RPi Pico 2W — Using I2C

The Pico 2 can act as I2C master or I2C slave. The default setting is to be master. The SDK controls the I2C hardware and provides functions to configure I2C and read/write to a slave.

First, add the hardware_i2c library to CMakeLists.txt for I2C support.

target_link_libraries(my_project pico_stdlib hardware_i2c)

Now the steps are:

• Configure GPIO pin(s) for I2C.
• Set the bus speed.
• Read data from an I2C peripheral — in this example an LM75 temperature sensor, which uses two bytes to represent the temperature.

void configureI2C()
{
    uint32_t baudrate = 10000;
    i2c_init(i2c_default, baudrate);

    // configure GPIO pins
    // This example will use I2C0 on the default SDA and SCL pins (GP4, GP5 on a Pico)
    gpio_set_function(PICO_DEFAULT_I2C_SDA_PIN, GPIO_FUNC_I2C);
    gpio_set_function(PICO_DEFAULT_I2C_SCL_PIN, GPIO_FUNC_I2C);

    // set pull-up on pins
    gpio_pull_up(PICO_DEFAULT_I2C_SDA_PIN);
    gpio_pull_up(PICO_DEFAULT_I2C_SCL_PIN);
}

The i2c_read_blocking function can be used to read data from a peripheral into a buffer. It handles I2C START and STOP signalling internally.

Tip

Check the return value from the read function to know if data was read and can be used. The function returns PICO_ERROR_GENERIC for all errors, so set a breakpoint inside its implementation in i2c.c to investigate what the actual error was.

void readTempFromLM75(uint8_t address)
{
    // the temperature is two bytes
    uint8_t byte[2];

    int8_t rc; // to hold the return code

    rc = i2c_read_blocking(i2c_default, address, &(byte[0]), 2, false);

    if (rc != PICO_ERROR_GENERIC)
    {
        printf("byte0: %d, byte1: %d\r\n", byte[0], byte[1]);
    }
    else
    {
        printf("I2C error, return value was: %d\r\n", rc);
    }
}