# The driver at a glance

STEPFOC is a field-oriented controller for bipolar stepper motors. It sits on the back of a NEMA 17 motor, reads
the rotor position from a magnet on the motor shaft, and drives the two coils with sinusoidal currents the way a
servo drive does, instead of the fixed step currents of a conventional stepper driver. The result is a stepper that
runs quietly and efficiently, never loses steps, reports its position, and can be commanded in position, speed,
current (torque) or impedance.

![The STEPFOC driver, 39 × 39 mm, with the encoder chip in the middle of the board.](https://docs.source-robotics.com/STEPFOC-docs/108/assets/STEPFOC.webp){ width="520" }

## What is on the board

| Part | What it does |
|---|---|
| STM32F103CB microcontroller | runs the control loops at 6250 Hz and the CAN and UART interfaces |
| 14-bit magnetic encoder (MT6816) | reads the position of the magnet on the motor shaft: 16384 ticks per revolution |
| H-bridge driver with inline current sensing | drives the two coils with up to 2.8 A and measures the phase currents |
| 16 Kbit EEPROM | keeps the configuration and the calibration over a power cycle |
| Two CAN connectors and two power connectors | daisy-chain several drivers on one bus and one supply |
| CAN termination switch | the 120 Ω resistor the first and the last node need |
| UART connector (3.3 V) | the serial terminal: every setting, calibration, tuning |
| Thermistor connector | an NTC on the motor for temperature protection |
| ADDITIONAL2 pin | an input or output you can use: HALL mode homes against a sensor wired here |
| JTAG pads (1.27 mm) | the programming adapter, for installing the bootloader or for development |
| Status LED | normal operation, calibration or error, see below |

The connectors and their pinout are in [Connectors and pinout](https://docs.source-robotics.com/STEPFOC-docs/108/03-technical-data/connectors/).

## What the firmware does

The firmware closes three nested loops on the motor: a current (torque) loop, a velocity loop around it and a
position loop around that, plus an impedance (PD) loop that makes the motor behave like a spring and damper.
A command picks the loop: a current setpoint, a speed, a position or a PD setpoint. Open-loop stepping, a
voltage-torque mode and a homing mode against a sensor exist beside them; [Control modes](https://docs.source-robotics.com/STEPFOC-docs/108/06-operation/modes/)
describes each.

The driver protects itself and the motor: it stops on a supply voltage outside the limits, on an encoder fault (no
magnet), on a motor over-temperature (with the thermistor), on an E-stop command and on a watchdog that runs out.
The status LED and `#Error` tell which; see [Errors and recovery](https://docs.source-robotics.com/STEPFOC-docs/108/06-operation/errors/). It does not monitor
its driver stage: a fault there is not reported.

## How it is controlled

- **UART**, a 3.3 V serial port at 256000 baud: human-readable commands such as `#P 16384`, and the only way to
  run the calibration. See [Over UART](https://docs.source-robotics.com/STEPFOC-docs/108/06-operation/uart/).
- **CAN**, 1 Mbit/s, up to 14 drivers on one bus (node IDs 0 to 13): compact frames for real-time control from a PC with the CANvas
  adapter, a microcontroller or a robot control box. See [Over CAN](https://docs.source-robotics.com/STEPFOC-docs/108/06-operation/can/).
- **Python**, the `Spectral-BLDC` library on top of CAN. See [With Python](https://docs.source-robotics.com/STEPFOC-docs/108/06-operation/python/).
- **Motor GUI**, in the browser, over serial or CAN: the usual tool for setup, testing and tuning. See [Motor GUI](https://docs.source-robotics.com/STEPFOC-docs/108/05-setup/motor-gui/).

## The status LED

| Pattern | Meaning |
|---|---|
| three short flashes and a pause | normal operation: no error, calibrated |
| fast flashing, about ten times a second | the calibration routine is running |
| on, not flashing | error mode: at least one error is active, or the motor is not calibrated |

The LED can be turned off with `#LED 0`.

| Normal operation | Calibration | Error mode |
|---|---|---|
| ![Three short flashes and a pause](https://docs.source-robotics.com/STEPFOC-docs/108/assets/Normal.webp){ width="180" } | ![Fast flashing during calibration](https://docs.source-robotics.com/STEPFOC-docs/108/assets/Calib.webp){ width="180" } | ![On, steady](https://docs.source-robotics.com/STEPFOC-docs/108/assets/Error2.webp){ width="180" } |