stm32 timer output comparison function different frequency

Output comparison: Turn on a TIMx counter, and then turn on one or several output comparators of TIMx (4 in total). After all are configured, the counter starts counting. When the value in the counter is equal to the value in the comparison register, an output comparison interrupt is generated. In the interrupt, the value in the counter is read out, added to the rollover period, and written to the comparison register, so that it has the same rollover period as the next event.

The general meaning is that after the counter is turned on, the count value continues to increase. When it increases to the value of the comparison register, the level flips and a comparison interrupt is generated. In the comparison interrupt, the counter value is read out and added to the period, and written into the comparison register together, so that they have the same flip period.

For example, if the TIM clock frequency is set to 12 MHZ, and the self-load value in the output compare register is 600 (high level or low level count value), the output PWM frequency is

                   frequency = 12MHZ/(600*2)=10KHZ.

But the initial value can be set by yourself, in other words, the phase can be determined by yourself.

1. TIM3 clock and corresponding IO clock are enabled. 

TIM3 is mounted under APB1, so we enable TIM3 through the enable function under APB1 bus. The called function is: 

RCC_APB1PeriphClockCmd(RCC_APB1Periph_TIM3, ENABLE); // Enable timer 3 mounted under APB1 clock

RCC_APB2PeriphClockCmd(RCC_APB2Periph_GPIOB, ENABLE); // Enable channel 1 PB1 of timer 3 and enable PB clock

2. GPIO structure initialization Because it is a timer output, it is a multiplexed push-pull output AF-PP

GPIO_InitTypeDef GPIO_InitStructure;

//Step1. Set PB.1 port as OC4 output port of TIM3

GPIO_InitStructure.GPIO_Pin = GPIO_Pin_1;

GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF_PP;

GPIO_InitStructure.GPIO_Speed ​​= GPIO_Speed_50MHz;

GPIO_Init(GPIOB, &GPIO_InitStructure);

3. Set the timer mode to output compare mode

TIM_TimeBaseInitTypeDef TIM_TimeBaseStructure;

TIM_OCInitTypeDef TIM_OCInitStructure;

TIM_TimeBaseStructure.TIM_Prescaler = 71; //The frequency division coefficient is 71, so the actual time base unit is 72M/(71+1)=1M

TIM_TimeBaseStructure.TIM_CounterMode = TIM_CounterMode_Up; //Count up

TIM_TimeBaseStructure.TIM_Period = 0xffff; //Self-loaded value is 65535 special value

TIM_TimeBaseStructure.TIM_ClockDivision = 0;

TIM_TimeBaseInit(TIM3,&TIM_TimeBaseStructure);

TIM_OCStructInit(&TIM_OCInitStructure);

TIM_OCInitStructure.TIM_OCMode = TIM_OCMode_Toggle; //Trigger event level flip

TIM_OCInitStructure.TIM_OutputState = TIM_OutputState_Enable; // Enable output

TIM_OCInitStructure.TIM_Pulse = t; //initial value 10000

TIM_OCInitStructure.TIM_OCPolarity = TIM_OCPolarity_High; //Output polarity is high

TIM_OC4Init(TIM3,&TIM_OCInitStructure); //Channel 4

4. Because you want to enable the timer interrupt OC and set the interrupt priority

NVIC_InitTypeDef NVIC_InitStructure;

//Step 3. Enable TIM3 output compare match interrupt

NVIC_InitStructure.NVIC_IRQChannel = TIM3_IRQn;

NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 1;

NVIC_InitStructure.NVIC_IRQChannelSubPriority = 1;

NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE;

NVIC_Init(&NVIC_InitStructure);

5. Clear the interrupt flag and enable the interrupt to enable timer 3

TIM_ClearFlag(TIM3, TIM_FLAG_CC4); // Clear interrupt flag

TIM_ITConfig(TIM3, TIM_IT_CC4, ENABLE); //TIM3 interrupt source setting, enable capture comparison interrupt of corresponding channel

TIM_Cmd(TIM3, ENABLE); //First disable timer 3

6. Interrupt service function: void TIM3_IRQHandler(void)

7. First check whether the output comparison interrupt response is set    

if(TIM_GetITStatus(TIM3,TIM_IT_CC4) == SET)

8. Clear interrupt flag

TIM_ClearITPendingBit(TIM3, TIM_IT_CC4 );

9. Extract the current count value of Timer_CNT

capture = TIM_GetCapture4(TIM3); //Extract the current count value of Timer3

10. Reassign CCR. Next timer interrupt time.

TIM_SetCompare4(TIM3, capture + t); //need to accumulate the current count value

Frequency output calculation method: PCLK2/(TIM_Prescaler+1)/(CCR)/2=f

Assuming CCR is 1000 72M/(71+1)/1000/2=500HZ   

TIM_SetCompare4(TIM3, capture + t); Changing t means changing CCR.