Hardware and bring-up

Doppelstep runs on the STMicroelectronics B-G431B-ESC1 Discovery kit, a small electronic speed controller (ESC) board for drone motors, with a detachable debugger. This page describes the board, how the stepper motor is wired to it, how the firmware scales the measurements, and the steps to bring a new setup up safely.

The board

Part	Detail
Microcontroller	STM32G431CBU6: Arm Cortex-M4F, 170 MHz, 128 KiB flash, 32 KiB SRAM
Power stage	L6387 gate drivers, STL180N6F7 MOSFETs, three half-bridges (legs U, V, W)
Current sensing	Three 3 mΩ low-side shunts, amplified by the internal OPAMPs of the MCU
Debugger	ST-LINK/V2-1 on a detachable part of the board, with a Virtual COM Port
Pinout	UM2516 Rev 4, Table 4 (boards/st/b_g431b_esc1/doc/index.rst)

The board definition is part of this repository (boards/st/b_g431b_esc1/), because Zephyr 4.5 does not include it: 170 MHz from the 8 MHz crystal, the pinout of the user manual, and the flash runners OpenOCD (default), J-Link, STM32CubeProgrammer and pyOCD.

Motor wiring

The board has three half-bridges, while a two-phase stepper has two coils. Doppelstep uses the shared-leg topology of Stepper-C: each coil sits between its own leg and the common leg W.

Wiring of the 17HS19-2004S1 to the B-G431B-ESC1: coil A between U and W, coil B between V and W, with the MCU pins of every leg

Motor lead (17HS19-2004S1)	Coil end	ESC1 output	Leg pins (TIM1)	Current sense
Black	A+	U	CH1 / CH1N: PA8 / PC13	OPAMP1 → PA2 (ADC1_IN3)
Green	A−	W	CH3 / CH3N: PA10 / PB15	OPAMP3 → PB1 (ADC1_IN12)
Red	B+	V	CH2 / CH2N: PA9 / PA12	OPAMP2 → PA6 (ADC2_IN3)
Blue	B−	W	CH3 / CH3N: PA10 / PB15	OPAMP3 → PB1 (ADC1_IN12)

The lead colours are those of the motor datasheet. Other motors, and some batches, use other colours: identify the two coils with an ohmmeter before connecting (about 1.6 Ω within a coil, open between the coils). Swapping the two ends of one coil reverses the sign of its current; bring-up step 3 checks it.

W carries the sum of both coil currents, iA + iB, up to √2 times the phase current. The firmware measures its shunt too and switches the outputs off when |iW| exceeds the trip current (aux-trip of the motor node).

The supply goes to the power input pads of the board (+ and −). The firmware accepts 10–28 V (vbus-min-mv, vbus-max-mv) and is tuned for 24 V; use a laboratory supply with a current limit for the first runs. The USB cable to the ST-LINK carries the DYNAMIXEL bus, the debugger and the RTT log.

The tested motor is a 17HS19-2004S1 (NEMA 17, 2.0 A rated, 1.6 Ω and 3.0 mH per coil, 200 steps per revolution). Another motor needs its values in the motor node (Configuration).

Current and voltage scaling

The scaling comes from the power stage node of the devicetree (app/boards/b_g431b_esc1.overlay):

Quantity	Value
Shunt	3 mΩ
Amplifier gain	−64/7 ≈ −9.14 (OPAMP PGA 16 with VINM0 as the bias input, as in ST MCSDK, NuttX and SimpleFOC)
Current full scale	±60.2 A with a 3.3 V reference, about 29 mA per ADC count
VBUS divider	18 kΩ / (18 kΩ + 169 kΩ)
VBUS full scale	34.3 V

The DYNAMIXEL bus

The bus is USART2 (PB3 TX, PB4 RX), for now through the ST-LINK Virtual COM Port of the debugger, at 1 Mbps. A DYNAMIXEL X430-T has no direction pin either: for a TTL half-duplex bus later, USART2 can run in single-wire mode (single-wire; on &usart2, TX pin only). Because USART2 carries the bus, the console and the log use SEGGER RTT through the debug probe.

Bring-up checklist

Follow these steps in order on a new board or after a change to the power stage. They follow the integration guide of Stepper-C.

1. Dead time (required). Enter the value of the B-G431B-ESC1 MCSDK (Motor Control Workbench) project in dead-time-ns of app/boards/b_g431b_esc1.overlay. While it is 0 the firmware runs, but never enables the gate drivers: Torque Enable answers Result Fail and the log says "enable refused". Check the gate driver input polarity (TIM1 CHx/CHxN active high) against the same project.
2. Outputs off. The RTT log shows the current offsets (about 2048 counts) and the ADC readings. Compare Present Input Voltage with a multimeter.
3. Polarity. Set voltage-drive; and a low run-current-milliamp in the motor node. After Torque Enable the measured phase A current must be positive. If not, set current-sense-invert; in the power stage node.
4. Current loop. Back to current drive with a low run current: check that the current settles at the run current.
5. Motion. Small Goal Position steps (DYNAMIXEL Wizard or the GUI), then the OpenMANIPULATOR-X profile.
6. Protection. Lower trip-current-milliamp and vbus-max-mv, then check the trips and the Hardware Error Status.

After a change to the drivers or the threads, also run the hardware checklist: it covers what the PC tests cannot see.