comms: UART / I²C / SPI
| bus | wires | pros | notes |
|---|---|---|---|
| UART | TX,RX | simple point-to-point | baud, parity, levels (3.3/5 V) |
| I²C | SCL,SDA | two wires, many devices | pull-ups, 7-bit addr, bus caps |
| SPI | SCLK,MOSI,MISO,CS | fast | one CS per device; mode 0/1/2/3 |
caution
I²C needs pull-ups to the correct rail; don’t mix 5 V pull-ups with 3.3 V parts.