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The Brain (ESP32-S2)

The ESP32-S2 runs the meltybrain control loop. You program it over the USB-C port with the Arduino IDE. The first upload needs a boot jumper. After that, uploads are one click.

1. Install the tools

  1. Install Arduino IDE 2.

  2. Open File → Preferences and add this to Additional boards manager URLs:

    https://espressif.github.io/arduino-esp32/package_esp32_index.json

  3. Open Tools → Board → Boards Manager, search for esp32, and install esp32 by Espressif Systems.

2. Board settings

Select Tools → Board → esp32 → ESP32S2 Dev Module, then set:

Tools menu item Value
USB CDC On Boot Enabled
Upload Mode Internal USB
USB DFU On Boot Disabled
USB Firmware MSC On Boot Disabled
Flash Size 4MB (32Mb)
Partition Scheme Default 4MB with spiffs
PSRAM Disabled
CPU Frequency 240MHz
Upload Speed 921600

USB CDC On Boot is what makes Serial print over the USB port and lets the IDE reset the board into the bootloader for you. Leave it enabled in every sketch you upload.

3. First flash

A fresh ESP32-S2 has no firmware that knows how to reset itself, so you force it into download mode by hand once.

  1. Short S2_BOOT (J1 pin 5, or test pad TP4) to GND (J1 pin 1, or TP3). A jumper wire in the JST-SH cable works, as does a pair of tweezers held across the pads while you plug in.
  2. Plug in USB-C. The board enumerates as a new serial port.
  3. In the IDE, pick that port under Tools → Port.

  4. Open File → Examples → 01.Basics → Blink and change the LED pin to the green status LED:

    #define LED_BUILTIN 41

  5. Click Upload.

  6. Unplug USB, remove the boot jumper, and plug USB back in.
  7. The green LED blinks.

From now on you can upload without the jumper. The running sketch's USB CDC port lets the IDE reboot the chip into the bootloader.

When you need the jumper again

Use the boot jumper any time the port disappears or an upload fails to connect. Typical causes are a sketch that crashes before USB comes up, or a sketch built with USB CDC On Boot disabled. The jumper always works because it bypasses whatever is in flash.

Reset after the first flash

After a jumper-assisted upload the board does not restart on its own. Unplug and replug USB. Uploads done without the jumper reset automatically.

4. Install SimpleMelt

  1. Open Tools → Manage Libraries, search for SimpleMelt, and install it. Accept the prompt to install its dependencies, AlfredoCRSF and SparkFun LIS331 Accelerometers.
  2. Open File → Examples → SimpleMelt → Rotini-Example.

If SimpleMelt is not in the Library Manager yet, clone AlfredoSystems/SimpleMelt into your Arduino/libraries folder and install the two dependencies from the Library Manager.

5. Configure and upload the example

The pin constants at the top of Rotini-Example.ino already match Rotini V3. The values you tune for your robot are in setup():

Field Meaning
melty_led_offset_CW / _CCW Angle in radians between the accelerometer and the heading LED, for each spin direction
turn_speed Heading rotation rate in rotations per second at full stick
accelerometer_radius Distance from the spin axis to the accelerometer, in metres
radius_trim Fine adjustment to the radius, also adjustable from the transmitter

Upload it exactly as you uploaded Blink. Open Tools → Serial Monitor at 115200 baud to see output.

The example expects the receiver to be running ExpressLRS. Once the S2 is programmed, flash the receiver through it as described in The Receiver. Without a link, the robot stays in the disconnected state and the motors are held off.

Motors off while programming

Disconnect ESC power or remove the weapon and drive motors before uploading anything. The FOO and BAR pins float while the bootloader runs, and a badly behaved ESC can read that as a command.

Transmitter mapping used by the example

The example was written for a RadioMaster Zorro. Channels are CRSF channel numbers.

Control Channel Action
Right stick X 1 Rotation
Left stick Y 3 Throttle
SWA 6 Back: stop, middle: arcade, forward: melty
SWD 5 Spin direction
SWB 12 Selects what the left arrows trim: radius, CW LED offset, CCW LED offset
Bumpers 11 Spin power down / up in 2 % steps
Right arrows 7, 8 Spin power presets: 0 %, 18 %, 100 %
Left arrows 9, 10 Trim the value selected by SWB

Troubleshooting

Symptom Fix
No serial port appears Try another cable, some are power only. Then use the boot jumper
Port appears only with the jumper The sketch in flash is not bringing up USB. Check USB CDC On Boot is enabled and re-upload
Upload fails with No serial data received Board is not in download mode. Use the boot jumper
Serial.print shows nothing USB CDC On Boot is disabled, or the Serial Monitor is on the wrong port
Board resets when the ESC arms Pack voltage spike. See the power limits on the Overview