Motorola DSP Developer's Kit

angle-c.asm

Y = mot###_angle( X )

Description

This function returns the inverse tangent (arctangent) of the input complex vector X

Input/Output

Input: Complex vector X (includes the real part Xr and the imaginary part Xi)

Output: Vector Y

Algorithm

Use the CORDIC algorithm. For each input vector, x represents the real part, and y represents the imaginary part of the input vector.

int z = 0, X, Y, Z =0, i; 
        if (x == (unsigned frac)0)
           return (frac)0;
        X = x = x>>2 ;
        Y = y = y>>2 ;
    /* Circular Function */
        for (i = 0; i <= fracbits; ++i)
       {
            x = X >> i;
            y = Y >> i;
            z = atan[i];
            if (Y <= 0)
           {
                X -= y;
                Y += x;
                Z -= z;
           }
            else
           {
                X += y;
                Y -= x;
                Z += z;
           }
       }
        Z = Z << 2;
        return Z;

Memory & Register

Memory allocation:

• Label IN stores the start address of input vector X
• Label INREAL stores the start address of real data of input vector
• Label INIMAG stores the start address of complex data of input vector
• Label OUTREAL stores the start address of output vector
• Variable X, Y is saved starting from label XYZ

X0 as x

X1 as y

Y1 as z

B as Z

Register usage:

• Register R6 stores the number of items of the input array
• Register R2 stores the fraction bits +1

In the case of DSP563, R2 stores 22

In the case of DSP566, R2 stores 14

• Register R3 stores the value of the atan table
• Registers R0 and R5 store the label IN(OUT)
• Register R7 stores the start address of label XYZ
• Register R1 stores the loop counter
• Register Y0 is mainly used as shift number register (i.e., variable 'i' in the algorithm description)

Status Register

The assembly function angle-c.asm does not explicitly set any status registers/bits during the function execution.

Data Size Limit

The length of vector X is limited by the size of available continuous data memory.

Data Range Limit

The input vector X range is [-1.0, +1.0].

Precision

In the case of DSP563, precision is 17 bits.

In the case of DSP566, precision is 10 bits.

Performance Limit

In the case of DSP563:

• When the input vector is a real vector, there are 25 cycles for each element of input data.
• When the input vector is a complex vector, there are 628 cycles for each element of input data.

In the case of DSP566:

• When the input vector is a real vector, there are 25 cycles for each element of input data.
• When the input vector is a complex vector, there are 609 cycles for each element of input data.

abs-c.asm

conv-r.asm