Block FIR 16-bit filter

UM0304

10/19

3.3.2

Calling the function from Assembly

Use the following registers equivalent to arguments/variables.

●

Move N to R3, N is output filter length or output vector length, a multiple of 6

●

Move SP to R2, comparable &p as in C function above h[T] = filter coefficient vector

with T number of taps, T is an integer multiple of 6

●

Set R1 to the address of x, comparable as in x[M] filter input vector with total M

samples, i.e., (M = N + T - 1)

●

Set R0 to the address of y, comparable as in y[N] filter output vector with total N

samples

Then use BL instruction to call the function.

3.4

Algorithm

Following is the C equivalent of the assembly code. Note that the assembly code is hand

optimized and restrictions may apply.

Block FIR Filter Reference C Source

void fir_16by16_arm9e(short input, int nt, short h[], short z[])

{

int j;

short acc;

/* store input at the beginning of the delay line */

z[0] = input;

/* calc FIR */

acc = 0;

for (j = 0; j < nt; j++) {

acc += h[j] * z[j];

}

/* shift delay line */

for (j = nt - 2; j >= 0; j--) {

z[j + 1] = z[j];

}

return acc;

}

3.5

Assembly code

The assembly code for this block FIR 16-bit fixed-point filter algorithm is validated in STR91x

environment for ARM966E-S. The assembly code is for 16-bit fixed point applications, and