Implementation
UART Communication
Setup Diagram
To enable UART connection, the RX/TX pins of the Datalogger are respectively connected to the TX/RX pins of the data-source device. To prevent interference, the devices' GRD have to be connected as well.
It is possible to connect the Datalogger to more than one UART source. The Datalogger cannot distinguish between the different sources and will treat all the messages the same.
Note: in a multiple-sources situation messages collision may occur.
Communication Interface
The Datalogger waits for incoming UART messages. Its settings are configured to baud-rate of 115200 and message size of a single measurement.
Message Handling
Upon incoming message, the UART callback is executed. Once arrived, the message is pushed to the queue and the green LED is toggeled. Following that, the semaphore upon which the UART task is pending on is posted.
The UART task dequeues one message at a time. Each message is then stored to the external flash memory.
External Flash Management
Data Storage
For data storage, the Datalogger uses the address space between 0x1000 and 0xFEFF0.
This address space is divisible by 8, which represents the measurement size in bytes, thus providing an aligned file structure.
We have not used the first sector since it contains OTP area.
The last sector is dedicated to Power Failure Resistance section (see below).
The address space stated above is considered as a single file, where data is written in a cyclic manner.
Power Failure Resistance
When the program is first uploaded to the MCU, we run a unique initialization code that sets the cyclic pointers, which are located in the last sector.
The pointers are updated upon either write operation or BLE indication acknowledgement, to provide power failure resistance.
BLE Data Transfer
Communication Interface
The Datalogger exposes a data service, which allows to download the stored data from the flash. The service comprises a single characteristic, with an "Indicate" property. Registration to this characteristic initiates the downloading of the data, which is being sent through indications.
Every indication has maximum length of 24 bytes, which cover 3 data measurements in our case, since every measurement is of 8 bytes size. As the default ATT_MTU value is 23 bytes, we increased the MAX_PDU_SIZE parameter, accordingly.
Data Integrity
By using indications, an acknowledgment is sent back for every indication received by the peer device. In our implementation, data integrity is guaranteed by sending the next indication only when the privious one has been acknowledged.
Moreover, data is being erased from the flash, only after it has been acknowledged.
Concretly, the first indication is sent immidiately after a registration event from the peer device. Every following indication would be sent after receiving a GATT event ATT_HANDLE_VALUE_CFM.
Security Measurements
Connecting to the Datalogger requires a pincode, which is hardcoded in the firmware. Any attempt to download the data without providing the pincode would fail, as the service characteristic requires an authenticated connection.
Authentication is required for per every connection to the Datalogger, as every bond of a peer device is erased upon a disconnection.
Datalogger supports only one connection at any given time.
External Data Source
Temperature Measurement
Every 10 seconds a clock interrupt awakens the MCU. Once up, a measurement of the surrounding temperature is taken, using the build-in BATMON module. To indicate temperature measuring, the red LED is toggled.
Message Structure
The message consists of two parts with a total length of 8 bytes.
The first 4 bytes represent the index assigned for the measurement. When the index reaches maximum value, it wraps-around back to zero.
The last 4 bytes are the temperature measurement, taken from BATMON module and converted to a string.
Full Documentation
You can find our project on GitHub
