Software/bitbang method of baud rate calibration for micro without built-in UART, App note from Holtek. Link here (PDF)
Not every HOLTEK MCU contains a Universal Asynchronous Receiver/Transmitter function, otherwise known as a UART. If this function is required, it can be emulated using software. However, the software accuracy will change according to the system frequency error. This application note will illustrate how to use a software method to calibrate the baud rate and how to avoid errors between the two communicating parties. It will also show how to readjust to the same transmission frequency of each other when transmission errors occur.
Taking the HT66F4540 as an example, this application note describes the UART baud rate calibration method.