In a program that seems to work well I have the following declarations:
#include <stdio.h>
#include "system.h"
signed char input[4]; /* The 4 most recent input values */
char get_q7( void );
void put_q7( char );
void firFixed(signed char input[4]);
const int c0 = (0.0299 * 128 + 0.5); /* Converting from float to Q7 is multiplying by 2^n i.e. 128 = 2^7 since we use Q7 and round to the nearest integer*/
const int c1 = (0.4701 * 128 + 0.5);
const int c2 = (0.4701 * 128 + 0.5);
const int c3 = (0.0299 * 128 + 0.5);
const int half = (0.5000 * 128 + 0.5);
enum { Q7_BITS = 7 };
void firFixed(signed char input[4])
{
int sum = c0*input[0] + c1*input[1] + c2*input[2] + c3*input[3];
signed char output = (signed char)((sum + half) >> Q7_BITS);
put_q7(output);
}
int main(void)
{
int a;
while(1)
{
for (a = 3 ; a > 0 ; a--)
{
input[a] = input[a-1];
}
input[0]=get_q7();
firFixed(input);
}
return 0;
}
But I don't understand how it's possibl to declare a fractional number as int which is done with the constants. It's supposed to be Q7 notation but why is it done this way and how can the compiler accept an int that is a fractional number? Will it simply take the integer part of the fraction and if so why isn't a cast required?
