The clock of stm32f103 SPI is different from theoretic calculation after initialization - stm32

Question

I am a new to stm32f103c8t6. Now I am trying to learn the SPI function and finished the code from the RCC to SPI initialization. I used oscilloscope to measure the SPI1 SCK signal due to test equipment limitation. Indeed that is clock signal but much lower than my RCC configuration. I tried to search the reason, but still failed. So may I ask if someone knows the answer.
RCC_DeInit();
RCC_HSEConfig(RCC_HSE_ON);
Errsts = RCC_WaitForHSEStartUp();
if (Errsts == SUCCESS)
{
FLASH_PrefetchBufferCmd(FLASH_PrefetchBuffer_Enable);
RCC_PLLConfig(RCC_PLLSource_HSE_Div2, RCC_PLLMul_9);
RCC_PLLCmd(ENABLE);
while (RCC_GetFlagStatus(RCC_FLAG_PLLRDY == RESET))
{
}
FLASH_SetLatency(FLASH_Latency_1);
RCC_SYSCLKConfig(RCC_SYSCLKSource_PLLCLK);
RCC_HCLKConfig(RCC_SYSCLK_Div1);
RCC_PCLK1Config(RCC_HCLK_Div2);
RCC_PCLK2Config(RCC_HCLK_Div1);
while (RCC_GetSYSCLKSource() != 0x08)
{
}
}
The code above shows my configuration about RCC, I used HSE as clock and the final frequency is 8Mhz/2*9=36Mhz. Below is my SPI and GPIO initialization:
SPI_InitTypeDef SPI_InitStructure;
GPIO_InitTypeDef GPIO_InitStructure;
RCC_APB2PeriphClockCmd(RCC_APB2Periph_GPIOB, ENABLE);
RCC_APB2PeriphClockCmd(RCC_APB2Periph_GPIOA, ENABLE);
RCC_APB2PeriphClockCmd(RCC_APB2Periph_SPI1, ENABLE);
GPIO_InitStructure.GPIO_Pin = GPIO_Pin_5| GPIO_Pin_6| GPIO_Pin_7;
GPIO_InitStructure.GPIO_Speed = GPIO_Speed_10MHz;
GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF_PP;
GPIO_Init(GPIOA, &GPIO_InitStructure);
GPIO_InitStructure.GPIO_Pin = GPIO_Pin_4;
GPIO_InitStructure.GPIO_Speed = GPIO_Speed_10MHz;
GPIO_InitStructure.GPIO_Mode = GPIO_Mode_Out_PP;
GPIO_Init(GPIOA, &GPIO_InitStructure);
GPIO_WriteBit(GPIOA, GPIO_Pin_4,Bit_SET);
SPI_InitStructure.SPI_Direction = SPI_Direction_2Lines_FullDuplex;
SPI_InitStructure.SPI_Mode = SPI_Mode_Master;
SPI_InitStructure.SPI_DataSize = SPI_DataSize_8b;
SPI_InitStructure.SPI_CPOL = SPI_CPOL_Low;
SPI_InitStructure.SPI_CPHA = SPI_CPHA_1Edge;
SPI_InitStructure.SPI_NSS = SPI_NSS_Soft;
SPI_InitStructure.SPI_BaudRatePrescaler = SPI_BaudRatePrescaler_4;
SPI_InitStructure.SPI_FirstBit = SPI_FirstBit_MSB;
SPI_InitStructure.SPI_CRCPolynomial = 7;
SPI_Init(SPI1, &SPI_InitStructure);
/* Enable SPI2 */
SPI_Cmd(SPI1, ENABLE);
So SPI_BaudRatePrescaler=4 means my SPI clock is 36Mhz/4=9Mhz, but actually the oscilloscope shows the frequecy is around 200Khz, it is much lower the the expectation. Also I tried to config SPI_BaudRatePrescaler with different value, it seems that the outcome is not linear.
Therefore I would like to ask if someone knows the answer. Thanks a lot!

Answer 1

Double check your prescaler value in your configuration. According to the reference manual, a value of 4 in the baud rate field will result in a divider of 32. To get to a clock of around 200 kHz, you need a value in that register of 6 or 7, which results in a divider of 128 or 256 (281 kHz or 140 kHz). If you want a divider of 4, you'll need to put a value of 1 in there. Check and see if SPI_BaudRatePrescaler_4 results in a value of 4 in the register, or a value of 1.
The outcome is not linear because they are using it as an exponent of 2. A value of 0 is 2^(0+1), while 1 is 2^(1+1), etc, etc.