Infinite Impulse Response Filters - Digital Signal Processing and Applications with the C6713 and C6416 DSK - page 204

// Sinegencasm.c Sine generation using DE calling ASM function

#include "dsk6713_aic23.h" //codec-DSK support file

Uint32 fs=DSK6713_AIC23_FREQ_8KHZ; //set sampling rate

short y[3] = {0, 15137,11585}; //y(1)=sinwT (f=1.5kHz)

short A =12540; //A=2*coswT * 2^14

short n = 2;

interrupt void c_int11() //interrupt service routine

{

sinegencasmfunc(&y[0], A); //calls ASM function

output_sample(y[n]);

return;

}

void main()
{
comm_intr();

//init DSK, codec, McBSP

while(1); //infinite loop

}

FIGURE 5.24. C source program that calls an ASM function to generate a sine wave using
a difference equation ( sinegencasm.c ).

; Sinegencasmfunc.asm ASM function to generate sine using DE
;A4 = address of y array, B4 = A

.def _sinegencasmfunc ;ASM function called from C

_sinegencasmfunc:

LDH *+A4[0], A5 ;y[n-2]-->A5

LDH *+A4[1], A2 ;y[n-1]-->A2

LDH *+A4[2], A3 ;y[n]-->A3

NOP 3 ;NOP due to LDH

MPY B4, A2, A8 ;A*y[n-1]

NOP 1 ;NOP due to MPY

SHR A8, 14, A8 ;shift right by 14

SUB A8, A5, A8 ;A*y[n-1]-y[n-2]

STH A8, *+A4[2] ;y[n]=A*y[n-1]-y[n-2]

STH A2, *+A4[0] ;y[n-2]=y[n-1]

STH A8, *+A4[1] ;y[n-1] = y[n]

B B3 ;return addr to call routine

NOP 5 ;delays due to branching

.end
FIGURE 5.25. ASM function called from C to generate a sine wave using a difference
equation ( sinegencasmfunc.asm ).