Tiffany's Lab 2

Lab 2: Bluetooth

In our second lab, we establish and test the Bluetooth connection between the host laptop and the Artemis board and create a framework for bidirectional data transmission.

Prelab

The code for this lab was run on a virtual environment on my laptop titled "FastRobots_ble", and code for Bluetooth data transmittion was written in Arduino (for the Artemis board) and Jupyter notebook (on my laptop). The robot uses Bluetooth Low Energy, a wireless communication protocol designed for low power at low data rates. The Artemis board mainly acts as a peripheral device and my laptop as a central device, although both devices engage in bidirectional communication.

I ran the code from ble_arduino.ino which prints the MAC address of my Artemis board to the serial monitor:

Configurations

First, to establish a BLE connection between the Artemis board and my laptop, I configured the MAC address in the connections.yaml file to match the MAC address of my Artemis board. Then, I generated a unique UUID and configured the UUID of both the devices to match.

demo.ipynb

The demo.ipynb file provided at the start of the lab contains some helpful methods and explanations in addition to code that imports the necessary libraries and establishes a connection between my laptop and the Artemis board.

Send Echo Command

ECHO: When this command is sent to the Artemis, it echos the message I sent back through Bluetooth. For example, when I send "QUACK!" over Bluetooth, the robot says "QUACK!" back to me.

Get Time Command

GET_TIME_MILLIS: This command prompts the Artemis to write the time in milliseconds since startup to the string characteristic, where my laptop can read the time value. Time is expressed in the format T:666666, where 666666 represents the result of the millis() function call in Arduino.

Notification Handler

I wrote a notification handler called str_notify_handler() which handles new messages that the Artemis board writes to the BLEStringCharactersitic. When my laptop is notified of a new message, it will process the message as a time value and update the global time variable with the new updated time. This is more convenient and efficient compared to pollling for new messages using ble.receive_string(ble.uuid['RX_STRING']).

Get Temperature Command

Building off of the previous notification handler, I added functionality to handle time and temperature values. In the first video, a temperature value is transmitted every 1 second for an interval of 5 seconds. In the second video, temperature values are continuously sent at a rapid page for 5 seconds.

Limitations

When I was trying to send the 5 seconds of rapid temperature data at first, I kept running into an error. It turns out that the storage constraints of the Artemis board are an important aspect to consider when sending data. The Artemis board has 384 kB of RAM. Assuming that all of this RAM is being used towards data transmittion, that gives us 384,000 bytes of memory. If we also assume that we are transmitting 16-bit or 2-byte values at a frequency of 150 Hz, this means that every second we are sending 300 bytes/second, and in 5 seconds we use 5*300=1500 bytes. Dividing our total 384 kB by 1500 b, we get 256; thus, we can transmit 256 5-second intervals of data with our current memory limitations.

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