UART Troubleshooting Guide

UART is simple in principle but there are four common failure modes that trip up beginners. Here they are in order of likelihood.

Baud rate mismatch: the most common cause of garbage output. Both sides must agree exactly. The UART hardware generates timing from a crystal or oscillator and small frequency errors accumulate over a frame — at 3% error you'll see bit errors at high baud rates. Check both sides' clock source.

Logic level mismatch: a 5V MCU TX directly connected to a 3.3V MCU RX will damage the 3.3V device over time even if it appears to work. Use a voltage divider (two resistors) or a level shifter. Crossed TX/RX: TX goes to RX, RX goes to TX. It's obvious but it accounts for maybe 30% of beginner UART problems. Framing errors: wrong stop bit count (usually should be 1) or parity setting. Check that both sides are 8N1 or whatever format your device expects.