SCARA V1

Sensor-Actuator-Control-PCB

STM32F103C8 carrier board controlling 6 stepper motors, servos, limit switches, and IO expanders with UART, SPI, and I2C interfaces. Includes full pin mapping, dual-regulator power design, and detailed failure analysis with debugging insights and hardware lessons learned.

📘 STM32F103C8 Stepper Motor Controller

Carrier Board + Small Pill Architecture & Debug Report

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📑 Table of Contents

• Overview
• Hardware Architecture
• Pin Mapping (STM32 ↔ Carrier Board)
• Peripheral Interfaces
• Clock & Crystal
• Power System Design
• IO Expanders
• Failure Report & Debugging
• Root Cause Analysis
• Current Status
• Design Lessons & Prevention

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🔍 Overview

This project is a custom STM32F103C8-based PCB designed to control:

• Multiple A4988 stepper drivers
• Several servo outputs
• External IO via expanders

The board uses a Small Pill (STM32F103C8T6) mounted on a carrier board, providing structured routing for motion control and expansion.

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🧩 Hardware Architecture

• MCU: STM32F103C8T6 (Small Pill)
• Motor Drivers: A4988 (external)
• IO Expansion: PI4IOE5V6416
• Power Inputs:
  • USB (via L78L00 regulator)
  • External 5V (via AP2112K-3.3)
• Control Signals:
  • STEP / DIR for 6 axes
  • Servo PWM outputs
• Debug Interface:
  • SWD (SWCLK, SWDIO)

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🔌 Pin Mapping (STM32 ↔ Carrier Board)

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🔗 Peripheral Interfaces

UART

• TX: PA2
• RX: PA3

SPI

• SCK: PA5
• MISO: PA6
• MOSI: PA7

I2C

• SCL: PB6
• SDA: PB7

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⏱ Clock & Crystal

• Component: 25 MHz SMD Crystal

• Capacitance: 20pF

• Package: 3.2mm × 2.5mm

• DigiKey Part #: 3155-25M20P2/XT324-10/10CT-ND

• Manufacturer #: 25M20P2/XT324-10/10

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⚡ Power System Design

Regulators

1. L78L00

• Input: USB (~5V)
• Output: 3.3V (intended)
• Status: ❌ Damaged / unstable

2. AP2112K-3.3

• Input: External 5V
• Output: 3.3V
• Status: ✅ Functional

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Observed Behavior

• With motors connected:
  • 3.3V rail dropped to ~3.0V
• With L78L00 removed:
  • Stable 3.29V from AP2112K
• Backfeeding observed across regulators

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🔌 IO Expanders

• Model: PI4IOE5V6416
• Used for:
  • Microstepping control
  • Enable signals

⚠️ These lines interface with high-current drivers → risk area

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🧪 Failure Report & Debugging

Initial Symptoms

• PB12 LED stopped working
• MCU still flashable (initially)
• Reset did not recover functionality

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Voltage Measurements

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GPIO Testing

• PB12–PB15 stuck at ~0.12V
• PB15 toggle test failed
• Same issue after MCU replacement

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Regulator Testing

• L78L00:
  • Input ~4.8V
  • Output unstable / low
• AP2112K:
  • Output stable at ~3.29V

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Fuse

• 0685H9100-01 → OK (~0Ω)

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After L78L00 Removal

• System powered via AP2112K
• 3.3V stable
• ❌ Flashing failed:
  • Error in final launch sequence
  • Failed to start GDB server

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🔎 Root Cause Analysis

1. GPIO Damage

• PB12–PB15 permanently stuck low
• Cause: Backfeed / overvoltage from stepper drivers
• Likely triggered by:
• Swapping motor wires while powered

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2. Power Rail Instability

• L78L00 regulator damaged
• Caused rail drops under load

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3. Regulator Conflict

• Backfeeding between regulators
• L78L00 became ineffective

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4. Flashing Failure

• Likely due to:
• Unstable VDD reference
• Improper SWD voltage levels

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📉 Current Status

• ❌ Original MCU damaged (GPIO bank failure)
• ⚠️ Replacement MCU still showing abnormal GPIO behavior
• ✅ AP2112K regulator working
• ❌ Board cannot be flashed
• ❌ System non-functional

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🛠 Design Lessons & Prevention

Critical Rules

• ❌ Never swap stepper wires under power
• ❌ Avoid direct MCU exposure to driver signals

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Recommended Fixes

• Add series resistors on control lines
• Use optocouplers or buffers
• Isolate high-current paths
• Ensure single clean 3.3V source
• Avoid regulator backfeeding

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Debug Best Practices

• Verify rails before applying load
• Test MCU pins before connecting drivers
• Use current limiting during bring-up

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🚧 Final Note

This failure highlights a critical embedded design principle:

Power integrity and signal protection are just as important as logic design.

Future revisions should prioritize:

• Electrical isolation
• Robust power architecture
• Safer bring-up procedures

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