directly from the FPGA on bank 14, which is a bank powered to 3.3V with no VREF connected.

 

Avrum

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Moderator
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Registered: ‎09-18-2014

Re: Comparison of I/O standards and recommended uses

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Simchask,

 

They should be set up already. I mean it is a built reference/dev board. Anyways why don't you have look at the AC701 board user guide or the AC701 schematics? Both are public and easy to download/view from the Xilinx documentation site. Being it a built board there should be minimal set up needed for off-chip connections. Not only that Vref is nominally VCCO/2. So it doesn't really make sense to let it be configurable to other unless you also have a dynamically switch the IO bank VCCOs. Check the Vref pins for that part on the ASCII or even the user guide for AC701 pin tables may show you which pin is the VREF pin. Then see if the user guide or the schematic documents what it's connected to. 7-series also had Internal VREF that could be used as well. Although it may not be as accurate but it is handy when there is no board VREF connection made. Look into the IO user guide for more information on that.

 

Regards,

T

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Adventurer
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Registered: ‎04-04-2018

Re: Comparison of I/O standards and recommended uses

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@simchask, the AC701 xdc file shows the default IO standards for all the interfaces:

https://www.xilinx.com/member/forms/download/design-license.html?cid=396446&filename=ac701-xdc-rdf0212.zip

 

Additionally you can change the reference voltage on some banks via VADJ. 

https://www.xilinx.com/support/documentation/boards_and_kits/ac701/ug952-ac701-a7-eval-bd.pdf

 

I have been told the best way to do this is to use the TI software/programming cable:

http://www.ti.com/tool/FUSION_DIGITAL_POWER_DESIGNER?DCMP=hpa_pmp_general&HQS=fusiondocs

 

 

Steve Markgraf - Distinguished FPGA Design & Support Engineer E5-E
www.designlinxhs.com