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Rotorflight FC Design Requirements
This page collects the generic hardware requirements for all Flight Controller designs for Rotorflight.
The actual Reference Designs for Rotorflight-2 can be found here.
For an FC to be fully supported by Rotorflight-2 (RF2), it must follow one of the Reference Designs.
Rotorflight is based on Betaflight, and thus has the same basic requirements for the hardware.
Please read the Design Guidelines for Betaflight first.
The following features and peripherals are mandatory:
- Gyroscope & Accelerometer
- Blackbox (flash chip or SD card)
- Indicator LEDs (Power, GPIO)
- DFU button
- USB socket
- Regulated +5V for external devices (RPM sensor, GPS, Compass, LED strips, etc.)
- Voltage sensor (ADC) for Vx and +5V
Optional peripherals and features:
- Barometer
- Extra voltage/current sensors (ADC)
The supported gyro&acc chips are:
- BMI270
- ICM42688P
- ICM20689
- ICM20602
- MPU6000
Other models are not supported.
The supported barometer chips are:
- DPS310
- BMP280
- BMP388
Other barometer models - like BMP390 - will be supported in the future.
The minimum flash size for the Blackbox is 1Gbit (128Mb x 8).
The supported SPI flash chips are:
- W25N01G
Larger flash chips - like W25N02 and W25N04 - will be supported in the future.
A helicopter Flight Controller typically have two kinds of connectors. The servo and motors are connected to standard 0.1" pin headers, with 6V-8.4V power on the center pin. The power is usually provided by a BEC in the ESC. The other connectors are usually UARTs or multi-function connectors, with 5V power. Most commonly these are JST type connectors (XH,PA,GH,etc).
It is also very useful to have the motor/servo headers on one end of the FC, and the other connectors on the other. This helps to remember that the other side has BEC (>6V) voltage, and the other side has 5V.
The standard "JR" style servo plugs are thicker than 0.1", usually 2.65mm. If more than 4 headers are located side by side, an extra gap of 0.5mm is required after every 4x3 header block. Otherwise the servo plugs will not fit in properly.
Three servos are required for cyclic. The reference designs also have a pin for an optional fourth cyclic servo, which can be a solder pad on the PCB.
The servo pins should be selected so that they are on the same timer. DMA is not required for servos, but the pins should have it available for future use.
The servo header is a standard 0.1" pin header. It's common to have the cyclic servos side-by-side, as a 3x3 header block.
The tail servo pin should be selected so that it is on a different timer than the cyclic servos. For ESC/motor use, DMA must also be available without conflicts. The pin and timer selection must allow DShot BitBang.
Main ESC/motor pin should be selected so that it is on a different timer than the tail motor/servo. The pin must have DMA available without conflicts. The pin and timer selection must allow DShot BitBang.
The ESC telemetry pin can be an Rx on any UART.
If a combined ESC telemetry and RPM input pin is preferred, then the pin should be selected so that it can act as a UART Rx, and a timer input, for example PA1 or PA3.
The RPM input requires a free timer. The timer can't be shared with other functions.
TIM2 and TIM5 can accommodate 4 RPM inputs, one on each channel. Other timers can accommodate only one RPM input. It is preferred to use TIM2 or TIM5 for RPM inputs.
The RPM inputs require over-voltage protection diodes. Any pin header that provides the servo power (VX) and can act as an RPM input, must have the protection diode on the signal pin. This is due to a shortcoming in a commonly used HobbyWing RPM sensor.
The AUX connection headers are for any extra peripherals, including receivers.
The AUX headers are primarily for UARTs, but because of the STM32 flexibility, they can be configured for other functions too.
Possible functions for the AUX headers are:
- UART
- I2C
- RPM input
- LED strip
- Extra PWM (servo) output
- Extra DShot (motor) output
- Camera Control
- Voltage and current measurements (ADC)
- RSSI input
- External OSD
- Video Tx Control (VTX)
The physical connector should be 4-pin JST-GH [GND, 5V, RX, TX], or other JST style polarised connector.
It is preferable to have as many UARTs available as possible.
Typically, any of the UART connectors can be used for connecting the receiver.
If a dedicated receiver port is preferred, UART3/4 on PC10/PC11 is the best choice due to its flexibility.
Spektrum DSM can't be connected to a UART socket, as it requires 3.3V. The DSM socket is a JST-ZH with [3.3V GND DATA] pins.
The options for connecting a DSM receiver are:
- Provide a standard DSM socket
- Connect to a generic UART socket with 3.3V power (instead of 5V)
- Connect to a generic UART socket with an adapter, converting 5V to 3.3V
An external compass connects to an I2C interface, which can be provided by one of the multi-function headers (UART/I2C/...)
Usually a compass is integrated into an external GPS unit.
Note! The external I2C can't be shared with the internal barometer, as this would not allow using that pin header for anything else than I2C.
An external GPS connects to any of the UART (multi-function) headers.
If a GPS specific header is preferred, a 6-pin socket with [GND,SDA,SCL,TX,RX,5V] can be added (Pixhawk compatible).
The WS2812B LED strip requires a GPIO pin with a timer and DMA capability.
It can be provided on one of the multi-function header (shared with UART etc.)
At least one power LED and one indicator LED (GPIO) are mandatory.
Preferably, a two colour (green+red) indicator LED on two GPIO pins can be used.
There must be a bootloader DFU button on the PCB, accessible without opening the FC case.
The SWD pins PA13 and PA14 must be exposed on test pads on the PCB, along with the GND and +3.3V pads.
Two regulators are needed for 3.3V, one for the MCU and another one for the noise sensitive devices, like the gyro and the baro.
If the 3.3V is also used for DSM, it should be provided by the MCU regulator, which must be large enough for the extra current drawn by the DSM receiver.
The regulators should be fed from the VX power bus.
A 5V power must be provided for the external devices.
The regulator should be able to provide at last 1A current, preferably 2A.
The regulator should be fed from the VX power bus. An LDO regulator is needed, as the VX voltage range is 6..8.4V.
Some of the UART sockets may be powered from a 4.5V power bus, which is fed through diodes from both the 5V power and the USB power buses.
This is especially useful for the dedicated receiver socket, as it would allow setting up the system with with the USB power only.
The system is powered from the VX Power bus, which can be provided by a BEC or a 2S LiPo battery.
The VX power is fed into the FC via the ESC pin headers (Throttle, Telem, RPM).
Large ESCs usually have two or three pairs of BEC power wires, for increasing the current capacity. The FC should have input pins for all of them.
For very large helicopters, it may be reasonable to have a separate connector (XT30) for the VX power input.
DShot BitBang requires one "pacing" timer, either TIM1 or TIM8. In the design, one of them must be reserved for BitBang use.
One "pacing" timer channel and one DMA channel is required for each GPIO port (PA,PB,PC,PD) with motor pins. It is preferred to have all motor pins located on the same GPIO port, thus using only one timer and DMA channel.
Some FBL gyros and drone FCs have series resistors on the signal lines.
This is unnecessary, and will do more harm than good.
The signal pins on the MCU shall be connected directly to the pin headers or connectors, without series resistors.
Some pins may require Zeners for over-voltage protection, though.