Stm32f407DISCOVEY study uart serial port baud rate summary-EEWORLD

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I have been debugging Stm32f407DISCOVEY these days. Since there is no ready-made library function routine for the serial port, I used the routine on the Shenzhouwang development board to directly debug. As a result, the serial port output is always messy, 115200 baud rate; I tried to change it to 38400 and it can actually print, which is more than 3 times the difference; there is only one truth, after debugging and searching Baidu, I finally found the reason;

.The clock frequency is wrong. Since the clock frequency is automatically defined in the library file, I didn’t notice it at first, but the baud rate was always wrong. Later I found that the problem was that the external crystal oscillator defined in the library file was 25M, and the DISCOVERY board used an 8M crystal oscillator, so the generated main frequency was wrong. Two places need to be changed. ①So in stm32f4xx.h, change
#if !defined (HSE_VALUE)
#define HSE_VALUE ((uint32_t)8000000) /*!< Value of the External oscillator in Hz */
#endif /* HSE_VALUE */

In system_stm32f4xx.c, change
/* PLL_VCO = (HSE_VALUE or HSI_VALUE / PLL_M) * PLL_N */

#define PLL_M 8

The serial port prints normally;

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