USB-I2C/firmware/i2c.c

#include <avr/io.h>
#include <util/delay_basic.h>
#include <stdbool.h>
#include "i2c.h"

#define SCL_BIT PD3
#define SDA_BIT PD2
#define SCL_M   (1 << SCL_BIT)
#define SDA_M   (1 << SDA_BIT)

/* Open drain: PORT bits stay 0, DDR=1 drives low, DDR=0 releases (pull-up). */
#define SDA_LOW()  (DDRD |=  SDA_M)
#define SDA_HIGH() (DDRD &= ~SDA_M)
#define SCL_LOW()  (DDRD |=  SCL_M)
#define SCL_HIGH() (DDRD &= ~SCL_M)

static uint8_t dly = 4;          /* default ~400 kHz at 16 MHz - verify with a scope */
static bool    bus_err;

static inline void d(void) { if (dly) _delay_loop_1(dly); }

void i2c_set_delay(uint8_t v) { dly = v; }

void i2c_init(void)
{
    PORTD &= ~(SCL_M | SDA_M);
    DDRD  &= ~(SCL_M | SDA_M);
}

static void scl_release(void)    /* release SCL and honour clock stretching */
{
    SCL_HIGH();
    if (!(PIND & SCL_M)) {
        uint16_t t = 0;          /* ~16 ms max */
        while (!(PIND & SCL_M)) if (--t == 0) { bus_err = true; break; }
    }
}

static void start(void)
{
    SDA_HIGH(); d();
    scl_release(); d();
    SDA_LOW(); d();
    SCL_LOW();
}

static void stop(void)
{
    SDA_LOW(); d();
    scl_release(); d();
    SDA_HIGH(); d();
}

static bool wr(uint8_t b)        /* returns true on ACK */
{
    for (uint8_t i = 0; i < 8; i++) {
        if (b & 0x80) SDA_HIGH(); else SDA_LOW();
        b <<= 1;
        d(); scl_release(); d(); SCL_LOW();
    }
    SDA_HIGH();
    d(); scl_release(); d();
    bool ack = !(PIND & SDA_M);
    SCL_LOW();
    return ack;
}

static uint8_t rd(bool ack)
{
    uint8_t v = 0;
    SDA_HIGH();
    for (uint8_t i = 0; i < 8; i++) {
        d(); scl_release(); d();
        v = (v << 1) | ((PIND & SDA_M) ? 1 : 0);
        SCL_LOW();
    }
    if (ack) SDA_LOW(); else SDA_HIGH();
    d(); scl_release(); d(); SCL_LOW();
    SDA_HIGH();
    return v;
}

/* Diagnostic: optionally enable weak internal pull-ups, return bit0=SCL, bit1=SDA levels */
uint8_t i2c_lines(uint8_t pullup)
{
    if (pullup) PORTD |= (SCL_M | SDA_M); else PORTD &= ~(SCL_M | SDA_M);
    DDRD &= ~(SCL_M | SDA_M);
    d(); d();
    uint8_t p = PIND;
    return ((p & SCL_M) ? 1 : 0) | ((p & SDA_M) ? 2 : 0);
}

uint8_t i2c_recover(void)
{
    bus_err = false;
    for (uint8_t i = 0; i < 9 && !(PIND & SDA_M); i++) {
        SCL_LOW(); d(); d();
        scl_release(); d(); d();
    }
    stop();
    return (bus_err || !(PIND & SDA_M) || !(PIND & SCL_M)) ? I2C_BUS_ERR : I2C_OK;
}

uint8_t i2c_xfer(uint8_t addr, const uint8_t *w, uint8_t wlen, uint8_t *r, uint8_t rlen)
{
    uint8_t st = I2C_OK;
    bus_err = false;

    if (!(PIND & SDA_M) || !(PIND & SCL_M)) {
        if (i2c_recover() != I2C_OK) return I2C_BUS_ERR;
    }

    if (wlen || !rlen) {
        start();
        if (!wr(addr << 1)) { st = I2C_NACK_ADDR; goto done; }
        for (uint8_t i = 0; i < wlen; i++)
            if (!wr(w[i])) { st = I2C_NACK_DATA; goto done; }
    }
    if (rlen) {
        start();                               /* (repeated) START */
        if (!wr((addr << 1) | 1)) { st = I2C_NACK_ADDR; goto done; }
        for (uint8_t i = 0; i < rlen; i++)
            r[i] = rd(i < rlen - 1);
    }
done:
    stop();
    return bus_err ? I2C_BUS_ERR : st;
}