Ex-25: Inter-Integrated Circuit (I²C) Logger¶

In this exercise you will implement a temperature logger based on the LM75 Digital Temperature Sensor. The sensor data is read out via the I²C bus. The logger can be controlled by your computer via UART.

Description¶

Reading a sensor with an MCU and transmitting the data to a computer is a common task in practice. The MCU collects all the data from the sensors and sends it as a packet to the host. The host can set some parameters in the device. This scenario has to be implemented in this exercise.

UART is used as the communication method between the computer and the MCU. The MCU reads the LM75 Digital Temperature Sensor on the mbed application shield using I²C. When the reading is complete, the result is converted into a format that can be further processed by the MCU. Finally, the converted result is sent to the host.

The user can set the readout frequency and the accuracy of the received data via the host computer. For practical reasons, it is also necessary to be able to return the current settings.

Tasks¶

There are three steps to be taken to complete this exercise:

I²C configuration¶

When using I²C in a system, the two most important configurations are the timing behaviour and the address management. The specifications and information can be found in the datasheet.

What is still missing is which GPIO pins and I²C peripherals are used for the sensor. This can be determined by reading the schematic and checking which alternative function can be enabled on these pins for an I²C peripheral.

With this information the I²C configuration can be implemented.

I²C configuration

Transmission of data at regular intervals¶

There are several ways to process a task with a specific period. It is desired that your solution does not block the system. You have learnt several methods to solve this problem, choose one of your choice.

In the I²C bus system, our MCU works as a master to read the temperature data. To interact with the registers in the sensor, several methods are needed. The datasheet LM75 starting on page 14 describes these possibilities.

HAL provides an abstraction of these functionalities with the write and read functions. These have been implemented in the I2C input and output operation function group.

The received temperature data is represented as an 11 bit 2’s complement value. This needs to be converted into a format that can be handled by the MCU. Finally, the converted data can be sent to the computer via the UART. It is up to you how to send the message.

UART message protocol¶

There is no default standard for communicating with the UART bus. This must be designed and implemented by the developer. This will result to optimised communication for his device, but also requires a well-documented and bug-free implementation, which is difficult.

For this exercise, you are free to choose how this communication protocol should look like. The communication only has to meet the following requirements:

• The receiving part must not block the system

• The following commands and responses are expected

  • Set readout frequency

    \(\rightarrow\) Returns the current readout frequency as an acknowledgement, e.g. set to 0.1, 0.5, 1, 2 … Hz.

  • Set current data accuracy

    \(\rightarrow\) Returns the current accuracy as a confirmation, e.g. set to 1, 0.5, 0.25 … °C.

  • Get current settings

    \(\rightarrow\) Returns current frequency and accuracy.

• Return an error message if the message could not be read.

A basic solution is given below for those who need some assistance with this.

/**
 * @brief Stores the incoming character from UART.
 *
 */
typedef struct {
    char messageBuffer[128]; ///< message buffer to collect the messages
    bool processMessage;     ///< flag to process the message in the superloop
    uint32_t curPosition;    ///< stores the current position to add the new character
    char rxData;             ///< stores the current received character from UART
} uartRxHandlerType;

Implementation¶