BlackWing Blinky - SAMD51 LED Blink Project

A bare-metal C firmware for the SAMD51J20A microcontroller that blinks an LED at PA22 every second. This project serves as the foundation for the BlackWing CubeSat mission.

Project Overview

• Microcontroller: ATSAMD51J20A (Cortex-M4)
• LED Pin: PA22
• Blink Rate: 1 second ON / 1 second OFF
• Development Environment: VS Code + WSL + ARM toolchain
• Programmer: Microchip PICkit5

Hardware Setup

SAMD51 Board Connections

• PA22 → LED (with current-limiting resistor)
• GND → Ground
• 3.3V → Power supply

PICkit5 to SAMD51 (5-pin SWD connector)

• J2 Program PICKit5

---

• Pin 1: 3.3V Target ------- Pin 2: VDD (Sense)
• Pin 2: SWDIO ------------- Pin 8: SWDIO (TDO)
• Pin 3: SWCLK ------------- Pin 5: SWCLK (TCK)
• Pin 4: NRST -------------- Pin 1: NRST (Reset)
• Pin 5: GND --------------- Pin 3: GND

•                        Pin 4: SWO (TRACE) NC
  

•                        Pin 6: NC
  

•                        Pin 7: NC
  

---

Important: Pull-up resistors (10kΩ) on SWDIO and SWCLK to VDD are recommended but not required.

Software Setup

Prerequisites

• WSL (Windows Subsystem for Linux)
• ARM GNU Toolchain: arm-none-eabi-gcc
• Microchip Studio (for programming via PICkit5)
• make (installed in WSL)

Build

wsl make clean && wsl make all

Output files:

• blink.elf - ELF executable
• blink.hex - Intel HEX format
• blink.bin - Binary format

Programming via PICkit5

Critical Settings (MUST BE CONFIGURED)

These settings are essential for SAMD51 programming to work:

1. Open MPLAB IPE v6.25
2. Operate → Device Programming
3. Tool: Select "PICkit 5"
4. Device: ATSAMD51J20A
5. Advanced Settings
6. Comms SWD Speed: Change from 2.00 MHz to 0.100 MHz ⚠️
7. Program Speed: Change from NORMAL to LOW ⚠️

Programming Steps

1. Build firmware: wsl make all
2. Open blink.hex in Microchip Studio
3. Tools → Device Programming
4. Apply the settings above
5. Click Program
6. Wait for "Programming complete"
7. Look for blue horizontal bar moving back and forth
8. If not moving, PICKit5 programmer and/or is stuck

Troubleshooting

Issue: "Error: open failed" or connection timeout

• Solution: Reduce SWD speed to 0.100 MHz and set Program Speed to LOW

Issue: PICkit5 hangs or device not detected

• Solution: Unplug the PICkit5 USB cable, wait 5 seconds, replug

Issue: LED doesn't blink after programming

• Verify:
  • Board is powered (check 3.3V)
  • PA22 pin is connected to LED
  • LED has current-limiting resistor (~330Ω recommended)
  • LED polarity is correct (anode to PA22, cathode to GND)

Code Architecture

Files

• main.c - Main application (LED toggle logic)
• startup_samd51.c - Cortex-M4 vector table and reset handler
• linker.ld - Linker script (memory layout)
• include/samd51.h - Register definitions and memory-mapped addresses
• Makefile - Build configuration

Execution Flow

1. Power On/Reset → CPU loads SP and PC from vector table (0x00000000)
2. Vector Table → Points to Reset_Handler
3. Reset Handler → Calls main()
4. main() → Configures PA22 as output, enters infinite toggle loop

Register Access

The current implementation uses direct memory-mapped register access:

/* Set PA22 as output */
*(volatile uint32_t *)(0x41008000 + 0) = (1 << LED_PIN);

/* Toggle PA22 */
*(volatile uint32_t *)(0x41008000 + 0x1C) = (1 << LED_PIN);

PORT Base Address: 0x41008000

• DIRSET (offset 0x00) - Set pin as output
• OUTTGL (offset 0x1C) - Toggle output pin

Future Enhancements

Phase 2: MicroPython

• Flash MicroPython firmware to SAMD51
• Use same PICkit5 settings (0.100 MHz SWD speed)
• Develop CubeSat mission logic in Python

Phase 2: CircuitPython ✅ COMPLETED

• ✅ Downloaded CircuitPython firmware for SAMD51
• ✅ Converted firmware.uf2 to firmware.hex
• ✅ Programmed via PICkit5 with 0.100 MHz SWD speed
• Have issues with USB enumeration as comm port for ciruitPython
• Verified REPL access via USB
• Created custom board definitions
• See CircuitPythonOnSAMD.md for detailed setup

Phase 3: Advanced Features

• Add sensor interfaces (I2C, SPI)
• Implement communications protocols
• Integrate with PyCubed framework

Known Issues & Workarounds

Issue	Cause	Workaround
SWD connection fails	Default 2.00 MHz too fast for SAMD51	Set to 0.100 MHz in Microchip Studio
PICkit5 hangs during programming	USB power issue	Unplug/replug USB cable
LED doesn't toggle	Clock not enabled or PORT not configured	Verify 3.3V supply and PA22 connections

Build Information

• Compiler: arm-none-eabi-gcc
• CPU: Cortex-M4 (default 1 MHz internal oscillator)
• Optimization: -O0 (no optimization, for debugging)
• Flash Memory: 1 MB (only ~240 bytes used)
• RAM: 256 KB (currently unused)

References

• SAMD51 Datasheet
• Microchip Studio
• ARM Cortex-M4 Documentation
• PyCubed CubeSat Framework

Author & Project

Terry Carpenter 12/19/2025 PyCubed V5 Bring Up

BlackWing CubeSat Project - SAMD51-based satellite control system