mbed LPC1768 Digital Low-Pass Filter

#include "mbed.h"
#include <cstdlib>

// preprocessor directives
#define M 19                        // number of samples processed at any given time
#define N_SAMPLES 1000              // number of samples to output to file
#define SAMPLE_RATE 20000           // sample rate of the DAC
#define InOutMode 0                 // operation mode: 0 = DAC (real-time), 1 = file (non-real-time)

// object declarations
AnalogIn a_in(p15);                 // analog input pin
AnalogOut a_out(p18);               // analog output pin
DigitalOut d_out(p21);              // digital output pin
LocalFileSystem local("local");     // create local filesystem under "local"
Ticker trigger;

// function prototypes
void shiftLocalArrayElements();     // shifts x_local array elements up
void writeToLocalArray();           // writes the analog input pin value to the last element of x_local
double processInputSignal();        // applys filter processing to x_local, defined by the coefficients in b
void filter();                      // a consolidation of the functions used in the filtering process

// global variables
double x_local[M] = {0.0f};         // sample buffer array
double b[M] = {0.0f};               // weighting array of low-pass filter coefficients

int main()
{

    // initialise p21 as low (logic 0)
    d_out = 0;

    // low-pass filter coefficients
    b[0] = b[18] = -0.002693840;
    b[1] = b[17] = -0.002519748;
    b[2] = b[16] = 0.005014695;
    b[3] = b[15] = 0.015641050;
    b[4] = b[14] = 0.000000000;
    b[5] = b[13] = -0.046914239;
    b[6] = b[12] = -0.048021820;
    b[7] = b[11] = 0.083481298;
    b[8] = b[10] = 0.294332820;
    b[9] = 0.400000000;

// output to file
#if InOutMode

    printf("InOutMode 1: file\n");

    // create input and output signal files
    FILE *s_in = fopen("/local/signalInput.txt", "w");
    FILE *s_out = fopen("/local/signalOutput.txt", "w");

    // error checking
    if ((s_in == NULL) || (s_out == NULL)) {

        printf("Program terminated due to file error\n");
        exit(-1);
    }

    else {

        printf("Signal input and output files ready for write\n");
    }

    printf("Writing to files\n");

    // write input and output signals to files N_SAMPLES times
    for (int i = 0; i < N_SAMPLES; i++) {

        shiftLocalArrayElements();
        writeToLocalArray();

        double input_signal = x_local[M - 1];
        double output_signal = processInputSignal();

        fprintf(s_in, "%.4d %8.4f\n", i, input_signal);
        fprintf(s_out, "%.4d %8.4f\n", i, output_signal);
    };

    printf("Writes complete\n");
// close files
    fclose(s_in);
    fclose(s_out);
    printf("Files closed\n");

// output to DAC
#else

    printf("InOutMode 0: DAC\n");

    // attach filter function to the Ticker object; to be called every sample period
    trigger.attach(&filter, 1 / SAMPLE_RATE);

    // loop infinitely
    while(1) {

    }

#endif

    return 0;
}

void shiftLocalArrayElements()
{

    // shift index value i to i - 1
    for (int i = 1; i < M; i++) {

        x_local[i - 1] = x_local[i];
    }
}

void writeToLocalArray()
{

    // set p21 high (logic 1)
    d_out = 1;
    // write analog input pin value to the last element of x_local
    x_local[M - 1] = a_in.read();
    // set p21 low
    d_out = 0;
}

double processInputSignal()
{

    double y = 0.0f;

    // apply filtering on x_local elements, based on the coefficients stored in b
    for (int i = 0; i < M; i++) {

        y += b[i] * x_local[(M - 1) - i];
    }

    // return the filtered sample
    return y;
}

void filter()
{

    shiftLocalArrayElements();
    writeToLocalArray();
    double output_sample = processInputSignal();
    // write the filtered sample to the Analog output pin
    a_out.write(output_sample);
}