FreeRTOS two CPU clocks

Question

Dependent on a HW assembly variant my firmware should operate with 2.1 or 4.2 MHz. In FreeRTOS configCPU_CLOCK_HZ is already set while compile time. Is there any possibility to set this frequency while initialisation time?

Answer 1

configCPU_CLOCK_HZ seems to be used in vPortSetupTimerInterrupt() function, which simply configures SysTick hardware registers (if you're not using tickless mode). I guess it should be possible to configure these register manually even when the scheduler is running (but I'm not sure).

But there is probably a better way: vPortSetupTimerInterrupt() is defined with __attribute__((weak)) in the source code. It means that, if you provide your own version of vPortSetupTimerInterrupt(), it will replace the original one. In your own version, simply load SysTick CTRL & LOAD registers with the appropriate values.

Here is the original version of vPortSetupTimerInterrupt() (This can vary depanding on the uC model):

/*
 * Setup the systick timer to generate the tick interrupts at the required
 * frequency.
 */
__attribute__( ( weak ) ) void vPortSetupTimerInterrupt( void )
{
    /* Calculate the constants required to configure the tick interrupt. */
    #if ( configUSE_TICKLESS_IDLE == 1 )
        {
            ulTimerCountsForOneTick = ( configSYSTICK_CLOCK_HZ / configTICK_RATE_HZ );
            xMaximumPossibleSuppressedTicks = portMAX_24_BIT_NUMBER / ulTimerCountsForOneTick;
            ulStoppedTimerCompensation = portMISSED_COUNTS_FACTOR / ( configCPU_CLOCK_HZ / configSYSTICK_CLOCK_HZ );
        }
    #endif /* configUSE_TICKLESS_IDLE */

    /* Stop and clear the SysTick. */
    portNVIC_SYSTICK_CTRL_REG = 0UL;
    portNVIC_SYSTICK_CURRENT_VALUE_REG = 0UL;

    /* Configure SysTick to interrupt at the requested rate. */
    portNVIC_SYSTICK_LOAD_REG = ( configSYSTICK_CLOCK_HZ / configTICK_RATE_HZ ) - 1UL;
    portNVIC_SYSTICK_CTRL_REG = ( portNVIC_SYSTICK_CLK_BIT | portNVIC_SYSTICK_INT_BIT | portNVIC_SYSTICK_ENABLE_BIT );
}

You can probably just copy the original one (without the weak attribute of course) and replace configCPU_CLOCK_HZ with some global variable you set in your code.