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

Question asked by wang.hao.005 on Aug 6, 2016
Latest reply on Aug 6, 2016 by wang.hao.005
Hi all, 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.

Here is my RCC configuration:
     Errsts = RCC_WaitForHSEStartUp();

     if (Errsts == SUCCESS)
          RCC_PLLConfig(RCC_PLLSource_HSE_Div2, RCC_PLLMul_9);

          while (RCC_GetFlagStatus(RCC_FLAG_PLLRDY == RESET))
          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!