Quad-encoder monitoring code

encoder.c

#define ENC_INPUT_A 0
#define ENC_INPUT_B 1
#define ENC_INPUT_I 2

typedef struct enc_state {
    int8_t idx, ab;
    int8_t dir;
    int32_t pos_count;
    int32_t isr_count;
    int8_t enc_error_flag;
    int32_t enc_ppr;
    int32_t update_period_ms;
    int32_t pulse_freq;
} enc_state_t;

volatile enc_state_t cur_state;

int8_t enc_matrix[4][4] = 
{
    {0, -1, 1, 2},
    {1, 0, 2, -1},
    {-1, 2, 0, 1},
    {2, 1, -1, 0},
};

/**
 * called every time the XOR signal rises, the isr
 * freq is 2 times pulse freuency
 */
void enc_xor_isr()
{
    int8_t prev_ab;
    isr_count++;

    prev_ab = cur_state.ab;
    cur_state.ab = (read_input(ENC_INPUT_A) << 1) | read_input(ENC_INPUT_B);

    if (enc_matrix[prev_ab][cur_state.ab] == 2)  {
        enc_error_flag = 1;
    } else if (enc_matrix[cur_state.ab][ab] != 0) {
        dir = enc_matrix[cur_state.ab][ab];
        pos_count += enc_matrix[cur_state.ab][ab];
    }
}

/* this function should be called at a fixed update interval
 *  f = isr_count / 2T;
 */
void enc_update()
{
    cur_state.pulse_freq = (isr_count * 500)/ cur_state.update_period_ms;
}