UART in Depth
A UART has no clock wire. That single fact generates every interesting property of the peripheral and every way it fails. SPI and I²C both ship a clock alongside the data, so the receiver is told exactly when to look; a UART receiver is told nothing. It sees a falling edge, starts its own counter, and from that moment guesses where the bit centres are using an oscillator the transmitter has never met. Everything below — the divider arithmetic, the oversampling modes, the tolerance budget, the overrun flag — is machinery built around that one guess.