Using the RC Hat
If you’re converting an off-the-shelf RC car to full autonomy with Donkeycar, one of the questions you may have is why you can’t use the RC controller the car came with instead of using a Bluetooth gaming joypad. The old answer was that the RC signal has to go through the RaspberryPi and be processed or bypassed by the Python code, depending on whether you’re in manual or autonomous mode, and there’s no good way to just insert the RaspberryPi into that process (even if you could plug your RC receiver cables into the RaspberryPi, it can’t properly handle those signals nor those necessary to drive your servo or motor controller output). Thus our old advice was to throw out your RC transmitter and receiver and use the gaming joypad instead along with an I2C servo controller board.
But now we have a better answer! The new RC Hat, which plugs on top of the Raspberry Pi, allows you to use your RC transmitter and receiver as is, without needing a joypad or servo driver board. Not only does that save you money, but the RC gear has way longer range, which is great for outdoors racing or racing around lots of other cars.
The RC hat consists of a circuit board that has a RaspberryPi 2040 Zero board on it, along with an OLED screen and the pins to plug in your RC receiver, your servos, as well as wheel encoders, other I2C devices and anything that needs 5v power. The 2040 board handles all the low-level reading of RC input and servo output and communicates with the RaspberryPi over hardware serial. The CircuitPython code for the RP2040 is here, although that’s already loaded on the board so you shouldn’t need to mess with it. 
Simply use female-to-female 3-wire RC cables to connect your RC receiver into the “In 1”, “In 2” and (if you have a third channel) “In 3” pin headers. Plug your steering servo into “Steer” and your motor controller/ESC into “Throt”. Make sure you get the orientations right — black (ground) should always be on the outside next to the board edge.
To use it the hat, you’ll need to edit your myconfig.py file in your mycar directory. Make the following changes:
To enable the OLED screen, uncomment these lines and change the values as shown in the relevant areas of the myconfig.py file:
USE_SSD1306_128_32 = TrueSSD1306_RESOLUTION = 2 To enable the RC hat as output, in the “Drive train” section, uncomment and edit to this:
DRIVE_TRAIN_TYPE = "MM1"To enable the RC hat as input, in the “Joystick” section, uncomment and edit to this:
USE_JOYSTICK_AS_DEFAULT = True
CONTROLLER_TYPE = 'MM1'In the “Robohat MM1 controller” section, uncomment and edit to this:
MM1_SERIAL_PORT = '/dev/ttyAMA0'Since you probably only have 2 or 3 channels on your RC gear, all other commands and mode changes will be done via the web interface (on joypads you use dedicated buttons) via Wifi. Instructions for that are here.
You can also set the car to show its IP address at startup on the OLED screen, assuming it’s connected to a Wifi network. Instructions for doing that are here. Otherwise, the screen will come on and show the Donkey state when you start Donkey via the usual “python manage.py drive” command.