Motorola DSP Developer's Kit

ifft-c.asm

Y = mot###_ifft( X )

Description

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

Input/Output

Input: Complex vector X, including vector Xr (real input) and vector Xi (imaginary input)

Output: Complex vector Yr (real output), and vector Yi (imaginary output)

Algorithm

Note    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    The 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

Above two symbols are also used as output symbols

Register usage:

• R3 stores the address of the input vector
• R7 is used for passnum
• R2 is used for the scaling exponent

Status Register

The assembly function ifft-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.

ifft-r.asm

interp-r.asm