STM8S定时器的预分频器问题

PSCR contains the value which is loaded in the active prescaler register at each UEV (including when the counter is cleared through the UG bit of the TIM1_EGR register or through the trigger controller when configured in trigger reset mode).

A UEV must be generated so that a new prescaler value can be taken into account.

这是摘自手册中的一段话，就是说写入的预分频值要想生效，就必须产生一个更新事件，但是我看了固件库中的有些例子，在定时器初始化时，写入了预分频值，但是就没有置位UG产生更新事件使写入的预分频值生效，这怎么解释啊?下面是两个定时器初始化的例子，一个置位UG，另一个没有：

static void TIM2_Config(void)

{

  /* Time base configuration */        

TIM2_TimeBaseInit(TIM2_PRESCALER_2048, 65535);  

/* Prescaler configuration，这里就是置位UG，使得写入的预分频值生效 */

TIM2_PrescalerConfig(TIM2_PRESCALER_2048, TIM2_PSCRELOADMODE_IMMEDIATE);

/* Output Compare Active Mode configuration: Channel1 */

  /*    TIM2_OCMode = TIM2_OCMODE_ACTIVE    TIM2_OutputState = TIM2_OUTPUTSTATE_ENABLE    TIM2_Pulse = CCR1_Val    TIM2_OCPolarity = TIM2_OCPOLARITY_HIGH  */  

TIM2_OC1Init(TIM2_OCMODE_ACTIVE, TIM2_OUTPUTSTATE_ENABLE,CCR1_Val, TIM2_OCPOLARITY_HIGH);

TIM2_OC1PreloadConfig(DISABLE);  

/* Output Compare Active Mode configuration: Channel2 */

   /*TIM2_Pulse = CCR2_Val  */

   TIM2_OC2Init(TIM2_OCMODE_ACTIVE, TIM2_OUTPUTSTATE_ENABLE,CCR2_Val, TIM2_OCPOLARITY_HIGH);  

TIM2_OC2PreloadConfig(DISABLE);   

/* Output Compare Active Mode configuration: Channel3 */

  /*  TIM2_Pulse = CCR3_Val  */  

TIM2_OC3Init(TIM2_OCMODE_ACTIVE, TIM2_OUTPUTSTATE_ENABLE,CCR3_Val, TIM2_OCPOLARITY_HIGH);  

TIM2_OC3PreloadConfig(DISABLE);

TIM2_ARRPreloadConfig(ENABLE);   

/* TIM2 enable counter */  

TIM2_Cmd(ENABLE);

}

static void TIM2_Config(void)

{

  /* Time base configuration 这里就没有置位UG，使得写入的预分频值生效*/

TIM2_TimeBaseInit(TIM2_PRESCALER_1, 999);  

/* PWM1 Mode configuration: Channel1 */

  TIM2_OC1Init(TIM2_OCMODE_PWM1, TIM2_OUTPUTSTATE_ENABLE,CCR1_Val, TIM2_OCPOLARITY_HIGH);

TIM2_OC1PreloadConfig(ENABLE);  

/* PWM1 Mode configuration: Channel2 */   

TIM2_OC2Init(TIM2_OCMODE_PWM1, TIM2_OUTPUTSTATE_ENABLE,CCR2_Val, TIM2_OCPOLARITY_HIGH);

TIM2_OC2PreloadConfig(ENABLE);  

/* PWM1 Mode configuration: Channel3 */      

  TIM2_OC3Init(TIM2_OCMODE_PWM1, TIM2_OUTPUTSTATE_ENABLE,CCR3_Val, TIM2_OCPOLARITY_HIGH);

  TIM2_OC3PreloadConfig(ENABLE);  

TIM2_ARRPreloadConfig(ENABLE);

  /* TIM2 enable counter */

TIM2_Cmd(ENABLE);

}