Trouble using I2C on ESP32 to read temperature/humidity sensor (SI7020)

Question

I am trying to read a temperature and humidity sensor (SI7020) using I2C on the ESP32.

I modified their example code to read just from one slave device (0x40) but some commands do not work. I tried resetting the device with 0xFE as well.

I can read the device ID (0xFA):

*******************
TASK[0]  MASTER READ SENSOR( SI7020 )
*******************
data_h: 48
data_l: 52
sensor val: 15428.333333

However if I try and read the temperature (0xE3), I get:

data_h: 00
data_l: 00

If I use 0xE0 I always get:

data_h: 39
data_l: 9f

Why am I not able to read the temperature from SI7020?

The code I'm using:

#include <stdio.h>
#include "driver/i2c.h"


#define DATA_LENGTH          512  /*!<Data buffer length for test buffer*/
#define RW_TEST_LENGTH       127  /*!<Data length for r/w test, any value from 0-DATA_LENGTH*/
#define DELAY_TIME_BETWEEN_ITEMS_MS   2000 /*!< delay time between different test items */

#define I2C_EXAMPLE_MASTER_SCL_IO    19    /*!< gpio number for I2C master clock */
#define I2C_EXAMPLE_MASTER_SDA_IO    18    /*!< gpio number for I2C master data  */
#define I2C_EXAMPLE_MASTER_NUM I2C_NUM_1   /*!< I2C port number for master dev */
#define I2C_EXAMPLE_MASTER_TX_BUF_DISABLE   0   /*!< I2C master do not need buffer */
#define I2C_EXAMPLE_MASTER_RX_BUF_DISABLE   0   /*!< I2C master do not need buffer */
#define I2C_EXAMPLE_MASTER_FREQ_HZ    100000     /*!< I2C master clock frequency */

#define SI7020_SENSOR_ADDR  0x40    /*!< slave address for SI7020 sensor */
#define SI7020_CMD_READ    0xE0    /*!< Command to set measure mode */
//#define ESP_SLAVE_ADDR 0x28         /*!< ESP32 slave address, you can set any 7bit value */
#define WRITE_BIT  I2C_MASTER_WRITE /*!< I2C master write */
#define READ_BIT   I2C_MASTER_READ  /*!< I2C master read */
#define ACK_CHECK_EN   0x1     /*!< I2C master will check ack from slave*/
#define ACK_CHECK_DIS  0x0     /*!< I2C master will not check ack from slave */
#define ACK_VAL    0x0         /*!< I2C ack value */
#define NACK_VAL   0x1         /*!< I2C nack value */

xSemaphoreHandle print_mux;

/**
 * @brief test code to write esp-i2c-slave
 *
 * 1. set mode
 * _________________________________________________________________
 * | start | slave_addr + wr_bit + ack | write 1 byte + ack  | stop |
 * --------|---------------------------|---------------------|------|
 * 2. wait more than 24 ms
 * 3. read data
 * ______________________________________________________________________________________
 * | start | slave_addr + rd_bit + ack | read 1 byte + ack  | read 1 byte + nack | stop |
 * --------|---------------------------|--------------------|--------------------|------|
 */
static esp_err_t i2c_example_master_sensor_test(i2c_port_t i2c_num, uint8_t* data_h, uint8_t* data_l)
{
    i2c_cmd_handle_t cmd = i2c_cmd_link_create();
    i2c_master_start(cmd);
    i2c_master_write_byte(cmd, SI7020_SENSOR_ADDR << 1 | WRITE_BIT, ACK_CHECK_DIS);
    i2c_master_write_byte(cmd, SI7020_CMD_READ, ACK_CHECK_EN);
    i2c_master_stop(cmd);
    int ret = i2c_master_cmd_begin(i2c_num, cmd, 1000 / portTICK_RATE_MS);
    i2c_cmd_link_delete(cmd);
    if (ret == ESP_FAIL) {
        return ret;
    }
    vTaskDelay(1000 / portTICK_RATE_MS);

