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Visitor
Visitor
5,910 Views
Registered: ‎12-04-2014

Artix 7 Hi-Z pullup/down

Hi,

 

I have a couple specific questions about the Artix 7 IOBs. I cannot seem to find the answers to these anywhere in the provided Xilinx documentation. If you know of where these answer are in the documentation, I would love to know. I appreciate the help. 

 

Nick

 

 

1. How is the Hi-Z implemented, hardware wise, in the IOB? Is it implemented with a resistor or FET?

 

2. Can I directly drive I2C lines with the FPGA output directly (they require open-drain interfaces)?

 

3. What resistance is used when the PULLUP constraint is used?

 

4. What resistance is used when the PULLDOWN constraint is used?

 

 

 

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Scholar
Scholar
5,905 Views
Registered: ‎02-27-2008

Re: Artix 7 Hi-Z pullup/down

1.  fet

2.  yes (open drain is tristate control is used as the data -- tristate is '1', and not tristate with a zero as input to the IOB is '0')

3.  In the data sheet

4.  In the data sheet

 

Be sure the Vcco of the IO bank used for I2C is equal to the I2C component's Vdd.

 

 

Austin Lesea
Principal Engineer
Xilinx San Jose
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Advisor
Advisor
5,896 Views
Registered: ‎02-12-2013

Re: Artix 7 Hi-Z pullup/down

Nick,

 

Those I/O pullups tend to be in the range of 50K Ohms to 100K Ohms.  That kind of resistance is too high to provide the pullup for an I2C interface.  I2C uses a pullup resistor to privide the rising edge on signals.  Normally you want 1K Ohms or less for that.

 

Put resistors on your board.

 

You can then infer your Tri-state I/O like this.

 

if (whatever) then

    SDA <= '0';

else

    SDA <= 'Z';

end if;

 

Also, I am not sure about what Austin says about Vcco.  I2C was conceived to allow chips of different I/O standards to communicate.  The idea is that the on-board pullup resistor establishes your logic 1 voltage.  Chips on the bus only pull down to ground.  As long as the FPGA pin can be safely pulled up the logic 1 level you should be ok.

 

  Pete

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DSP in hardware and software
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Visitor
Visitor
5,889 Views
Registered: ‎12-04-2014

Re: Artix 7 Hi-Z pullup/down

Thanks. Which data sheet has that info, I couldn't find it.

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Voyager
Voyager
5,875 Views
Registered: ‎04-21-2014

Re: Artix 7 Hi-Z pullup/down

‎08-27-2015 09:09 PM


@nicklabianco wrote:

Thanks. Which data sheet has that info, I couldn't find it.


 

Try this.

 

Then control F, search for "pull"

 
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Highlighted
Advisor
Advisor
5,792 Views
Registered: ‎02-12-2013

Re: Artix 7 Hi-Z pullup/down

Page 3 of DS181 shows Irpu and Irpd, the currents that the I/O pull resistors can source/sink.

Dividing the Vcco by the current gives you the resistance. It ranges from 36K to 100K Ohms.

That is not what you want for I2C.
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DSP in hardware and software
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