A UART link produces consistent garbage. What's the most likely cause?

A baud-rate mismatch, consistent garbage (the same wrong bytes every time for given input) is the signature, because the framing is being decoded but the sampling is systematically off. The two ends are configured for different baud rates, or one end computed its baud divider from the wrong assumed peripheral clock (a clock-tree misconfiguration, e.g., assuming the wrong APB frequency or missing the timer/UART clock relationship). Other consistent-garbage causes in the same family: mismatched frame format (data bits, parity, or stop bits differ between ends) so the receiver mis-aligns the fields. By contrast, intermittent garbage points more at noise, a missing/poor common ground, or marginal levels. The debugging order: verify both ends' baud and frame settings agree, confirm the actual UART clock matches what the baud calculation assumed (scope the bit period, at 115200 each bit is ~8.68 µs), and check the common ground. A scope or logic analyzer measuring the actual bit width immediately reveals a baud error.