The clock of stm32f103 SPI is different from theoretic calculation after initialization
<p>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.</p>

<pre><code>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)
    {
    }
}
</code></pre>

<p>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:</p>

<pre><code>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, &amp;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, &amp;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, &amp;SPI_InitStructure); 
/* Enable SPI2 */ 
SPI_Cmd(SPI1, ENABLE); 
</code></pre>

<p>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. </p>

<p>Therefore I would like to ask if someone knows the answer. Thanks a lot!</p>