PWMPulseWidthSet不太理解这个函数是怎么实现一定的占空比的设置的

void
PWMPulseWidthSet(uint32_t ui32Base, uint32_t ui32PWMOut,
                 uint32_t ui32Width)
{
    uint32_t ui32GenBase, ui32Reg;

    //
    // Check the arguments.
    //
    ASSERT((ui32Base == PWM0_BASE) || (ui32Base == PWM1_BASE));
    ASSERT(_PWMOutValid(ui32PWMOut));

    //
    // Compute the generator's base address.
    //
    ui32GenBase = PWM_OUT_BADDR(ui32Base, ui32PWMOut);

    //
    // If the counter is in up/down count mode, divide the width by two.
    //
    if(HWREG(ui32GenBase + PWM_O_X_CTL) & PWM_X_CTL_MODE)
    {
        ui32Width /= 2;
    }

    //
    // Get the period.
    //
    ui32Reg = HWREG(ui32GenBase + PWM_O_X_LOAD);

    //
    // Make sure the width is not too large.
    //
    ASSERT(ui32Width < ui32Reg);

    //
    // Compute the compare value.
    //
    ui32Reg = ui32Reg - ui32Width;

    //
    // Write to the appropriate registers.
    //
    if(PWM_IS_OUTPUT_ODD(ui32PWMOut))
    {
        HWREG(ui32GenBase + PWM_O_X_CMPB) = ui32Reg;
    }
    else
    {
        HWREG(ui32GenBase + PWM_O_X_CMPA) = ui32Reg;
    }
}