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Custom Controller Board

Overview
A compact, USB-C–enabled controller board built around the ATmega32U4 MCU for dual-axis stepper motor control with encoder feedback, sensor integration, and flexible communication interfaces.

Features

  • Microcontroller: ATmega32U4 (44-pin TQFP) with native USB support
  • Power: USB-C or external 5V supply with automatic source selection
  • Motor Control:
    • Two PUL/DIR/EN outputs for CL57T V4.1 stepper drivers
    • Differential encoder inputs via SN75157DR line receivers
  • Sensors:
    • I²C interface to Adafruit BNO055 9-DOF IMU
    • Three analog cliff sensors
    • Dual-channel battery voltage monitoring
  • Communication & Debug:
    • USB-C (full-speed USB 2.0)
    • SPI programming header
    • 10-pin JTAG header
  • User Interface: Status LEDs and push-button inputs
  • Signal Integrity: Controlled-impedance differential routing, proper termination, ground planes

Technical Challenges

Designing this custom controller board involved overcoming several critical technical challenges:

Encoder Signal Acquisition for MCU Integration

Industrial environments are electrically noisy, stepper motors and long cables can introduce substantial electromagnetic interference, leading to false encoder readings, glitches, and motor missteps.

  • Solution:
    • Utilize dedicated buffer ic for splitting encoder signals, which offer high input impedance and proper conversion to logic-level signals for MCUs without loading or distorting the encoder output
    • Avoid single-ended buffers, preventing signal integrity issues and logic contention
    • Employ differential receivers (SN75157) to reject common-mode noise before signals reach the MCU, ensuring robust, error-free pulse counting.

High-Speed Differential Signal Integrity

  • Controlled-impedance routing (90–120 Ω) for encoder A+/A– and B+/B– pairs
  • Length matching within ±0.1 mm to prevent skew
  • Avoided sharp bends and stubs; routed over continuous ground plane

Mixed-Signal Noise Isolation

  • Physical separation of analog (cliff sensors, battery monitors) and noisy digital/motor control traces
  • Dedicated ground planes and star-point connections for analog vs. digital returns
  • Shielded, twisted-pair encoder cables with chassis-grounded shields

USB-C Integration & Protection

  • Matched-length differential pair routing on D+/D–
  • CC1/CC2 pull-down resistors (5.1 kΩ) for UFP detection
  • ESD suppressors
  • UVCC/UCAP decoupling (0.1 µF + 1 µF) for internal USB regulator stability

Reliable Power Source Selection

  • Power selection header between USB-C VBUS and external 5 V input to prevent backfeed
  • Resettable fuse and bulk decoupling on both power sources
  • Voltage divider and filter network for 5 V VBUS sensing at MCU’s VBUS pin

Firmware Programming & Debug Accessibility

  • Standard 10-pin SPI header and 10-pin JTAG header with clear silkscreen labels
  • Test points for USB D+/D–, SPI lines, and key analog nets
  • Buffer footprints for optional level-shifting or signal conditioning

User Interface Debouncing & Visibility

  • Software-debounced GPIO inputs for push buttons; optional RC filter footprint
  • Active-low LED indicators with 330 Ω current-limiting resistors

Compact, Manufacturable Layout

  • Hierarchical schematic blocks for modularity and maintainability
  • Strategic component placement: MCU centered, connectors at edges, decoupling caps adjacent to IC power pins
  • 2-layer PCB stack-up for noise control and routing efficiency

License

This project is licensed under the MIT License.