Crawler schematic

MITE - SCX24 Electronic Speed Controller

MITE (Micro Inline Terrain ESC)

A from-scratch Electronic Speed Controller for SCX24 rock crawlers, built around the STM32F411. It has two parts:

• Crawler: receives commands over a 2.4 GHz nRF24L01+ link and drives a brushed 050 motor through a TB6612FNG H-bridge plus a steering servo. Its custom board is MITE v1, shown above.
• Controller: a handheld transmitter that reads a 2-axis joystick and sends throttle and steering to the crawler.

Both firmwares are bare-metal C++17: no vendor HAL and no CubeMX-generated code. They're built on a register-level HAL (STM32-HAL-Library, included as a submodule) and a custom nRF24L01+ driver.

Status

• Crawler and controller firmware run on prototype (two Black Pill boards).
• MITE v1 (custom crawler PCB): layout about 90% done, antenna needs impedance matching, then going through CDR, not yet fabricated. The firmware pin map has to be updated for the new board (see Pin maps).
• Controller PCB: schematic captured, layout not started.

Contents

• Repository layout
• Building and flashing
• Radio link
• Crawler control behavior
• Pin maps
• Hardware: MITE v1 crawler board
• Hardware: controller
• First power-up checklist (MITE v1)
• Roadmap
• License

Repository layout

.
├── controller/            # Transmitter firmware (joystick → nRF24 TX)
│   ├── main.cpp
│   └── startup.cpp        # Vector table + Reset_Handler
├── crawler/               # Receiver firmware (nRF24 RX → motor + servo)
│   ├── main.cpp
│   └── startup.cpp
├── drivers/
│   ├── include/nrf24.hpp  # nRF24L01+ driver
│   └── src/nrf24.cpp
├── hal_lib/               # Submodule: SySch415/STM32-HAL-Library (GPIO, RCC, SPI, ADC, TIM)
├── hardware/
│   ├── crawler_schematic/     # KiCad project: MITE v1 crawler board
│   ├── controller_schematic/  # KiCad project: controller
│   └── original-schematic-STM32F411CEU6_WeAct_Black_Pill_V2.0.pdf  # reference
├── docs/images/           # README images
├── linker.ld              # STM32F411CE: 512 KB flash, 128 KB SRAM
└── CMakeLists.txt         # Builds controller.elf and crawler.elf

Building and flashing

Requirements

• arm-none-eabi-gcc toolchain
• CMake ≥ 3.20
• OpenOCD and an ST-Link (V2 or clone)

Clone

The HAL is a git submodule, so clone recursively:

git clone --recursive https://github.com/SySch415/stm32-crawler.git
# or, in an existing clone:
git submodule update --init

Build

cmake -B build
cmake --build build

This produces controller.elf/.bin and crawler.elf/.bin in build/.

Flash

With the ST-Link connected to the target's SWD pins:

cmake --build build --target flash-crawler
cmake --build build --target flash-controller

Each target erases, writes to 0x08000000, verifies, and resets the MCU through OpenOCD.

Radio link

The controller transmits continuously, and the crawler listens on pipe 0.

Payload format (little-endian):

Crawler control behavior

Throttle → TB6612FNG channel A

• A dead band of ±100 counts around center (2048) gives neutral: AIN1 = AIN2 = high and PWM = 0, which puts the bridge in short brake. That's what keeps a crawler from rolling back down a slope.
• Above the dead band: forward (AIN1 high, AIN2 low), with duty scaled to the distance from center.
• Below the dead band: reverse (AIN1 low, AIN2 high).
• Motor PWM: timer ARR = 15999, prescaler = 0.

Steering → servo

• Standard RC servo frame of about 50 Hz, with a pulse of about 1.0–2.0 ms mapped linearly from the steering axis.
• Within the steering dead band the servo holds center (1.5 ms).

Status LEDs (prototype firmware): throttle forward/reverse and steering left/right indicators.

