STM32 system operating frequency and CAN working baud rate setting based on library function version

1. System operating frequency setting

    The register version and the library version of the STM32 system operating frequency setting are quite different. The library version of the system operating frequency is set by the SystemInit() function in system_stm32f10x.c, and other configurations are mainly in the stm32f10x_rcc.c file. For the system clock, by default, it is determined in the SetSysClock() function of the SystemInit function, and the setting is set through macro definition.

static void SetSysClock(void)

{

#ifdef SYSCLK_FREQ_HSE

  SetSysClockToHSE();

#elif defined SYSCLK_FREQ_24MHz

  SetSysClockTo24();

#elif defined SYSCLK_FREQ_36MHz

  SetSysClockTo36();

#elif defined SYSCLK_FREQ_48MHz

  SetSysClockTo48();

#elif defined SYSCLK_FREQ_56MHz

  SetSysClockTo56(); 

#elif defined SYSCLK_FREQ_72MHz

  SetSysClockTo72();

#endif

}

The higher the operating frequency, the higher the system power consumption. You do not need to update the delay_ms() function after changing the operating frequency, but you should pay special attention to the problem of setting the CAN baud rate. Changing the operating frequency will change the CAN baud rate nonlinearly.

2. CAN baud rate setting

         Here, it is specially noted that the optional working frequencies in the firmware library are: 24MHz, 36MHz, 48MHz, 56MHz, 72MHz, which are set by macro definition. When set to 24MHz, 48MHz, the frequency used for calculation is 48MHz; when set to 36MHz, 72MHz, the frequency used for calculation is 72MHz; when set to 56MHz, the baud rate is calculated using 56MHz. For example, in the routine given by the battleship STM32, the working frequency is 72M and the default baud rate is 450kps (36000/[(7+8+1)*5]=450Kbps). When we change the working frequency to 36M, the baud rate is still 450kps. If the working frequency is changed to 24M, the baud rate becomes 24000/[(7+8+1)*5]=300Kbps. Why is this change, the principle is not clear yet.

    The figure also gives the calculation formula of CAN baud rate. We only need to know the settings of BS1 and BS2, and the clock frequency of APB1 (usually 36Mhz, that is, the working frequency of APB1 under 72M working frequency), and we can easily calculate the baud rate. For example, if TS1=6, TS2=7 and BRP=4 are set, under the condition that the APB1 frequency is 36Mhz, the baud rate of CAN communication can be obtained = 36000/[(7+8+1)*5]=450Kbps. Set the function to CAN_Mode_Init(CAN_SJW_1tq,CAN_BS2_8tq,CAN_BS1_7tq,5,CAN_Mode_Normal);//Normal mode

Optional parameters are:

#define CAN_SJW_1tq                 ((uint8_t)0x00)  /*!< 1 time quantum */

#define CAN_SJW_2tq                 ((uint8_t)0x01)  /*!< 2 time quantum */

#define CAN_SJW_3tq                 ((uint8_t)0x02)  /*!< 3 time quantum */

#define CAN_SJW_4tq                 ((uint8_t)0x03)  /*!< 4 time quantum */

#define CAN_BS1_1tq                 ((uint8_t)0x00)  /*!< 1 time quantum */

#define CAN_BS1_2tq                 ((uint8_t)0x01)  /*!< 2 time quantum */

#define CAN_BS1_3tq                 ((uint8_t)0x02)  /*!< 3 time quantum */

#define CAN_BS1_4tq                 ((uint8_t)0x03)  /*!< 4 time quantum */

#define CAN_BS1_5tq                 ((uint8_t)0x04)  /*!< 5 time quantum */

#define CAN_BS1_6tq                 ((uint8_t)0x05)  /*!< 6 time quantum */

#define CAN_BS1_7tq                 ((uint8_t)0x06)  /*!< 7 time quantum */

#define CAN_BS1_8tq                 ((uint8_t)0x07)  /*!< 8 time quantum */

#define CAN_BS1_9tq                 ((uint8_t)0x08)  /*!< 9 time quantum */

#define CAN_BS1_10tq                ((uint8_t)0x09)  /*!< 10 time quantum */

#define CAN_BS1_11tq                ((uint8_t)0x0A)  /*!< 11 time quantum */

#define CAN_BS1_12tq                ((uint8_t)0x0B)  /*!< 12 time quantum */

#define CAN_BS1_13tq                ((uint8_t)0x0C)  /*!< 13 time quantum */

#define CAN_BS1_14tq                ((uint8_t)0x0D)  /*!< 14 time quantum */

#define CAN_BS1_15tq                ((uint8_t)0x0E)  /*!< 15 time quantum */

#define CAN_BS1_16tq                ((uint8_t)0x0F)  /*!< 16 time quantum */

#define CAN_BS2_1tq                 ((uint8_t)0x00)  /*!< 1 time quantum */

#define CAN_BS2_2tq                 ((uint8_t)0x01)  /*!< 2 time quantum */

#define CAN_BS2_3tq                 ((uint8_t)0x02)  /*!< 3 time quantum */

#define CAN_BS2_4tq                 ((uint8_t)0x03)  /*!< 4 time quantum */

#define CAN_BS2_5tq                 ((uint8_t)0x04)  /*!< 5 time quantum */

#define CAN_BS2_6tq                 ((uint8_t)0x05)  /*!< 6 time quantum */

#define CAN_BS2_7tq                 ((uint8_t)0x06)  /*!< 7 time quantum */

#define CAN_BS2_8tq                 ((uint8_t)0x07)  /*!< 8 time quantum */   

    In actual engineering applications, the design of SJW, BS1, BS2, and BRP is involved. The meanings of the four can be found in the definition in the figure below. In the CAN initialization function:

CAN_Mode_Init(CAN_SJW_1tq,CAN_BS2_8tq,CAN_BS1_7tq,5,CAN_Mode_Normal); //Normal mode

The "5" in the STM32 firmware library function is the direct frequency division coefficient, and does not require +1, which is different from the figure below. In order to achieve reliable long-distance transmission, it is necessary to consider the reasonable combination of four parameters. The theoretically allowable transmission delay is determined by the position of the sampling point, so the selection of the sampling point position within a bit period is very important. The sampling point at the back will allow a larger transmission delay error t. , so that the system can transmit farther; on the contrary, selecting a sampling point at the front will allow a larger clock tolerance. Selecting a crystal oscillator with a small clock tolerance can make the sampling point selection position later. The CAN sampling point in STM32 is between BS1 and BS2, so setting BS1 and BS2 to a larger value can achieve maximum reliability.