Introduction¶

Stepper-C is a C11 library that drives a two-phase hybrid stepper motor with closed-loop sinusoidal current control. Given the measured phase currents and bus voltage, it computes the PWM compare values of the power stage, and it turns motion commands (move to a position, run at a velocity, stop) into a smooth microstep trajectory. It runs on any MCU: the hardware is reached only through callbacks, and the numeric backend is either fixed point (no FPU needed) or single-precision float.

What the library does¶

Controls the phase currents. One PI controller per phase with back-EMF feed-forward and anti-windup, gains derived from the motor's R and L and the requested bandwidth. An open-loop voltage mode is available for boards without current sensing.
Generates the microstep angle. Sine/cosine current references from a 32-bit electrical angle, 1 to 256 microsteps per full step.
Plans the motion. Trapezoidal profile with separate acceleration and deceleration, no overshoot, smooth replacement of a running command; constant-velocity mode; stop.
Fits the voltage to the power stage. Two full H-bridges (4 legs) or three half-bridges with a shared leg (3 legs); the request is limited to what the topology and the measured bus voltage can deliver, keeping its direction.
Protects the hardware. Overcurrent, overvoltage and undervoltage trips in the fast loop, hardware-callback errors, external faults; outputs off within the same period.
Detects a stall without a sensor. A back-EMF observer reports a rotor that stops, falls out of synchronism or slips at standstill (SSTEP_FAULT_STALL, optional).
Reduces the current at standstill. Hold current after a configurable delay.

What the library does not do¶

Concern	Why it is not in the library
Timer, PWM and ADC drivers	Hardware-specific; you start the conversions and pass raw ADC counts in, and write the compare values out in a callback.
Interrupt and task setup	Every system has its own; the library only defines which function runs at which rate. See the integration guide.
Position feedback (encoder)	The library is an open-loop positioner with closed-loop current; the rotor position is not measured. Sensorless stall detection reports a lost synchronism but does not correct the position.
Homing, limit switches, G-code, multi-axis coordination	Application logic on top of `sstep_move_to()`, `sstep_set_position()` and the status.
Motor parameter identification	R, L and Kt come from the datasheet or a measurement.
Communication (UART, CAN, Modbus…)	Out of scope; call the API from your protocol handler.

Who it is for¶

Firmware for stepper drives on a general-purpose MCU with gate drivers, for example STMicroelectronics B-G431B-ESC1 or a custom board with two H-bridges.
Machines with several axes, one driver instance per motor, no shared state.
Projects that must be reviewed or certified: MISRA C:2012, 100 % coverage, no heap, bounded execution time.

Key properties¶

Property	Value
Language	C11, no warnings with `-Wall -Wextra -Wpedantic -Wconversion`
Dependencies	`<stddef.h>`, `<stdint.h>`, `<stdbool.h>` only
Numeric backends	Fixed point Q15/Q31 (integer only) or single-precision float, chosen at build time
Memory	No heap, no global or static mutable data (`.data` and `.bss` are empty)
RAM per instance	432 B fixed point, 456 B float on Cortex-M4 (details)
Flash	9.0 KiB fixed point, 9.7 KiB float (Cortex-M4, `-Os`, both topologies)
Stack	`sstep_fast_loop()` 216 B, `sstep_slow_loop()` 152 B worst case (Cortex-M4 `-Os`, details)
Coding standard	MISRA C:2012, 0 Cppcheck addon violations, 1 deviation
Tests	100 % line and branch coverage of `src/` in both backends; closed-loop tests on a motor model
Thread safety	One fast-loop context and one slow-loop context per instance, by design; see Threading
License	Apache License 2.0

Terminology¶

Full step: 1/200 of a revolution for a 1.8° motor; a quarter of an electrical period.
Microstep: A full step divided by microsteps (power of two, 1 to 256). Positions and velocities of the API are in microsteps.
Electrical angle: Angle of the current vector in the two phases; one electrical period is four full steps. Held as a 32-bit unsigned number, one turn = \(2^{32}\).
Per unit (pu): Internal signals are relative to a base value: currents to i_full_scale_ma, voltages to vbus_nominal_mv. 1.0 pu is the base.
Fast loop: sstep_fast_loop(), called from the PWM/ADC interrupt at fast_loop_hz (typically 20 kHz): current control and PWM.
Slow loop: sstep_slow_loop(), called at slow_loop_hz (typically 1 kHz): trajectory, angle, currents, events.
Leg: One half-bridge (high-side and low-side switch) of the power stage, driven by one PWM compare value.
Load angle: Electrical angle between the commanded current vector and the rotor; it grows with the load torque and the motor loses steps when it exceeds 90° (180° to the next stable position).

Conventions in this documentation¶

Engineering units in the API are integers with the unit in the field name: _mohm, _uh, _ma, _mv, _hz, _ms, _ns.
"Fixed point" and "float" name the two numeric backends; numbers are given for both when they differ.
The example motor is a 17HS19-2004S1 (1.6 Ω, 3.0 mH, 2.0 A, 0.21 N·m/A, 200 steps) on a 24 V supply; the example board is a B-G431B-ESC1 (three half-bridges).