Wiring — the reference build¶
Every signal in the aircraft, grouped by the thing it connects to. This is the page to have open while you solder — flight battery disconnected throughout.
How to read these tables¶
Pins come from config/32raven.config — the checked-in reference build, not a suggestion.
Most of them are Kconfig choices, not fixed silicon. If your board routes a signal
differently, change it in make 32raven-menuconfig rather than reworking the PCB — the
build validates every pin against ST's silicon data and aborts on an impossible
(pin, alternate-function) pair.
Signal overview¶
flowchart LR
RC[CRSF receiver] -->|USART6| STM
GPS[u-blox M10] <-->|USART2| STM
IMU[ICM42688P] <-->|SPI2 + INT| STM
FLASH[SPI flash / EEPROM] <-->|SPI1| STM
STM[STM32F407<br/>flight computer] -->|TIM1 DShot ×4| ESC[4× AM32 ESC]
ESC -->|USART3 RX| STM
BATT[Battery sense] -->|ADC1| STM
STM <-->|USART1 — FcLink| ESP[ESP32-C3<br/>bridge]
ESP -->|BOOT0 / NRST| STM
ESP <-->|I2C| OLED[SSD1306 OLED]
ESP <-->|UART| TELEM[Telemetry radio]Inter-MCU link — FcLink¶
The one connection that must be right before anything else works. Cross TX to RX.
| Signal | ESP32-C3 | STM32F407 |
|---|---|---|
| FcLink TX → FC RX | GPIO4 |
PA10 |
| FcLink RX ← FC TX | GPIO3 |
PA9 |
| Ground | GND |
GND |
Tip
Keep this link short. The reference build runs the link at 921600 baud, and long
unshielded wire inside a frame full of ESC switching noise is a good way to spend an
evening debugging handshake failures. Twist TX/RX with ground, and if the run has to be
long, drop the baud under make 32raven-menuconfig → Common → FcLink rather
than fight the noise. Both ends read that one symbol, so there is no pair to keep in sync.
STM32 programming lines¶
The two lines the bridge drives to put the flight computer into its ROM bootloader. This is how the STM32 gets its firmware: its own USB port is never used, so every update reaches it through the bridge.
| Signal | ESP32-C3 | STM32F407 |
|---|---|---|
| BOOT0 | GPIO0 |
BOOT0 |
| NRST | GPIO1 |
NRST |
BOOT0 is reachable two ways, and NRST only one — see
where these signals live.
IMU — ICM42688P (SPI2)¶
| Signal | STM32F407 |
|---|---|
| SCK | PB13 |
| MISO (SDO) | PB14 |
| MOSI (SDI) | PB15 |
| CS | PA4 |
| INT (data ready) | PB10 |
The INT line is not optional — the control loop is driven by the IMU's data-ready edge, not by a timer. An unconnected INT is a control loop that never runs.
Note
Mounting matters more than wiring. Orientation and vibration isolation dominate IMU performance — see mounting for how the sensor goes down and which way its axes end up pointing.
GPS — u-blox M10 (USART2)¶
| Signal | STM32F407 |
|---|---|
| TX → GPS RX | PA2 |
| RX ← GPS TX | PA3 |
The driver configures the module at runtime (nav rate, message set), so the GPS does not need pre-programming with u-center.
RC receiver — CRSF (USART6)¶
| Signal | STM32F407 |
|---|---|
| RX ← receiver TX | PC7 |
| TX → receiver RX | PC6 |
CRSF runs at 420000 baud (CONFIG_STM32_RC_RECEIVER_UART_BAUD_420000). It is a
half-duplex-style protocol over two wires; binding and channel mapping are covered in the RC
stage, not here.
Motors — TIM1 DShot¶
Motor numbering follows the mixer, which is not the same as the physical arm order on every frame. Wire by number now; confirm the mapping on the bench with props off.
| Motor | Timer channel | STM32F407 |
|---|---|---|
| 1 | TIM1_CH1 | PE9 |
| 2 | TIM1_CH2 | PE11 |
| 3 | TIM1_CH3 | PE13 |
| 4 | TIM1_CH4 | PE14 |
ESC telemetry (USART3)¶
One shared RX-only line back from the ESCs.
| Signal | STM32F407 |
|---|---|
| Telemetry RX | PB11 |
The pin is configured with an internal pull-up because the line idles high and no ESC drives it until one is polled.
Battery sense (ADC1)¶
| Signal | STM32F407 |
|---|---|
| Pack voltage | PC0 |
| Pack current | PC1 |
Warning
Divider first, pin second. These go to a divided and clamped sense node, never to
the pack. 6S is ~25 V fully charged; the F407 ADC input is 3.3 V tolerant. Verify the
divider output with a meter before the wire ever reaches PC0.
The reference build measures ×10.762 at PC0 — STM32_BATTERY_VOLTAGE_MULTIPLIER_MILLI=10762,
calibrated 2026-08-19 against a balance meter at two load points; the nominal 11000 read
Onboard flash (SPI1)¶
Settings storage. Not flight-critical to have wired for a first power-on, but tuning will not persist across reboots without it.
| Signal | STM32F407 |
|---|---|
| SCK | PB3 |
| MISO | PB4 |
| MOSI | PB5 |
| CS | PA15 |
STM32 user I/O¶
| Signal | STM32F407 |
|---|---|
| Status LED | PA1 |
| User button | PA0 |
Polarity for both is a separate Kconfig switch (CONFIG_STM32_LED_ACTIVE_LOW,
CONFIG_STM32_BUTTON_ACTIVE_LOW) — set it to match your wiring instead of adding a
transistor.
ESP32-C3 peripherals¶
| Signal | ESP32-C3 |
|---|---|
| Status LED | GPIO8 |
| Button | GPIO9 |
| Buzzer | GPIO10 |
| OLED SDA | GPIO5 |
| OLED SCL | GPIO6 |
| Telemetry TX | GPIO20 |
| Telemetry RX | GPIO21 |
On the reference bridge board (the 0.42" OLED module), the first five rows are already wired on the board — the LED, the BOOT button and the OLED are not yours to connect. Only the buzzer and the telemetry UART leave the board.
Caution
The board's RX/TX labels are inverted relative to this firmware. The silkscreen marks
GPIO20 as RX and GPIO21 as TX, following the chip's default UART0 assignment.
This firmware drives them the other way round — GPIO20 is telemetry TX
(CONFIG_ESP32_PINMAP_TELEM_UART_TX_GPIO_NUM) and GPIO21 is RX.
The ESP32-C3 routes any UART to any pin through its GPIO matrix, so this is legal and works. It also means wiring by the printed label gives you a dead link with no error and nothing in the logs. Wire by GPIO number.
GPIO9 is the ESP32-C3's strapping pin for download mode. Holding the user button through a
reset will drop the bridge into the ROM bootloader instead of running firmware — inconvenient
in the field, so consider where you mount it.
Before you power anything¶
- [ ] Continuity-check every ground between the two boards.
- [ ] Confirm no pin appears twice across the tables above — the build enforces this for pin-map entries, but the two off-map UARTs are yours to check.
- [ ] Confirm the battery divider output with a meter at full pack voltage.
Next: power and ESC configuration (not yet written).