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- FFT 7.1 C Model gives blk_exp error when using C_H...

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argento

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06-11-2012 09:55 AM

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Registered:
06-11-2012

FFT 7.1 C Model gives blk_exp error when using C_HAS_BFP = 1

Hello Folks,

I'm running the C Modelexample provided with the package.

I'm using my data. I used Scheduled Scaling, but when I'm going to test Block Fixed-Point I get an error:

ERROR: blk_exp is incorrect: expected 14, actual 7

Some outputs from simulation are incorrect

Output of FFT is unusable, and very far from results I obtain with Scheduled Scaling.

Why is this happening?

How can I solve it?

5 Replies

ywu

Xilinx Employee

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06-12-2012 06:54 AM

4,229 Views

Registered:
11-28-2007

Re: FFT 7.1 C Model gives blk_exp error when using C_HAS_BFP = 1

The blk_exp checking in the C model example is only valid for the input data used in the original example. If you use your own data, the blk_exp will be different, so you will need to remove the blk_exp checking in your C model.

The core automaically adjusts scaling in block floating mode, so blk_exp output can be different for each frame depending on the input data. You will need to left shift the FFT output by blk_exp to reconstruct the output data.

@argento wrote:

Hello Folks,

I'm running the C Modelexample provided with the package.

I'm using my data. I used Scheduled Scaling, but when I'm going to test Block Fixed-Point I get an error:

ERROR: blk_exp is incorrect: expected 14, actual 7

Some outputs from simulation are incorrect

Output of FFT is unusable, and very far from results I obtain with Scheduled Scaling.

Why is this happening?

How can I solve it?

Cheers,

Jim

Jim

argento

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06-14-2012 08:44 AM

4,220 Views

Registered:
06-11-2012

Re: FFT 7.1 C Model gives blk_exp error when using C_HAS_BFP = 1

Ok, I'll remove that check.

But anyway, I'd be glad if you can teach me why blk_exp should be equal to nfft, if it is possible.

Thanks in advance

ywu

Xilinx Employee

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06-17-2012 01:54 PM

4,210 Views

Registered:
11-28-2007

Re: FFT 7.1 C Model gives blk_exp error when using C_HAS_BFP = 1

The blk_exp doesn't always equal to nfft. It happens to be nfft in the provided example when the input is a constant 0.5.

@argento wrote:

Ok, I'll remove that check.

But anyway, I'd be glad if you can teach me why blk_exp should be equal to nfft, if it is possible.

Thanks in advance

Cheers,

Jim

Jim

argento

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06-19-2012 01:38 PM

4,201 Views

Registered:
06-11-2012

Re: FFT 7.1 C Model gives blk_exp error when using C_HAS_BFP = 1

Ok, I managed to use BFP.

One problem was I was using bad data.

Block Floating-Point seems to work now.

**What is faster** for the LogiCORETM IP Fast Fourier Transform v7.1,

to use a **Scaling Schedule or BPF ?**

ywu

Xilinx Employee

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06-19-2012 07:16 PM

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Registered:
11-28-2007

Re: FFT 7.1 C Model gives blk_exp error when using C_HAS_BFP = 1

If by "faster" you mean Fmax, the difference between scaled and block floating point is negligible. You can easily generate two netlists and compare the Fmax yourself.

@argento wrote:

Ok, I managed to use BFP.

One problem was I was using bad data.

Block Floating-Point seems to work now.

What is fasterfor the LogiCORETM IP Fast Fourier Transform v7.1,to use a

Scaling Schedule or BPF ?

Cheers,

Jim

Jim