Circular Buffer FIR Filter

This FIR filter implements a circular buffer. Your own coefficients in yourfiltercoeffs.h can be created using fdatool in MATLAB.

DIP1 down = Unfiltered

DIP1 up = Filtered

// These coefficients are used to filter
#include "yourfiltercoeffs.h"

// scale factor, S_h = 2^K.
#define K	15

int	filterout = 0; // In fixed-point implementation, the accumulator is 32-bit int
short delay[LENGTH];  // Short is the 16-bit integer variable

interrupt void isr()	 // Interrupt function
{
	short 	i;
	int 	Sx = 1;
	
	//  Do the filtering as per fixfilt if DIP switch 1 up
	if (get_DIP1() == 1) {
		// Direct-Form FIR
		delay[0] = Sx * get_sample();     	// input for filter
		filterout = hMax[0] * delay[0];   	// Set up filter sum
		// Notice, C-arrays go from 0..LENGTH-1
		for (i = LENGTH-1; i > 0; i--){		// Get sum of products
     			filterout += hMax[i] * delay[i];	
		     	delay[i] = delay[i-1];      	// Renew input array
		}			
		filterout = (filterout>>K); //Move into the lower 16 bits, reverses scaling
			//Sign extension carries the upper bit down if negative
	 	send_output(filterout);		// output for filter
	} else { // If DIP switch 1 down, == 0, then just pass through signal.
		send_output(get_sample());
	}

 	return;						// return from interrupt
}

void main()
{
	short i;

 	for (i=0; i<= LENGTH-1; i++) { 
		delay[i] = 0;          	// init filter processing array
    	}
	init_all();            		// init all
	while(1);    		   	// infinite loop
}