Motorola DSP Developer's Kit

fft-c.asm

Motorola DSP Developer's Kit

fft-c.asm

Motorola DSP Developer's Kit

fft-c.asm

Y = mot###_fft X )

Input/Output

Input: Complex vector X (includes the real input data of vector Xr, and the imaginary data input of vector Xi)

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

Algorithm

The algorithm is radix-2 DIT FFT.

Passnum = (int) (Log2(size) + 0.5);  //calculated by mex function
GroupPerPass= 1;
ButterflyPerGroup = size / 2;
Set coefficient table addressing mode as bit-reversed;
Clear scaling bit;
Set scaling down mode;
Scalexp = 1;  //scaling exponent
for(i = 0; i < Passnum; i++)
{
    pA = 0;  /*address pointer of the first input of butterfly */
    pB = pA + ButterflyPerGroup;/*address pointer of the second 
input of butterfly */
    pC = 0;  /*address pointer of coefficient lookup table C */
    for (j = 0; j < GroupPerPass; j++)
    {
        for (k = 0; k < ButterflyPerGroup; k++)
       {
            Xr[pA] = Xr[pA] + Xr[pB]*Cr[pC] + Xi[pB]*Ci[pC];
            Xi[pA] = Xi[pA]  + Xi[pB]*Cr[pC] - Xr[pB]*Ci[pC]; 
            Xr[pB] = 2*Xr[pA] - Xr[pA];
            Xi[pB] = 2*Xi[pA] - Xi[pA];
            pA ++;
            pB ++;
       
}
Clr scaling down mode;
if has overflow
{
Set scaling down mode;
Clr scaling bit;
Scalexp++;
}
pA += ButterflyPerGroup;
pB += ButterflyPerGroup;
pC += size / 4; /* bit-reverse */
}
ButterflyPerGroup >>= 1;
GroupPerPass<<= 1;
  }
  Clear scaling bit;
  Set no scaling mode;
  Convert bit reverse order to normal order in-place;

Note    Actual output data should be scaled up by 2^(Scalexp-1). This will be done by the MEX function.

Memory & Register

Memory allocation:

• X memory: IN_REAL stores the start address of the real part of the input data
• Y memory: IN_IMAG stores the start address of the imaginary part of the input data

Note    The above two symbols are also used as output symbol

Register usage:

• R3 stores the length of the input vector
• R7 stores the passnum
• R2 stores the scaling exponent

Status Register

The assembly function fft-c.asm does not explicitly 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.

fft-r.asm

filter-r.asm