STM32 I2C HAL FXOS8700 communication problem - arm

Question

I have a STM32F103RE MCU that is connected to a fxos8700cq sensor through I2C2 port.
I've set up the peripherals using stm32cubemx and I'm trying to simply test the communication with the sensor. But as I debug the code, it seems that data transfer is not happening correctly.
These are the i2c settings:
hi2c2.Instance = I2C2;
hi2c2.Init.ClockSpeed = 100000;
hi2c2.Init.DutyCycle = I2C_DUTYCYCLE_2;
hi2c2.Init.OwnAddress1 = 0;
hi2c2.Init.AddressingMode = I2C_ADDRESSINGMODE_7BIT;
hi2c2.Init.DualAddressMode = I2C_DUALADDRESS_DISABLE;
hi2c2.Init.OwnAddress2 = 0;
hi2c2.Init.GeneralCallMode = I2C_GENERALCALL_DISABLE;
hi2c2.Init.NoStretchMode = I2C_NOSTRETCH_DISABLE;
and here is the main code:
uint8_t send_buffer[1];
uint8_t receive_buffer[1];
uint8_t FXOS_ADDR = (FXOS8700_DEVICE_ADDR_SA_11 << 1);
send_buffer[0] = FXOS8700_WHO_AM_I;
receive_buffer[0] = 0;
// chip select for fxos
HAL_GPIO_WritePin(GPIOC, GPIO_PIN_2, GPIO_PIN_SET);
HAL_Delay(500);
HAL_GPIO_WritePin(GPIOC, GPIO_PIN_2, GPIO_PIN_RESET);
HAL_Delay(500);
ret = HAL_I2C_Master_Transmit(&hi2c2, FXOS_ADDR, send_buffer, 1, 100);
//ret = HAL_I2C_Master_Transmit_IT(&hi2c2, FXOS_ADDR, send_buffer, 1);
HAL_Delay(2);
ret = HAL_I2C_Master_Receive(&hi2c2, FXOS_ADDR, receive_buffer, 1, 100);
//ret = HAL_I2C_Master_Receive_IT(&hi2c2, FXOS_ADDR, receive_buffer, 1);
if (receive_buffer[0] == FXOS8700_WHO_AM_I_PROD_VALUE)
HAL_GPIO_WritePin(GPIOA, GPIO_PIN_8, GPIO_PIN_SET);
else
HAL_GPIO_WritePin(GPIOA, GPIO_PIN_10, GPIO_PIN_SET);
but after I debug the code, and watch the i2c registers after executing the HAL_I2C_Master_Transmit(&hi2c2, FXOS_ADDR, send_buffer, 1, 100); function I get these values. That means the TXE bit in SR1 register is not set.
I also tried using the interrupt mode but that's not working either. the values of i2c registers after the ret = HAL_I2C_Master_Transmit_IT(&hi2c2, FXOS_ADDR, send_buffer, 1); line are like this.
In all cases the functions return HAL_OK and there is no error.
I've tested the sensor and the board communication using mbed and it works fine. So the hardware has no problem.

Answer 1

You should not need to toggle a chip select when using the device's I2C interface.
The device's chip select signal is only for the SPI interface.
You should have the SA0 and SA1 pins pulled high or low to use the I2C interface and select the device's I2C address. See table 11 and section 10.2.3 in the datasheet.
HAL_I2C_Master_Transmit() and HAL_I2C_Master_Receive() are not the best functions for reading a register value from the device. These functions transmit a start condition, the device address, the data, and finally a stop condition. But this is not what the FXOS8700CQ datasheet recommends in section 10.1.2, "Single-byte read". It says transmit start condition, device address with W bit, register address, repeat-start condition (notice no stop before repeat start), device address with R bit, then the data, and finally a stop condition.
The HAL_I2C_Mem_Read() function can be used to perform the single-byte read as described in the datasheet. (i.e., write register address, repeat-start, read data.) Try something like this.
uint8_t data_byte = 0;
HAL_StatusTypeDef hal_status = HAL_I2C_Mem_Read(&hi2c2, FXOS_ADDR, FXOS8700_WHO_AM_I, I2C_MEMADD_SIZE_8BIT, &data_byte, 1, 100);
If that doesn't help, then check the return value from the HAL library calls.
If that doesn't help, then examine the I2C clock and data signals with a logic analyzer or oscilloscope.