File content as of revision 0:972f3778c19c:

#include <stdio.h>
#include "i2c_api.h"
#include "gpio_api.h"
#include "nrf_delay.h"

static i2c_t mbed_i2c_object = {0,};
PinName mbed_i2c_sda_pin = NC;
PinName mbed_i2c_scl_pin = NC;

void mbed_i2c_init(PinName sda, PinName scl)
{
    if (mbed_i2c_sda_pin == NC) {
        mbed_i2c_sda_pin = sda;
        mbed_i2c_scl_pin = scl;
    }

    i2c_init(&mbed_i2c_object, sda, scl);
    i2c_frequency(&mbed_i2c_object, 100000);
}

int mbed_i2c_write(unsigned char slave_addr,
                   unsigned char reg_addr,
                   unsigned char length,
                   unsigned char const *data)
{
    int i;
    slave_addr = slave_addr << 1;
    i2c_start(&mbed_i2c_object);
    i2c_byte_write(&mbed_i2c_object, slave_addr);
    i2c_byte_write(&mbed_i2c_object, reg_addr);
    for (i = 0; i < length; i++) {
        i2c_byte_write(&mbed_i2c_object, data[i]);
    }
    i2c_stop(&mbed_i2c_object);
    return 0;
}

int mbed_i2c_read(unsigned char slave_addr,
                  unsigned char reg_addr,
                  unsigned char length,
                  unsigned char *data)
{
    slave_addr = slave_addr << 1;
    i2c_write(&mbed_i2c_object, slave_addr, &reg_addr, 1, 0);
    return (i2c_read(&mbed_i2c_object, slave_addr, data, length, 1) == length) ? 0 : 1;
}

int mbed_i2c_clear(PinName sda, PinName scl)
{
    gpio_t sda_io, scl_io;

    if (mbed_i2c_object.i2c) {
        mbed_i2c_object.i2c->PSELSCL = 31;
        mbed_i2c_object.i2c->PSELSDA = 31;
    }

    gpio_init_in(&sda_io, sda);

    if (!gpio_read(&sda_io)) {
        printf("sda is always 0, i2c bus is not released\r\n");
        printf("try to clear i2c bus\r\n");
        gpio_init_out(&scl_io, scl);

        // Clock max 18 pulses worst case scenario(9 for master to send the rest of command and 9
        // for slave to respond) to SCL line and wait for SDA come high.
        for (int i = 0; i < 18; i++) {
            gpio_write(&scl_io, 0);
            nrf_delay_us(4);
            gpio_write(&scl_io, 1);
            nrf_delay_us(4);
        }

        if (!gpio_read(&sda_io)) {
            printf("warning! sda is still 0,, i2c bus is not released\r\n");
        }
    }


    if (mbed_i2c_object.i2c) {
        mbed_i2c_init(sda, scl);
    }

    return 0;
}

void mbed_i2c_enable(void)
{
    mbed_i2c_object.i2c->POWER = 1;

    mbed_i2c_init(mbed_i2c_sda_pin, mbed_i2c_scl_pin);
}

void mbed_i2c_disable(void)
{
    gpio_t sda_io, scl_io;

    mbed_i2c_object.i2c->PSELSCL = 31;
    mbed_i2c_object.i2c->PSELSDA = 31;


    mbed_i2c_object.i2c->ENABLE       = TWI_ENABLE_ENABLE_Disabled << TWI_ENABLE_ENABLE_Pos;
    mbed_i2c_object.i2c->POWER = 0;

    gpio_init_in(&sda_io, mbed_i2c_sda_pin);
    gpio_init_in(&scl_io, mbed_i2c_scl_pin);
}