Concepts

Cross-board concept guides. Each one answers a single question and grounds every claim in the fixed sources on the board pages — no unsourced generalities.

• Chip, module, board — the three layers of every dev board
• Pin numbers, GPIO numbers and pin functions
• 3.3 V and 5 V — power rails, logic levels and the one diode rule
• UART, I2C and SPI — what the three buses are