[技术问答] 新唐M0516定时器例程疑问

/* Peripheral clock source */

    CLK->CLKSEL1 = CLK_CLKSEL1_UART_S_PLL |

                   CLK_CLKSEL1_TMR0_S_HXT | CLK_CLKSEL1_TMR1_S_HCLK | CLK_CLKSEL1_TMR2_S_HIRC | CLK_CLKSEL1_TMR3_S_HXT;

 /* Open Timer3 frequency to 4 Hz in periodic mode, and enable interrupt */

    TIMER_Open(TIMER3, TIMER_PERIODIC_MODE, 8);

    TIMER_EnableInt(TIMER3);

/**

  * [url=home.php?mod=space&uid=247401]@brief[/url]      Open Timer in specified mode and frequency

  *

  * @param[in]  timer       The base address of Timer module. It could be TIMER0, TIMER1, TIMER2, TIMER3.

  * @param[in]  u32Mode     Operation mode. Possible options are

  *                         - \ref TIMER_ONESHOT_MODE

  *                         - \ref TIMER_PERIODIC_MODE

  *                         - \ref TIMER_TOGGLE_MODE

  *                         - \ref TIMER_CONTINUOUS_MODE

  * @param[in]  u32Freq     Target working frequency

  *

  * [url=home.php?mod=space&uid=266161]@return[/url]     Real Timer working frequency

  *

  * [url=home.php?mod=space&uid=1543424]@Details[/url]    This API is used to configure timer to operate in specified mode and frequency.

  *             If timer cannot work in target frequency, a closest frequency will be chose and returned.

  * [url=home.php?mod=space&uid=536309]@NOTE[/url]       After calling this API, Timer is \b NOT running yet. But could start timer running be calling

  *             \ref TIMER_Start macro or program registers directly.

  */

uint32_t TIMER_Open(TIMER_T *timer, uint32_t u32Mode, uint32_t u32Freq)

{

    uint32_t u32Clk = TIMER_GetModuleClock(timer);

    uint32_t u32Cmpr = 0, u32Prescale = 0;



    // Fastest possible timer working freq is (u32Clk / 2). While cmpr = 2, pre-scale = 0.

    if(u32Freq > (u32Clk / 2))

    {

        u32Cmpr = 2;

    }

    else

    {

        if(u32Clk >= 0x4000000)

        {

            u32Prescale = 7;    // real prescaler value is 8

            u32Clk >>= 3;

        }

        else if(u32Clk >= 0x2000000)

        {

            u32Prescale = 3;    // real prescaler value is 4

            u32Clk >>= 2;

        }

        else if(u32Clk >= 0x1000000)

        {

            u32Prescale = 1;    // real prescaler value is 2

            u32Clk >>= 1;

        }



        u32Cmpr = u32Clk / u32Freq;

    }



    timer->TCSR = u32Mode | u32Prescale;

    timer->TCMPR = u32Cmpr;



    return(u32Clk / (u32Cmpr * (u32Prescale + 1)));

}