osc_detector.cpp File Reference

Detailed Description

This logic automatically detects the speed of the oscillator or crystal connected to HSE.

Date

12 July 2021

It works by first using the reasonably-precise HSI oscillator (16MHz) to measure LSI (nominally 32khz, but wide tolerance). Then, it switches the system clock source to HSE, and repeats the same measurement. The inaccurate LSI will not drift significantly in the short period of time between these two measurements, so use it as a transfer standard to compare the speed of HSI and HSE. The ratio between the measured speed of LSI when running on HSE vs. HSI will give the ratio of speeds of HSE and HSI themselves. Since we know the value of HSI (16mhz), we can compute the speed of HSE.

Lastly, the PLL is reconfigured to use the correct input divider such that the input frequency is 1MHz (PLLM is set to N for an N-MHz HSE crystal).

Definition in file osc_detector.cpp.

Functions
static uint32_t	getOneCapture ()
static uint32_t	getTimerCounts (size_t count)
static void	enableTimer ()
static void	disableTimer ()
static void	reprogramPll (uint8_t roundedHseMhz)
void	__late_init ()

Variables
float	hseFrequencyMhz
uint8_t	autoDetectedRoundedMhz
static const float	rtcpreDivider = 63

Function Documentation

__late_init()

void __late_init

(

void

)

Definition at line 145 of file osc_detector.cpp.

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disableTimer()

static void disableTimer ( )

static

Definition at line 68 of file osc_detector.cpp.

                           {
    RCC->APB2ENR &= RCC_APB2ENR_TIM17EN;
}

Referenced by __late_init().

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enableTimer()

static void enableTimer ( )

static

Definition at line 64 of file osc_detector.cpp.

                          {
    RCC->APB2ENR |= RCC_APB2ENR_TIM17EN;
}

Referenced by __late_init().

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getOneCapture()

static uint32_t getOneCapture ( )

static

Definition at line 32 of file osc_detector.cpp.

                                {
    // wait for input capture
    while ((TIMER->SR & TIM_SR_CC1IF) == 0);
 
    // Return captured count
    return TIMER->CCR1;
}

Referenced by getTimerCounts().

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getTimerCounts()

static uint32_t getTimerCounts ( size_t  count )

static

Definition at line 40 of file osc_detector.cpp.

                                             {
    // Burn one count
    getOneCapture();
 
    uint32_t firstCapture = getOneCapture();
    uint32_t lastCapture = 0;
 
    for (size_t i = 0; i < count; i++)
    {
        lastCapture = getOneCapture();
    }
 
    return lastCapture - firstCapture;
}

Referenced by __late_init().

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reprogramPll()

static void reprogramPll ( uint8_t  roundedHseMhz )

static

Definition at line 72 of file osc_detector.cpp.

                                                {
    // Switch system clock to HSI to configure PLL (SW = 0)
    RCC->CFGR &= ~RCC_CFGR_SW;
 
    // Stop all 3 PLLs
    RCC->CR &= ~(RCC_CR_PLL1ON | RCC_CR_PLL2ON | RCC_CR_PLL3ON);
 
    // H7 is configured for 2MHz input to PLL
    auto pllm = roundedHseMhz / 2;
 
    // Set PLLM for all 3 PLLs to the new value, and select HSE as the clock source
    RCC->PLLCKSELR =
        pllm << RCC_PLLCKSELR_DIVM1_Pos |
        pllm << RCC_PLLCKSELR_DIVM2_Pos |
        pllm << RCC_PLLCKSELR_DIVM3_Pos |
        RCC_PLLCKSELR_PLLSRC_HSE;
 
    // Enable PLLs
    RCC->CR |= RCC_CR_PLL1ON | RCC_CR_PLL2ON | RCC_CR_PLL3ON;
 
    // Wait for PLLs to lock
    auto readyMask = RCC_CR_PLL1RDY | RCC_CR_PLL2RDY | RCC_CR_PLL3RDY;
    while ((RCC->CR & readyMask) != readyMask) ;
 
    // Switch system clock source back to PLL
    RCC->CFGR |= RCC_CFGR_SW_PLL1;
}

Referenced by __late_init().

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Variable Documentation

autoDetectedRoundedMhz

uint8_t autoDetectedRoundedMhz

Definition at line 24 of file osc_detector.cpp.

Referenced by __late_init(), and sayHello().

hseFrequencyMhz

float hseFrequencyMhz

Definition at line 23 of file osc_detector.cpp.

Referenced by __late_init(), and sayHello().

rtcpreDivider

static const float rtcpreDivider = 63

static

Definition at line 62 of file osc_detector.cpp.

Referenced by __late_init().

Go to the source code of this file.