STM32 MCU serial communication baud rate calculation method

1. What is baud rate?

No matter what microcontroller it is, when using serial communication, there is a very important parameter: baud rate. What is baud rate: baud rate is the number of bytes transmitted per second. The baud rate is consistent during data transmission between the two parties, which is the basic guarantee for successful communication. The following takes the STM32 microcontroller as an example to explain how to calculate the serial port baud rate.

2. STM32 baud rate related registers

The STM32 microcontroller has only one register for setting the baud rate: the USART_BRR register, as shown in the figure below.

The effective number of this register is 16 bits, the first 4 bits are used to store the decimal part, and the last 12 bits are used to store the integer part. After calculating the baud rate, fill in the value of this baud rate. The following describes how to calculate.

3. Baud rate calculation method

The STM32 data sheet gives the calculation method, there is a formula, as shown below:

In this formula, there are three variables, two of which we know, Fck and Tx/Rx baud rate. These two are known, and USARTDIV is unknown. From the description of the formula, we can see that if USART1 is used, then Fck is PCLK2=72MHz, otherwise it is PCLK1=36MHz, and the parameter of Tx/Rx baud rate is known. We only need to calculate the value of USARTDIV and assign it to the USART_BRR register. Taking 115200 as an example, after transforming the formula, we get: USARTDIV = 72×1000000/(16×115200) = 39.0625. That is, write 39.0625 into USART_BRR.

As mentioned above, the first 4 bits of USART_BRR store the decimal part, and the last 12 bits store the integer part.

The decimal part DIV_Fraction = 0.0625×16 = 1 = 0x01;

The integer part DIV_Mantissa = 39 = 0x27;

Then USART_BRR = 0X271;

The data sheet provides us with a data table:

In this data sheet, the commonly used baud rate values ​​have been calculated, and we can use them directly. However, if we want to make the baud rate calculation a parameter-passing function,

For example: USART_INIT(uint_t 16 Baud), when calling, you only need to write USART_INIT(9600), USART_INIT(115200), so how should the program be written?

4. Program Implementation

The following is the register version of the program.

The first step of the program, line 45: first calculate USART_DIV, which is a variation of the previous formula;

The second step of the program, line 46: Get the integer part of USART_DIV;

Program step 3, line 47: Get the decimal part of USART_DIV;

The fourth step of the program, line 48: Shift the integer part left by 4 bits, that is, store it in the last 12 bits of the BRR register;

The fifth step of the program, line 49: combine the integer and the decimal part, that is, reassemble the first 4 digits and the last 12 digits;

Program step 6, line 50: Assign the calculated value to the BRR register.

In this way, a function with baud rate as a parameter is completed. From the above, it seems that the calculation of baud rate is not very difficult.