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Choosing the USARTx baud rate

AN3155

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Choosing the USARTx baud rate

The calculation of the serial baud rate for USARTx, from the length of the first byte that is

received, is used to operate the bootloader within a wide range of baud rates. However, the

upper and lower limits have to be kept, in order to ensure proper data transfer.

For a correct data transfer from the host to the microcontroller, the maximum deviation

between the internal initialized baud rate for USARTx and the real baud rate of the host

should be below 2.5%. The deviation (fB, in percent) between the host baud rate and the

microcontroller baud rate can be calculated using the formula below:

, where fB ≤ 2.5%.

This baud rate deviation is a nonlinear function depending on the CPU clock and the baud

rate of the host. The maximum of the function (fB) increases with the host baud rate. This is

due to the smaller baud rate prescale factors, and the implied higher quantization error.

2.1

Minimum baud rate

The lowest tested baud rate (BLow) is 1200. Baud rates below BLow would cause the

SysTick timer to overflow. In this event, USARTx would not be correctly initialized.

2.2

Maximum baud rate

BHigh is the highest baud rate for which the deviation still does not exceed the limit. All baud

rates between BLow and BHigh are below the deviation limit.

The highest tested baud rate (BHigh) is 115 200.

STM32 baud rate Host baud rate

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STM32 baud rate

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