How to Flash STM32F103-Based Flight Controllers with ST-Link

JTAG (Joint Test Action Group) and SWD (Serial Wire Debug) are standardized debugging protocols that enable communication with microcontrollers for programming and debugging. JTAG uses four essential signals (TDI, TDO, TCK, TMS) plus two optional signals (TRST, RTCK), supports boundary scanning for detecting soldering defects, and works with multiple microcontroller architectures. SWD is simpler with only two signals (SWDIO, SWCLK) plus an optional trace output (SWO), offers higher data transfer rates, but only works with ARM architecture microcontrollers. The choice between them depends on application needs: SWD is preferred for simple programming and debugging on ARM devices, while JTAG is better for complex systems, high-volume manufacturing, or when boundary scanning is required.

SWD is a serial protocol with two lines: clock (host-controlled) and data (bidirectional). Communication follows three phases: command byte, acknowledgement, and data transmission. Turnaround (TRN) cycles allow data line control transfer between host and target. Write operations transmit 32-bit data plus parity after acknowledgement. Read operations differ—no turnaround before data transmission since target already controls the line. The command byte includes AP/DP bits, read/write flags, and address bits. All communication goes through debug and access port registers, which act as intermediaries to physical peripherals.

Cortex-M0+ microcontrollers support Serial Wire Debug (SWD) using two pins: SWDIO (data I/O) and SWCLK (clock). The debug interface typically includes VDD, GND, NRST, SWDIO, SWCLK, and optionally SWO (trace output). Tag Connect cables provide pogo-pin interfaces that clamp to PCB pads without requiring soldered components. The debug header connects to programmer/debugger tools like J-Link or TI's XDS110.

JTAG (Joint Test Action Group) and SWD (Serial Wire Debug) are standardized debugging interfaces used in embedded systems development; JTAG provides a comprehensive serial communication protocol with multiple dedicated pins (test data in/out, test clock, and control signals) for accessing internal registers, memory, halting processors, and stepping through instructions, while SWD offers a more streamlined two-pin solution (data and clock) specifically designed for ARM-based microcontrollers, making it ideal for space-constrained designs while maintaining similar debugging capabilities such as processor halt, memory access, and breakpoint setting.

SWD (Serial Wire Debug) is a two-wire protocol for ARM debug interface, designed to reduce pin count from JTAG's four pins to just two. SWDIO carries debug commands and SWCLK provides the clock signal. The ST-Link circuitry manages these pins to program the microcontroller, access memory regions, add breakpoints, and control CPU execution. SWO is an optional third pin for trace functionality. JTAG was the traditional mechanism for ARM7/ARM9 families but SWD was introduced for Cortex-M family to reduce pin requirements.

Betaflight is an open-source firmware for flight controllers that control multirotors. The Betaflight Configurator is a graphical application for flashing and configuring firmware. Targets are configuration files describing how specific flight controllers are laid out. To flash firmware, connect the FC to the PC, select the appropriate target, and choose the desired version. After flashing, apply default settings and calibrate the accelerometer. The Installation tab displays firmware version, target information, detected sensors, and real-time battery voltage and CPU usage. The Ports tab manages UARTs for connecting peripherals like receivers, GPS units, and VTX devices. The Configuration tab displays gyro frequency and PID loop frequency, which is the core control system translating stick inputs into motor commands. Options include enabling stabilizers, barometers, and compasses. Users can enter model and pilot names for OSD display. The arm angle limit prevents accidental armament when the FC is tilted. Air Mode maintains minimum throttle when the throttle stick is released. GPS configuration allows configuring GPS-specific settings and orientation correction. Beep configuration allows using the onboard buzzer or motors to indicate conditions. The Presets tab allows loading pre-configured settings for specific components. The Failsafe tab configures quad behavior when radio signal is lost. Configuration files can be saved and restored as text files for backup and transfer between flight controllers.

