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

Posted on August 06, 2016 at 13:53

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:

 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!