UPGRADE YOUR BROWSER

We have detected your current browser version is not the latest one. Xilinx.com uses the latest web technologies to bring you the best online experience possible. Please upgrade to a Xilinx.com supported browser:Chrome, Firefox, Internet Explorer 11, Safari. Thank you!

cancel
Showing results for 
Search instead for 
Did you mean: 
Visitor hallovipin
Visitor
6,692 Views
Registered: ‎06-15-2010

Doubt regarding VCCAUX voltage level and its separate power plane

Hi,

 

I am using XC6SLX75 FGG484 spartan-6 FPGA in my design. I need only 30 IOs for my application for which I think only one bank will suffice. While deciding power supplies for the chip , I came across few doubts regarding VCCAUX.

 

1. What should be the level of VCCAUX if all my IOs are working with 3.3 Volts. I read if you are working with 3.3 V only application, connect VCCAUX to 3.3 Volts. 

 

2. I have read VCCAUX powers DCM; for which it demands higher noise immunity and hence can not be connected to VCCO. I want to know if any unused  I/O bank can be connected to VCCAUX to minimize number of  power planes? 

 

3. Is bank 2 (if we start nomenclature with bank 1 not bank 0) of the XC6SLX75 FGG484 is different from other banks for its internal functionality? I had a doubt because while I was using NATIONAL SEMICONDUCTOR'S WEBENCH® Power Architect tool for xilinx FPGA, it always recommended separate power supply for bank 2 (VCCIO # 2) even if we are using 3.3 V for all the banks. This is the reason I was asking to connect VCCO # 2 to VCCAUX. I will make sure I dont use this bank.

 

Thanx.

 

0 Kudos
5 Replies
Teacher eteam00
Teacher
6,686 Views
Registered: ‎07-21-2009

Re: Doubt regarding VCCAUX voltage level and its separate power plane

1. What should be the level of VCCAUX if all my IOs are working with 3.3 Volts. I read if you are working with 3.3 V only application, connect VCCAUX to 3.3 Volts.

 

Where did you read this?  I recommend that you open DS162 (Spartan-6 datasheet) and search all instances of VCCAUX.  There are some rules for VCCAUX, and it's a good idea to read them directly from the datasheet.

 

2. I have read VCCAUX powers DCM; for which it demands higher noise immunity and hence can not be connected to VCCO. I want to know if any unused  I/O bank can be connected to VCCAUX to minimize number of  power planes?

 

VCCAUX supplies the CMTs (which include both DCMs and PLLs).  There are some mild warnings about noise from VCCO rails polluting the VCCAUX supply, but read post #4, written by Ed McGettigan (mcgett) in this thread.

 

3. Is bank 2 (if we start nomenclature with bank 1 not bank 0)

 

Sorry, let's stick with the notation used in all of the Xilinx docs.  IO Bank numbering starts with 0, not 1.

 

of the XC6SLX75 FGG484 is different from other banks for its internal functionality? I had a doubt because while I was using NATIONAL SEMICONDUCTOR'S WEBENCH® Power Architect tool for xilinx FPGA, it always recommended separate power supply for bank 2 (VCCIO # 2) even if we are using 3.3 V for all the banks. This is the reason I was asking to connect VCCO # 2 to VCCAUX. I will make sure I dont use this bank.

 

Search UG380 (Configuration User Guide) for instances of VCCO_2.  The interactions between VCCO_2 and VCCAUX will become a bit more clear.  VCCO_2 supplies power to the pins used for configuration, so there is some dependency for VCCO_2 voltage on the specific configuration interface being used on your board.  VCCAUX supplies power for the JTAG interface.  All the details are in UG380.

 

-- 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.
0 Kudos
Visitor hallovipin
Visitor
6,668 Views
Registered: ‎06-15-2010

Re: Doubt regarding VCCAUX voltage level and its separate power plane

1. Where did you read this?  I recommend that you open DS162 (Spartan-6 datasheet) and search all instances of    VCCAUX.  There are some rules for VCCAUX, and it's a good idea to read them directly from the datasheet.

