MAX31855 no-OS Driver

Supported Devices

• MAX31855

Overview

The MAX31855 is a thermocouple-to-digital converter that provides cold-junction compensation and digitizes signals from K-, J-, N-, T-, S-, R-, or E-type thermocouples. It delivers data in a 14-bit, SPI-compatible, read-only format with 0.25°C resolution. The device measures temperatures between -270°C to +1800°C, achieving ±2°C accuracy for K-type thermocouples within -200°C to +700°C. Fault detection capabilities include sensing open thermocouples and short circuits to GND or VCC, ensuring reliability in industrial settings such as HVAC systems and process control.

Applications

• HVAC systems

• Process control equipment

• Industrial temperature monitoring

Operating Modes

Mode Name	Description	Configuration Bits	Typical Use Case
Normal Temperature	Reads temperature from thermocouple and board’s internal temperature sensor	IIO_TEMP address: 0 & 2	Typical industrial measurement and monitoring
Fault Detection	Checks for faults such as open thermocouple, short circuits to GND/ VCC	MAX31855 _GET_FAULT_BIT	Ensures reliability and safety in monitoring systems

Device Configuration

Initialization

The max31855_init function sets up the device descriptor and SPI communication for the MAX31855. It handles memory allocation and SPI initialization to ensure the system can interface with the thermocouple sensor.

Resource Management

max31855_remove handles the deallocation of resources used by MAX31855. It frees all allocated memory to maintain system stability and prevent memory leaks.

Data Reading

The max31855_read_raw function retrieves raw data from the MAX31855 register using SPI. It checks and filters data for faults before processing or returning it.

Temperature Conversion

max31855_read_temp processes raw register data to provide thermocouple and internal temperatures in Celsius. It includes sign extension and scaling for accurate temperature measurement, enabling easy application and display.

Driver Initialization Example

int basic_example_main()
{
    struct max31855_decimal thermocouple_temp;
    struct max31855_decimal ambient_temp;
    struct max31855_dev *dev;
    int ret;

    ret = max31855_init(&dev, &max31855_ip);
    if (ret)
        goto error;

    while (1) {
        pr_info("%c", 27);
        pr_info("%c", '[');
        pr_info("%c", '2');
        pr_info("%c", 'J');

        ret = max31855_read_temp(dev, &thermocouple_temp, &ambient_temp);
        if (ret)
            goto free_dev;

        pr_info("Thermocouple temperature %.02d.%.02d\n", thermocouple_temp.integer,
                thermocouple_temp.decimal);
        pr_info("Ambient temperature %.02d.%.02d\n", ambient_temp.integer,
                ambient_temp.decimal);

        no_os_mdelay(2000);
    }

free_dev:
    max31855_remove(dev);
error:
    pr_info("Error!\r\n");
    return 0;
}

IIO Device Configuration

Initialization and Resource Management

The max31855_iio_init initializes the MAX31855 IIO driver by setting up the necessary device descriptor infrastructure, including memory allocation and device configuration, and associates the iio_device structure with the device descriptor. The max31855_iio_remove handles cleanup, freeing resources allocated during initialization.

Data Handling and Reading

The max31855_iio_read_raw function manages fetching raw temperature data from the MAX31855 device, processes it, and formats the data into the provided buffer. max31855_iio_read_scale handles scale attribute requests, setting and formatting scale values for the channels to ensure accurate sensor reading interpretation.

Channel Management

The max31855_iio_update_channels updates active channels within the MAX31855 IIO device using a bitmask to modify the internal configuration and recalculate the count of active channels, allowing selective monitoring and data acquisition.

IIO Initialization Example

#include "iio_example.h"
#include "common_data.h"

#define DATA_BUFFER_SIZE 400

uint8_t iio_data_buffer[DATA_BUFFER_SIZE * sizeof(int32_t)];

int iio_example_main() {
    int ret;
    struct max31855_iio_dev *max31855_iio_dev;
    struct max31855_iio_dev_init_param max31855_iio_ip;
    struct iio_app_desc *app;
    struct iio_data_buffer dbuff = {
        .buff = (void *)iio_data_buffer,
        .size = DATA_BUFFER_SIZE * sizeof(int32_t)
    };

    struct iio_app_init_param app_init_param = {0};

    max31855_iio_ip.max31855_dev_init = &max31855_ip;
    ret = max31855_iio_init(&max31855_iio_dev, &max31855_iio_ip);
    if (ret)
        return ret;

    struct iio_app_device iio_devices[] = {
        {
            .name = "maxim_thermocouple",
            .dev = max31855_iio_dev,
            .dev_descriptor = max31855_iio_dev->iio_dev,
            .read_buff = &dbuff,
        },
    };

    app_init_param.devices = iio_devices;
    app_init_param.nb_devices = NO_OS_ARRAY_SIZE(iio_devices);
    app_init_param.uart_init_params = uip;

    // Initialize IIO application
    ret = iio_app_init(&app, &app_init_params);
    if (ret)
        goto error_remove_device;

    // Run the IIO application
    ret = iio_app_run(app);
    if (ret)
        goto error_remove_app;

    return ret;

error_remove_app:
    iio_app_remove(app);
error_remove_device:
    max31855_iio_remove(max31855_iio_dev);

    return ret;
}