    cmd = i2c_cmd_link_create();
    i2c_master_start(cmd);
    i2c_master_write_byte(cmd, SI7020_SENSOR_ADDR << 1 | READ_BIT, ACK_CHECK_DIS);
    i2c_master_read_byte(cmd, data_h, ACK_VAL);
    i2c_master_read_byte(cmd, data_l, NACK_VAL);
    i2c_master_stop(cmd);
    ret = i2c_master_cmd_begin(i2c_num, cmd, 1000 / portTICK_RATE_MS);
    i2c_cmd_link_delete(cmd);
    if (ret == ESP_FAIL) {
        return ESP_FAIL;
    }
    return ESP_OK;
}

/**
 * @brief i2c master initialization
 */
static void i2c_example_master_init()
{
    int i2c_master_port = I2C_EXAMPLE_MASTER_NUM;
    i2c_config_t conf;
    conf.mode = I2C_MODE_MASTER;
    conf.sda_io_num = I2C_EXAMPLE_MASTER_SDA_IO;
    conf.sda_pullup_en = GPIO_PULLUP_ENABLE;
    conf.scl_io_num = I2C_EXAMPLE_MASTER_SCL_IO;
    conf.scl_pullup_en = GPIO_PULLUP_ENABLE;
    conf.master.clk_speed = I2C_EXAMPLE_MASTER_FREQ_HZ;
    i2c_param_config(i2c_master_port, &conf);
    i2c_driver_install(i2c_master_port, conf.mode,
                       I2C_EXAMPLE_MASTER_RX_BUF_DISABLE,
                       I2C_EXAMPLE_MASTER_TX_BUF_DISABLE, 0);
}

static void i2c_test_task(void* arg)
{
    int i = 0;
    int ret;
    uint32_t task_idx = (uint32_t) arg;
    uint8_t* data = (uint8_t*) malloc(DATA_LENGTH);
    uint8_t sensor_data_h, sensor_data_l;

    while (1) {
        ret = i2c_example_master_sensor_test( I2C_EXAMPLE_MASTER_NUM, &sensor_data_h, &sensor_data_l);
        xSemaphoreTake(print_mux, portMAX_DELAY);
        printf("*******************\n");
        printf("TASK[%d]  MASTER READ SENSOR( SI7020 )\n", task_idx);
        printf("*******************\n");
        if (ret == ESP_OK) {
            printf("data_h: %02x\n", sensor_data_h);
            printf("data_l: %02x\n", sensor_data_l);
            printf("sensor val: %f\n", ( sensor_data_h << 8 | sensor_data_l ) / 1.2);
        } else {
            printf("No ack, sensor not connected...skip...\n");
        }
        xSemaphoreGive(print_mux);
        vTaskDelay(( DELAY_TIME_BETWEEN_ITEMS_MS * ( task_idx + 1 ) ) / portTICK_RATE_MS);

        //---------------------------------------------------
        for (i = 0; i < DATA_LENGTH; i++) {
            data[i] = i;
        }
    }
}

void app_main()
{
    print_mux = xSemaphoreCreateMutex();
    //i2c_example_slave_init();
    i2c_example_master_init();

    xTaskCreate(i2c_test_task, "i2c_test_task_0", 1024 * 2, (void* ) 0, 10, NULL);
    //xTaskCreate(i2c_test_task, "i2c_test_task_1", 1024 * 2, (void* ) 1, 10, NULL);
}

Answer 1

Your oscillogram shows an isolated read transaction, while the datasheet shows a write transaction prefixing a restart and the read transaction. I have already see some buggy I2C devices that only implement a subset of the possible I2C transactions, could be the case for this one. I suggest to try a transaction that look exactly as the datasheet show.

As I understand, you should use the 0xF3 command multiple times instead of a single 0xE0.