STM32F4 PLL Precision - stm32

Question

Im trying to configure clocks on STM32F4 Discovery for precise time measurement. I have this configuration:
int main(void)
{
NVIC_InitTypeDef nvici;
GPIO_InitTypeDef gpioi;
TIM_TimeBaseInitTypeDef timtbi;
SystemInit();
RCC_HSEConfig(RCC_HSE_ON);
RCC_PLLConfig(RCC_PLLCFGR_PLLSRC_HSE, 8, 320, 8, 8);
RCC_PLLCmd(ENABLE);
RCC_SYSCLKConfig(RCC_SYSCLKSource_PLLCK);
RCC_HCLKConfig(RCC_SYSCLK_Div1);
RCC_PCLK1Config(RCC_HCLK_Div1);
RCC_PCLK2Config(RCC_HCLK_Div1);
RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_GPIOD, ENABLE);
nvici.NVIC_IRQChannel = TIM2_IRQn;
nvici.NVIC_IRQChannelPreemptionPriority = 0;
nvici.NVIC_IRQChannelSubPriority = 1;
nvici.NVIC_IRQChannelCmd = ENABLE;
NVIC_Init(&nvici);
gpioi.GPIO_Pin = GPIO_Pin_15;
gpioi.GPIO_Mode = GPIO_Mode_OUT;
gpioi.GPIO_OType = GPIO_OType_PP;
gpioi.GPIO_Speed = GPIO_Speed_100MHz;
gpioi.GPIO_PuPd = GPIO_PuPd_NOPULL;
GPIO_Init(GPIOD,&gpioi);
RCC_APB1PeriphClockCmd(RCC_APB1Periph_TIM2, ENABLE);
timtbi.TIM_Period = 20000000;
timtbi.TIM_Prescaler = 0;
timtbi.TIM_ClockDivision = 0;
timtbi.TIM_CounterMode = TIM_CounterMode_Up;
timtbi.TIM_RepetitionCounter = 0;
TIM_TimeBaseInit(TIM2, &timtbi);
TIM_ClearITPendingBit(TIM2, TIM_IT_Update);
TIM_ITConfig(TIM2, TIM_IT_Update, ENABLE);
TIM_Cmd(TIM2, ENABLE);
GPIO_SetBits(GPIOD,GPIO_Pin_15);
while(1)
{
}
}
void TIM2_IRQHandler()
{
TIM_ClearITPendingBit(TIM2, TIM_IT_Update);
GPIO_ToggleBits(GPIOD,GPIO_Pin_15);
}
with this i should have TIM2 sourced with 20MHz clock, but it appears to have diffrent frequency (about 10-30% diffrent). This problem appears for all other PLL configurations i tried, but when i use HSE as SYSCLK directly it works just fine. Am i doing something wrong, or is it PLL that isn't reliable?

Answer 1

Can't say with 100% certainty whether that's the problem, but after enabling the HSE using RCC_HSEConfig(), you should call RCC_WaitForHSEStartUp() since it takes a while for the HSE to start oscillating, and check the return code to make sure the call was successful and the HSE actually initialized.
Also, if you're using the system_stm32f4xx.c file that comes with the Standard Peripheral Library, you can scrap your PLL initialization code and just use the code that's called by SystemInit(). There are a few #defines that control the PLL configurations, near the beginning of the file (#define PLL_M, #define PLL_N and so on; their purpose should be self-evident). I always initialize my clocks using the code there, and they're always precise to within the crystal's accuracy. Note that this code assumes a 25 MHz oscillator by using PLL_M equal to 25, so you should set it to 8 for use with the STM32F4DISCOVERY board -- exactly as you've already done in your code. I'm not suggesting this because I have any prejudices against your code, but the code there has been tested far and wide, and in my experience it can be trusted.