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Adventurer
Adventurer
1,168 Views
Registered: ‎02-04-2016

What happens to non-utilized LUTs after programming ?

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Hi everybody,

 

May I know what actually happens to the LUTs and registers that I am not utilizing in my program, after burning the FPGA with the bitstream? I am more interested in 7-series and + FPGAs. Since the bitstreams are always of same/similar size for that particular chip, I think Xilinx writes something to those un-utilized registers and LUTs while programming. I assume remaining SRAMs are initialized to reset condition (0) ? May I know that , if that is not a trade secret.

Goutham

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Scholar austin
Scholar
1,748 Views
Registered: ‎02-27-2008

Re: What happens to non-utilized LUTs after programming ?

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G,

 

The memory is designed to power on INTO the '0' state.

 

The startup programs every configuration memory cell from the bitstream, used, or unused, every bit is represented by a value appropriate to its use, or if unused, again '0'.

 

So, unused configurations starts at '0' and is re-written as '0' by the bitstream.

 

Why so interested, if I may ask?

Austin Lesea
Principal Engineer
Xilinx San Jose
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Voyager
Voyager
1,146 Views
Registered: ‎06-24-2013

Re: What happens to non-utilized LUTs after programming ?

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Hey Goutham,

 

Your assumption is correct, the majority primitives (LUTs, FFs, MUXes, etc) are initialized to a default (0) value which can be easily seen in the bitstream which contains large amounts of zeros when you only utilize a few of those resources. It is probably similar for the routing resources and I/O pins, although the default state there might not always be zero. I don't think it is a trade secret, but let's see what Xilinx has to say about it ...

 

Best,

Herbert

-------------- Yes, I do this for fun!
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Scholar austin
Scholar
1,749 Views
Registered: ‎02-27-2008

Re: What happens to non-utilized LUTs after programming ?

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G,

 

The memory is designed to power on INTO the '0' state.

 

The startup programs every configuration memory cell from the bitstream, used, or unused, every bit is represented by a value appropriate to its use, or if unused, again '0'.

 

So, unused configurations starts at '0' and is re-written as '0' by the bitstream.

 

Why so interested, if I may ask?

Austin Lesea
Principal Engineer
Xilinx San Jose
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Adventurer
Adventurer
1,063 Views
Registered: ‎02-04-2016

Re: What happens to non-utilized LUTs after programming ?

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Hi @austin and @hpoetzl 

Thanks for confirming my assumption. I am doing a research on low-level custom routing in FPGA and thats when this question popped up. Thanks for your time.

Goutham

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