How to calculate actual Baud Rate and its percent error?

Question

I am using an Arduino Uno, and I need to create a program that calculates the actual baud rate and the percent error for all standard baud rate in Arduino monitor program.

Answer 1

First, look at the code used by the Arduino core library to configure the USART baud rate register: it's in HardwareSerial::begin(). The general formula is

uint16_t baud_setting = (F_CPU / 4 / baud - 1) / 2;

but beware of the hard-coded exception. You can replicate that part of the code in your own.

Then, look at the ATmega328P datasheet. More specifically, at section 24.4.1. Internal Clock Generation – The Baud Rate Generator. Here you find the formula

BAUD = F_CPU / (8*(UBRR0 + 1))

assuming the U2X0 bit has been set (again, beware of the hard-coded exception). Combine both formulas and you get the actual baud rate.