Pin maps

Prototype (Black Pill), matching the current firmware

Crawler

Controller

MITE v1 board (as routed, pending firmware update)

Several pins moved during PCB layout. The crawler firmware has to be updated before it runs on MITE v1.

Hardware: MITE v1 crawler board

A 40 × 50 mm, 4-layer board that replaces the prototype's Black Pill, nRF module, H-bridge breakout and regulators. The KiCad project is in hardware/crawler_schematic/.

Main parts

Power architecture

flowchart LR
    BAT[2S LiPo] --> FET[AO4407A<br/>reverse polarity]
    FET --> BULK[47 µF bulk cap<br/>star point]
    BULK --> HB[TB6612FNG VM] --> MOT[050 motor]
    BULK --> B5[RY8336 → 5 V] --> SERVO[Steering servo]
    BULK --> B3[RY8336 → 3.3 V]
    B3 --> MCU[STM32F411]
    B3 --> RF[nRF24L01+]

Why it's arranged this way:

• The servo and the logic run on separate rails. On the prototype, servo current spikes and radio TX bursts browned out the shared 3.3 V supply. The servo now has its own 5 V buck, and the 3.3 V buck runs straight from the battery.
• The STM32 and nRF share one 3.3 V rail, so neither chip can be back-powered through the SPI pins during power-up or power-down.
• The battery supply splits at the bulk capacitor (star point), so motor switching current doesn't share copper with the buck inputs.

Stackup and layout rules

JLCPCB JLC04161H-3313, 1.6 mm, impedance controlled.

RF notes

• The antenna has a copper keepout on all four layers. Only the feed trace enters it, on F.Cu.
• Following Nordic's product specification, the nRF24L01+ exposed die pad is left unconnected. Ground goes through the VSS pins, each with its own via.
• Do a range test against a known-good nRF24 module before final assembly.

Fabrication and assembly

• Fab: JLCPCB, 4 layers, 1.6 mm, JLC04161H-3313 with impedance control, with a top-side stencil.
• Assembly: in-house. Solder paste and stencil, hot-plate reflow, top side only. Headers are hand-soldered afterward. QFN bridges are reworked with hot air.
• The Interactive HTML BOM KiCad plugin is used as the placement map.

SWD header (J3)

Hardware: controller

Currently a prototype-level build. The KiCad schematic is in hardware/controller_schematic/, and the PCB isn't laid out yet.

First power-up checklist (MITE v1)

1. Inspect under magnification: QFN bridges, and the orientation of the electrolytic cap, MOSFET and LEDs.
2. Power it from a current-limited bench supply (about 7.4 V, 200 mA limit) before connecting a LiPo.
3. Measure the rails: 3.3 V and 5 V.
4. Measure STM32 VCAP_1 ≈ 1.2 V, which confirms the internal regulator is running.
5. Connect the ST-Link and check that the MCU is detected (openocd -f interface/stlink.cfg -f target/stm32f4x.cfg).
6. Bring up the subsystems one at a time: radio, then servo, then motor (wheels off the ground).

Roadmap

• Impedance match the antenna (add pi network??)
• Finish MITE v1 routing and DRC, then fabricate
• Failsafe: stop the motor and center the steering if no packet arrives within a timeout
• Range test of the chip antenna vs. an nRF24 module
• Controller PCB layout
• Battery voltage monitoring and low-voltage cutoff

License

Firmware (controller/, crawler/, drivers/): MIT © 2026 Simon Schmidt
Hardware design files (hardware/): CERN-OHL-P v2 © 2026 Simon Schmidt

The Kovali name and logo are trademarks of Kovali, a California limited liability company. All rights reserved.

The ARCS name and logo belong to Avionics, Robotics, and Control Systems, a nonprofit corporation and IRS-recognized 501(c)(3) public charity, and are used with permission.

If you build, modify, or distribute this board, please remove both logos from the silkscreen. The logo footprints are excluded from the CERN-OHL license.