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Contributor
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Registered: ‎09-12-2010

Nexys 2 board variable supply to core

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Hi All,

 

I am performing an experiment where I need to subject FPGA core to less than 1.2V power supply. I referred to the power supply schematic of Nexys 2 board, but I am not able to figure it out. For Spartan 3E, I varied the supply using JP7. Could similar thing be done on Nexys 2 board ? I will appreciate it if someone can give me few pointers.

 

Thanks,

Kanu

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Instructor
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Registered: ‎07-21-2009

Re: Nexys 2 board variable supply to core

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I am performing an experiment where I need to subject FPGA core to less than 1.2V power supply

On Nexys2 board, VCC1V2 supply voltage is set by R78 and R79.  According to the schematics R78=205K, R79=412K.  Reducing the value of R78 will reduce the VCC1V2 supply voltage.

               R78 + R79

Vout = 0.8V * -----------

                  R79

- Bob Elkind

SIGNATURE:
README for newbies is here: http://forums.xilinx.com/t5/New-Users-Forum/README-first-Help-for-new-users/td-p/219369

Summary:
1. Read the manual or user guide. Have you read the manual? Can you find the manual?
2. Search the forums (and search the web) for similar topics.
3. Do not post the same question on multiple forums.
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Instructor
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Registered: ‎07-21-2009

Re: Nexys 2 board variable supply to core

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I am performing an experiment where I need to subject FPGA core to less than 1.2V power supply

On Nexys2 board, VCC1V2 supply voltage is set by R78 and R79.  According to the schematics R78=205K, R79=412K.  Reducing the value of R78 will reduce the VCC1V2 supply voltage.

               R78 + R79

Vout = 0.8V * -----------

                  R79

- Bob Elkind

SIGNATURE:
README for newbies is here: http://forums.xilinx.com/t5/New-Users-Forum/README-first-Help-for-new-users/td-p/219369

Summary:
1. Read the manual or user guide. Have you read the manual? Can you find the manual?
2. Search the forums (and search the web) for similar topics.
3. Do not post the same question on multiple forums.
4. Do not post a new topic or question on someone else's thread, start a new thread!
5. Students: Copying code is not the same as learning to design.
6 "It does not work" is not a question which can be answered. Provide useful details (with webpage, datasheet links, please).
7. You are not charged extra fees for comments in your code.
8. I am not paid for forum posts. If I write a good post, then I have been good for nothing.

View solution in original post

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Contributor
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Registered: ‎09-12-2010

Re: Nexys 2 board variable supply to core

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Hi Bob,

 

Thanks for your reply. I wanted to confirm few things to see if I am interpreting it right. Links for Spartan Board and Nexys 2 boards schematic are:

 

http://www.digilentinc.com/Data/Products/NEXYS2/Nexys2_sch.pdf

http://www.xilinx.com/support/documentation/boards_and_kits/ug230.pdf

 

 

 My questions are as follows:

 

- Looking at Power Supply and Regulators shcematic of Spartan board (Spartan board manual, Pg 141), one can see that JP6 or JP7 can be used to provide external variable supply. Similarly, I was searching for a jumper which can help me provide variable supply for Nexys 2 board. But when I refer to Nexys schematics (Nexys Schematic manual, Pg 3), I don't find any jumpers. So in short, there is no easy way out other than connecting external resistors to reduce the value of R78 ?

 

Please do let me know if I am proceeding right.

 

Regards,

Kanu

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Registered: ‎07-21-2009

Re: Nexys 2 board variable supply to core

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Looking at Power Supply and Regulators schematic of Spartan board (Spartan board manual, Pg 141), one can see that JP6 or JP7 can be used to provide external variable supply.

JP6 can be used to insert an external 2.5V supply

JP7 can be used to insert an external 1.2V supply

The board you are referencing is the Spartan 3e Starter Kit.

Similarly, I was searching for a jumper which can help me provide variable supply for Nexys 2 board. But when I refer to Nexys schematics (Nexys Schematic manual, Pg 3), I don't find any jumpers. So in short, there is no easy way out other than connecting external resistors to reduce the value of R78 ?

You are correct.  If you can find someone proficient with soldering small surface mount components, you can solder another resistor onto R78 to lower its effective value, which will lower the VCC1V2 supply voltage.

 

If you aren't sure how to calculate the value of the solder-on resistor, based on your desired target supply voltage, then find someone to help you who is schooled in electrical engineering basic principles such as Thevenin equivalents, Ohm's law, etc. etc.

 

If you guess wrong, or if the soldering re-work is done incorrectly, you could destroy your board.

 

- Bob Elkind

SIGNATURE:
README for newbies is here: http://forums.xilinx.com/t5/New-Users-Forum/README-first-Help-for-new-users/td-p/219369

Summary:
1. Read the manual or user guide. Have you read the manual? Can you find the manual?
2. Search the forums (and search the web) for similar topics.
3. Do not post the same question on multiple forums.
4. Do not post a new topic or question on someone else's thread, start a new thread!
5. Students: Copying code is not the same as learning to design.
6 "It does not work" is not a question which can be answered. Provide useful details (with webpage, datasheet links, please).
7. You are not charged extra fees for comments in your code.
8. I am not paid for forum posts. If I write a good post, then I have been good for nothing.
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Registered: ‎09-12-2010

Re: Nexys 2 board variable supply to core

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Hi Bob,

 

There seems to be a way out. Battery terminals are present on the Nexys 2 Board from which variable supply is possible. I varied this supply and observed that power led starts glowing after 4.1V. So I reduced the main supply voltage, this has effectively reduced the voltage across the FPGA chip. Only problem here is to determine how I will know as to what is the actual voltage at the chip.

