ExactBench · Engineering calculators

Baud Rate Error Calculator

UART · integer divisor

Baud rate error

-3.55

111111.1 baud actual vs 115200 requested

Tolerance

9.5 bit times to the stop bit

Divisor (integer)9

Ideal divisor8.6806

Actual baud rate111111.11

Error-3.549 %

Drift at stop bit33.7 % of a bit time

Sampling clock1.78 MHz

VerdictWill not work reliably

How this works. A UART divides its peripheral clock down to produce a sampling clock, usually 16× or 8× the bit rate, then divides again by the oversampling factor. The divisor is an integer, so most clock and baud combinations cannot be hit exactly — the error is whatever the rounding leaves behind. What matters is not the error itself but how much it accumulates: the receiver resynchronises on each start edge, then samples the middle of each bit, so the worst case is the stop bit about 9.5 bit times later. Drift there must stay well under half a bit, which is where the familiar ±2 % rule of thumb comes from. Both ends contribute, so two devices each 2 % off in opposite directions is already 4 %.

This error is above ±2 %. It may work between two accurate endpoints and fail against a device with its own error in the opposite direction — pick a clock that divides more cleanly if you can.

Common clock and baud combinations (16× oversampling)

11.0592 MHz → 115200	divisor 6 · 0.00 %	Exact. This is the entire reason that crystal exists
14.7456 MHz → 115200	divisor 8 · 0.00 %	Exact, and leaves more headroom for the core
16 MHz → 9600	divisor 104 · +0.16 %	Fine
16 MHz → 115200	divisor 9 · −3.55 %	Marginal. The classic Arduino mismatch
8 MHz → 9600	divisor 52 · +0.16 %	Fine
12 MHz → 115200	divisor 7 · −6.99 %	Will not work reliably
48 MHz → 115200	divisor 26 · +0.16 %	Typical USB-capable clock

The odd-looking crystals — 11.0592 and 14.7456 MHz — are integer multiples of 115200 × 16. They exist purely so the UART divides exactly, and they remain the cheapest fix for a link that drops bytes.

Clock and baud

Baud rate error

Tolerance

Common clock and baud combinations (16× oversampling)

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