Boot console

my-first-linux-board

my-first-linux-board is a small F1C200S Linux board built around removable microSD storage, USB-C power and USB device data, a 3.3 V UART console, and power/status LEDs. This repository contains the exact KiCad design used for the assembled board, the working Buildroot firmware source, the final JLCPCB fabrication package, and purchasing files.

The assembled board boots U-Boot 2026.01 and Linux 6.19.14 into a minimal BusyBox system. Both boot stages identify it as Clark's Board.

Hardware

Open hardware/boot-console.kicad_pro in KiCad 10. The BOM, pinout, and component-purpose guide describe the final circuit.

The schematic PDF and combined copper-layer image are convenient read-only views of the same final design.

Firmware

The firmware directory is a Buildroot external tree. On macOS or another Docker host:

cd firmware
./build-container.sh

The complete card image is generated at:

firmware/output/images/sdcard.img

The proven board configuration uses one-bit microSD transfers at 12.5 MHz. The physical board has four data traces, but Linux produced data errors in four-bit/25 MHz mode on the first assembly; the conservative setting boots reliably and is therefore the canonical configuration.

UART0 is 115200 baud, 8 data bits, no parity, and one stop bit. Linux also creates a composite USB device with a CDC ACM console and a CDC ECM Ethernet link. The board is 192.168.7.2/24; its small DHCP server assigns the attached host an address without advertising a default route or DNS server. On macOS, the gadget can enumerate before Network Settings creates a service. Find the interface whose MAC address is 02:00:00:00:07:01, assign the host side 192.168.7.1 for immediate access, and ask macOS to detect the new hardware:

iface=$(for interface in $(ifconfig -l); do
  ifconfig "$interface" | grep -q 'ether 02:00:00:00:07:01' && echo "$interface"
done)
sudo ifconfig "$iface" inet 192.168.7.1 netmask 255.255.255.0 up
sudo networksetup -detectnewhardware

This address lasts until the interface is removed or reconfigured. After networksetup -detectnewhardware creates the service, give it the persistent static address used by this direct link. The service on the tested Mac is named Clark's Board Console + Ethernet:

sudo networksetup -setmanual "Clark's Board Console + Ethernet" \
  192.168.7.1 255.255.255.0 0.0.0.0

If macOS uses a different service name, find it with networksetup -listallnetworkservices and substitute that name. DHCP did not activate this ECM link reliably, so keep the host address static.

Prometheus metrics are the only HTTP resource. Scrape them with:

curl http://192.168.7.2/cgi-bin/metrics

The endpoint uses the Prometheus 0.0.4 text format and reports board identity, time, CPU, load, memory, VM, pressure, filesystem, microSD, network, socket, entropy, LED, service, USB gadget, and available thermal, CPU-frequency, and watchdog data. The root URL and every other HTTP path intentionally return 404.

SSH is available as root with a blank password:

ssh root@192.168.7.2

This passwordless access is intended for a directly attached development cable. The first boot stores a generated SSH host key on the FAT boot partition so the host fingerprint remains stable while the read-only root stays unchanged.

The status LED appears at:

/sys/class/leds/boot-console:green:status

It uses the CPU activity trigger by default. Manual control:

LED=/sys/class/leds/boot-console:green:status
echo none > "$LED/trigger"
echo 1 > "$LED/brightness"
echo 0 > "$LED/brightness"

See firmware/README.md for build details and firmware/docs/BRINGUP.md for bench bring-up.

Manufacturing and purchasing

• fabrication/clarks-board-JLCPCB-gerbers.zip is the exact bare-board package used for the final revision.
• fabrication/assembly/clarks-board-JLCPCB-BOM.csv and fabrication/assembly/clarks-board-JLCPCB-CPL.csv are formatted for JLCPCB assembly upload.
• procurement/DigiKey-order.csv and procurement/LCSC-order.csv contain the final supplier split used during sourcing.
• hardware/assembly/ibom.html is an interactive assembly view.

The fabrication package retains the routed board exactly. USB impedance was not matched to a specific JLCPCB controlled-impedance stackup, so select and verify a stackup before changing USB routing or ordering a controlled-impedance revision.

Validation status

• The assembled board reaches a Linux login prompt over UART.
• The status LED works through the Linux LED subsystem after correcting duplicate PC0 pin ownership.
• KiCad ERC and DRC pass on the final source.
• The saved fabrication package matches the final KiCad source.
• Power/reset calculations and saved transient results are documented in docs/POWER-VERIFICATION.md.

Generated firmware images, Buildroot downloads, compiler output, KiCad backups, caches, and editor state are intentionally excluded from Git.