 

I have attached Nexys 2 board's reference manual. Whatever I have deduced is from Pg 2 of the manual. Please comment on my thinking process.

 

Regards,

Kanu

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Instructor
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Registered: ‎07-21-2009

Re: Nexys 2 board variable supply to core

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Look on page 3 of the Nexys2 board schematic, and verify that lowering the VU supply will accomplish what you desire.

 

For some reason, I believed that you wanted to reduce only the FPGA core supply voltage.  Apparently my understanding of your goal was incorrect.  Varying the VU supply will not accomplish this.

 

It may well be that you are in over your head on this.  You should consider finding someone close at hand, with better understanding of basic electricity and electronics, to help you with your experiment.

 

- Bob Elkind

SIGNATURE:
README for newbies is here: http://forums.xilinx.com/t5/New-Users-Forum/README-first-Help-for-new-users/td-p/219369

Summary:
1. Read the manual or user guide. Have you read the manual? Can you find the manual?
2. Search the forums (and search the web) for similar topics.
3. Do not post the same question on multiple forums.
4. Do not post a new topic or question on someone else's thread, start a new thread!
5. Students: Copying code is not the same as learning to design.
6 "It does not work" is not a question which can be answered. Provide useful details (with webpage, datasheet links, please).
7. You are not charged extra fees for comments in your code.
8. I am not paid for forum posts. If I write a good post, then I have been good for nothing.
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Re: Nexys 2 board variable supply to core

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Hi Bob,

 

I figured out few things. Varying VU also may not help as ultimatly the voltage goes through voltage regulator IC which is supposed to provide fixed voltage. So fianlly, I cannot vary the voltage for the core without using resistor :( .

 

Regards,

Kanu

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Re: Nexys 2 board variable supply to core

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Where I live and work, being able to change voltage with a simple resistor mod is considered "a very good day at the office". It's a 5-minute fix for the qualified nerd, if you haven't consumed too much caffeine to hold a soldering pencil steady.

 

Find the right person with an appropriate surface mount soldering station, order a few 0402 resistors from Digi-Key or RadioShack, and you can have a "good day" as well.  If you bring a fresh, piping hot Starbucks mocha with with real whipped cream, you should have no problem finding help.  And please check with someone who understands power supply regulators and Thevenin / Ohm stuff, to protect yourself from frying your board.

 

If you aren't comfortable hacking R78, here's another option for you:

Lift one side or the other of L2.  You can connect a variable output bench supply directly to the VCC1V2 net.  This may require the use of a soldering iron.

- Bob Elkind

SIGNATURE:
README for newbies is here: http://forums.xilinx.com/t5/New-Users-Forum/README-first-Help-for-new-users/td-p/219369

Summary:
1. Read the manual or user guide. Have you read the manual? Can you find the manual?
2. Search the forums (and search the web) for similar topics.
3. Do not post the same question on multiple forums.
4. Do not post a new topic or question on someone else's thread, start a new thread!
5. Students: Copying code is not the same as learning to design.
6 "It does not work" is not a question which can be answered. Provide useful details (with webpage, datasheet links, please).
7. You are not charged extra fees for comments in your code.
8. I am not paid for forum posts. If I write a good post, then I have been good for nothing.
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Registered: ‎09-12-2010

Re: Nexys 2 board variable supply to core

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Thanks for help Bob. Now I have one more basic question. As there are three supplies for the core VCCO, VCCINT and VCCAUX, which one do I vary to say that core is running on different voltage ? Earlier I was trying to search for varying only VCCINT, but now I really don't know.

 

Regards,

Kanu

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Re: Nexys 2 board variable supply to core

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As there are three supplies for the core VCCO, VCCINT and VCCAUX, which one do I vary to say that core is running on different voltage?

There is only one supply for the FPGA core.  In the Xilinx Spartan 3 family datasheet, this supply is called VCCINT.  On the Nexys2 board schematics, this supply is called VCC1V2.

Earlier I was trying to search for varying only VCCINT, but now I really don't know.

I'm not sure I know what you are trying to do.

 

Happy New Year!

 

- Bob Elkind

SIGNATURE:
README for newbies is here: http://forums.xilinx.com/t5/New-Users-Forum/README-first-Help-for-new-users/td-p/219369

Summary:
1. Read the manual or user guide. Have you read the manual? Can you find the manual?
2. Search the forums (and search the web) for similar topics.
3. Do not post the same question on multiple forums.
4. Do not post a new topic or question on someone else's thread, start a new thread!
5. Students: Copying code is not the same as learning to design.
6 "It does not work" is not a question which can be answered. Provide useful details (with webpage, datasheet links, please).
7. You are not charged extra fees for comments in your code.
8. I am not paid for forum posts. If I write a good post, then I have been good for nothing.
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Registered: ‎11-11-2012

Re: Nexys 2 board variable supply to core

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Hi,

 

I'm interested in the "L2 lift solution". Can you give me more details about it? I just lift one side of the inductor, and then connect it to a voltage supply different than the 0.8V (this is the output of the tension regulator)?

For example, I supply a voltage of 0.9V instead of 0.8V, and following the equation posted above, I'll get ~1.34V?

 

 

Thanks,

Pierpaolo

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