This tutorial provides a comprehensive overview of configuring flight controllers for RC aircraft, covering essential settings including sensor management (disabling unused sensors like accelerometer, barometer, and magnetometer to conserve resources), ESC and motor configuration (setting minimum and maximum throttle values to 1000-2000 for modern ESCs, calibrating ESCs properly, and understanding the importance of minimum throttle settings), receiver configuration (ensuring correct stick throws from 1000-1500-2000), PID tuning fundamentals, loop time optimization (typically 500us for most users), and CLI commands for system monitoring, backup, and restore operations. The presenter emphasizes that while Cleanflight is more stable, Betaflight offers newer features that haven't been ported back, making it preferable for staying current with developments.

Betaflight is flight controller software that connects to drones via USB for configuration and monitoring. Before connecting, remove propellers for safety and ensure VTX antenna is connected. Download the latest configurator (10.7.0) and connect your drone. The Setup Tab displays orientation with an arrow indicating the front, which should correspond to actual movement. UART ports are serial communication interfaces with receive (R) and transmit (T) pins for connecting receivers and smart audio devices. Motor direction configuration determines propeller spin direction, with front motors typically spinning inward. ESC configuration sets maximum throttle percentage, and the arming angle determines minimum tilt required before arming (default 25 degrees, often changed to 180 degrees for easier recovery). The Receiver Tab verifies receiver binding by moving throttle and checking yaw, pitch, and roll responses. Flight modes include Angle mode (self-leveling with 60-degree pitch limit), Acro mode (full rate control for flips and rolls), Horizon mode (self-leveling with flip capability), and Acro Trainer (learning mode with limited control).

Flight modes in Betaflight and Cleanflight are configured by mapping transmitter switches to auxiliary channels (typically channels 5-8), then assigning flight mode functions to those channels using range sliders in the modes tab, allowing users to control modes like Angle Mode, Horizon Mode, Air Mode, and Beeper through switch positions.

A flight controller contains a CPU, flash memory, wireless receiver, and control outputs connecting to Electronic Speed Controllers (ESCs) that regulate motors. The primary sensors include: gyroscope (measures rotation around X, Y, Z axes), accelerometer (measures acceleration for orientation), barometer (measures atmospheric pressure for altitude calculation), GPS (determines Earth position), magnetometer (measures Earth's magnetic field), and optional airspeed sensor. These sensors work together to provide comprehensive flight data for precise control and stable flight operations.

Cortex-M0+ microcontrollers support Serial Wire Debug (SWD) using two pins: SWDIO (data I/O) and SWCLK (clock). The debug interface typically includes VDD, GND, NRST, SWDIO, SWCLK, and optionally SWO (trace output). Tag Connect cables provide pogo-pin interfaces that clamp to PCB pads without requiring soldered components. The debug header connects to programmer/debugger tools like J-Link or TI's XDS110.

The SWD (Serial Wire Debug) interface requires connecting three pins: SWDIO (data line), SWCLK (clock line), and GND (ground). On the ESC circuit board, these pins are typically found as small pads adjacent to each MCU. Identifying which pads correspond to which functions may require visual inspection of trace routing from the pads to the MCU pins. Some ESC designs make this more obvious than others. Users must carefully connect the ST-Link programmer wires to the correct pads in the proper order (SWDIO and SWCLK have specific roles).

The microcontroller datasheet shows the pinout, including VCC (positive power) and GND (ground) pins. These pins are typically at opposite ends of the package. Tracing from these pins leads to the voltage regulator.

The ST Link programmer requires precise pin connections to the STM32 microcontroller for successful programming. The VCC (power) pin connects to the first pin on the left side of the board, near the USB module. The SWDIO (data output) pin connects to the second pin, identified by a notch mark on the board. The SWCLK (clock) pin connects to the third pin, adjacent to SWDIO. The GND (ground) pin connects to the last pin on the opposite side from VCC. These four pins must be connected using appropriately colored wires (red for VCC, orange for SWDIO, green for SWCLK, yellow for GND) to establish proper communication between the programmer and the microcontroller.

Each microcontroller family has its own specific connection method to programmer devices. For AVR microcontrollers, the connection uses VCC and ground pins for power, plus MOSI, MISO, and SCK pins forming the SPI port. For STM32 microcontrollers, the connection uses RST, SWCLK, SWDIO, and SWO pins forming the SWD protocol port. These pin configurations are standardized within each family, so understanding the specific pinout for your chosen MCU family is essential for proper connection.