 

I read it in a forum. They say that VCCAUX is used to power LVCMOS25 inputs, if it is set a voltage level of 2.5V . If CONFIG VCCAUX = 2.5, VCCAUX is used to power LVCMOS25 inputs. If CONFIG VCCAUX = 3.3, VCCO must be 2.5V for any bankswith LVCMOS25 inputs. but if you are not using any LVCMOS25 inputs set it to 3.3 Volts as you will avoid danger of getting VCCAUX below 2 volts which is very much needed for configuration data retention. I read DS162 they have mentioned all the instances but didnt make it clear where and when to use it. :smileysad: 

 

2. VCCAUX supplies the CMTs (which include both DCMs and PLLs).  There are some mild warnings about noise from VCCO rails polluting the VCCAUX supply, but read post #4, written by Ed McGettigan (mcgett) in this thread.

 

I read the thread and it says, VCCAUX and VCCIO rails can be shared but we have to ensure that 5% ripples criterion is not violated. This is well understood but my original doubt was;  if an unused bank can be connected to VCCAUX as it wont have any fluctuations.  

 

3. Search UG380 (Configuration User Guide) for instances of VCCO_2.  The interactions between VCCO_2 and VCCAUX will become a bit more clear.  VCCO_2 supplies power to the pins used for configuration, so there is some dependency for VCCO_2 voltage on the specific configuration interface being used on your board.  VCCAUX supplies power for the JTAG interface.  All the details are in UG380.

 

Sorry for walking the wrong road again,  but I was concerned with VCCO_1 (as per xilinx standard ). It was National Semiconductor's WEBENCH® Power Architect tool which starts numbering from 1 (so it says VCCO_1 as VCCO_2). Why does VCCO_1 (as per xilinx standard and I will use it now onwards ) require a separate supply as recommended by the tool.

 

Thanx for all your patience Bob

0 Kudos
Teacher eteam00
Teacher
6,657 Views
Registered: ‎07-21-2009

Re: Doubt regarding VCCAUX voltage level and its separate power plane

Every question and worry which you mention is addressed in either DS162 or UG380.  To repeat myself, I suggest you re-read the references to 'VCCAUX' which appear in DS162.

 

if an unused bank can be connected to VCCAUX as it wont have any fluctuations

 

You can connect a VCCO to the same supply as VCCAUX if the I/O bank is unused, and even if the I/O bank is used.  This practice is quite common.  If you think I/O bank output switching will provoke noise on the VCCO supply which the regulator and decoupling caps cannot handle, then by all means isolate VCCO from VCCAUX.  A simple L-C filter (ferrite bead) should suffice.

 

As there are absolutely no references to "VCCO_1" or "VCCO1" in DS162, I submit that the NatSemi tool is either in error or it is misunderstood.  The folks at NatSemi will no doubt explain or defend their tool much better than I can.

 

You seem like a clever and astute person.  You should have no problem using DS162 and UG380 to settle all the questions you have mentioned so far.

 

-- 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.
0 Kudos
Visitor hallovipin
Visitor
6,648 Views
Registered: ‎06-15-2010

Re: Doubt regarding VCCAUX voltage level and its separate power plane

Thnx a lot bob.. I will burn the midnight oil for sure to find out the answers.

 

What I concluded from all your posts that you dont help people in "WHAT IS THE ANSWER" but  "HOW TO FIND THE ANSWER" . This approach serves the bigger cause by helping them being wiser and independent. 

 

 YUPPIEEE I am ASTUTE and CLEVER :smileyhappy:

0 Kudos
Teacher eteam00
Teacher
6,643 Views
Registered: ‎07-21-2009

Re: Doubt regarding VCCAUX voltage level and its separate power plane

What I concluded from all your posts that you dont help people in "WHAT IS THE ANSWER" but  "HOW TO FIND THE ANSWER" . This approach serves the bigger cause by helping them being wiser and independent.

 

Exactly.  Plus it seems silly for me to find, copy, and paste document excerpts into a forum post when you can simply read them for yourself.

 

I suspect you would prefer to draw your own conclusions from reading the documents yourself, rather than accept any broad generalisations which Austin Lesea, Ed McGettigan, or I might offer.

 

And I'm flat out ignorant of the NatSemi tool's reasoning.

 

YUPPIEEE I am ASTUTE and CLEVER :smileyhappy:

 

I would expect no less of a Xilinx user.

 

-- 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.
0 Kudos