Your board and how to work with it

Rules that keep the board alive

• The header is 3.3 V. Never put 5 V on a GPIO pin; it is the one mistake that damages the chip for good. Power sensors and modules from a 3.3 V pin. The 5 V pin powers only a load switched through a MOSFET that draws under about 200 mA, such as lab 6's motor or a relay coil; anything larger gets its own supply, sharing only ground with the board. A servo runs from its own 5 V supply, sharing only ground with the board; only its control signal comes from a pin.
• Never run a motor, relay or solenoid from a GPIO pin. A pin gives a few milliamps; switch the load with a MOSFET, with a flyback diode across the load's coil, stripe to the positive supply.
• Power off before you rewire. Shut down with sudo poweroff and wait for the console to show the board has halted, then unplug the USB Type-C supply, change the wiring, check it, and power on. Pulling the plug on a running board can corrupt its boot media.
• Never connect the USB-to-UART adapter's power pin. Ground, transmit and receive only; the board has its own supply.
• Cross the serial lines. The adapter's transmit goes to the board's receive, and the adapter's receive to the board's transmit.
• Use only a 3.3 V USB-to-UART adapter. The console lines are at the board's 3.3 V logic level.

Writing an image

Write the course's image, the one your instructor chose for the term, unless a lab names another.

Use the Libre Computer app for Android: put the boot media in the board first (on Das Frite the USB drive in a USB Type-A port, on Solitude the microSD card in its slot), then connect your phone to the board's USB Type-C port, and the app identifies the board and writes the checksum-verified image you pick from the libre.computer catalogue.

Without an Android phone, use a desktop imager on the bench PC, such as Raspberry Pi Imager or balenaEtcher, with the image file kept on the bench PC (the lab network has no internet): write a USB drive for Das Frite or a microSD card for Solitude. Either way, check the image's SHA-256 against the value your instructor recorded, the one published with it on libre.computer downloads.

Reading while you work

The lab network joins your board and bench PC, not the internet. Open the GPIO, layout and power viewers, the schematics, datasheets and the kernel's documentation on your own laptop or phone, or on a bench PC that your instructor has also connected to the campus network. Datasheets for the station's parts are also kept as files on the bench PC, and each station has a printed map of your board's 40-pin header.

When it will not boot

Recovering a board is part of the course: every embedded engineer breaks a boot sooner or later. Work down the list, and tell your TA if you reach the last row.