Motorola DSP Developer's Kit

fft-r.asm

Y = mot###_fft( X )

Description

This function returns the discrete Fourier transform (DFT) of the input for real vector X

Input/Output

Input: Vector Xe (even index input data, located in X memory), and vector Xo (odd index input data, located in Y memory)

Output: Complex Vector Y (includes real output data of vector Yr, and imaginary data output of vector Yi)

Algorithm

First, use algorithm in FFT-C.ASM to calculate length/2 complex data FFT.

Then use split algorithm to calculate final result.

No scaling is required for the input data. The output data should be scaled up by 2^(r2+1).

For example, to get true FFT values, after the FFT is done and r2=7, every output item has to be shifted left 8 bits.

Note    The variable "size" used in the code below is the half size of the input for real vector X.

The split algorithm is described below:

for (k =0; k < size/2 -1; k++)
{
    H1r = (Xr[k] + Xr[size-k]) / 2;
    H1i = (Xi[k] - Xi[size-k])  / 2;
    H2r = (Xi[k] + Xi[size-k]) / 2;
    H2i = (Xr[size-k] - Xr[k]) / 2;
    Yr[k] = H1r + (C2r[k]*H2r - C2i[k]*H2i);
    Yr[size-k] = H1r - (C2r[k]*H2r - C2r[k]*H2i);
    Yi[k] = H1i + (C2i[k]*H2r- C2r[k]*H2i);
    Yi[size-k] = -(H1i) + (C2i[k]*H2r - C2r[k]*H2i);
}

Memory & Register

Memory allocation:

• In X memory: label IN_REAL stores the location of the input data (real part)
• In Y memory: label IN_IMAG stores the location of the input data (imaginary part)

Note    The above two symbols are also used as output symbols

• p:$F000 = half the length of the input real vector
• p:$F001 = address offset of coefficient lookup table C1 (used by Fft-c)
• p:$F002 = address offset of coefficient lookup table C2 (used by split)
• p:$F003 = address offset of output vector

Register usage:

• R7 is used for passnum
• R2 is used for scaling exponent

Status Register

The assembly function fft-r.asm does not set any status registers/bits during the function execution.

Data Size Limit

The length of vector X can't be larger than the continuous available data memory size.

Data Range Limit

The value of input vector X must be between -1.0 and +1.0.

diff-c.asm

fft-c.asm