The principle and use of STM32 PWM

1.What is PWM?

    It is pulse width modulation, or PWM for short. A very effective technique for controlling analog circuits using the digital output of a microprocessor is the control of pulse width.

    The pulse mentioned here is the square wave we generate. A square wave is a continuous generation of N such cycles.

The duration of the high level in one cycle is the pulse width (pulse width), and PWM (pulse width modulation) is to control the duration of the high level in one cycle.

----------------------------------------------------------------------------------------------------------------------------------------

2. Schematic diagram of simple PWM principle

CNT: is the current value register, count register.

ARR: is the automatic reload register (initialization setting).

CCRx: Comparison value register (TIM_SetCompare1() sets and modifies the duty cycle).

Assume the timer operates in up-counting PWM mode:

When CNT

When the value of CNT is less than CCRx, IO outputs low level (0).

When the CNT value is greater than or equal to CCRx, IO outputs a high level (1).

When the value of CNT reaches ARR, it will reset to zero and then count up again, repeating the cycle.

Changing the value of CCRx can change the duty cycle of the PWM output. Changing the value of ARR can change the frequency of the PWM output. This is the output principle of PWM.

----------------------------------------------------------------------------------------------------------------------------------------------------------------

3. Register workflow:

PWM Mode

The pulse width modulation mode can generate a signal whose frequency is determined by the TIMx_ARR register value and whose duty cycle is determined by the

Determined by the TIMx_CCRx register value.

By writing 110 (PWM mode 1) or 111 (PWM mode 2) to the OCxM bit in the TIMx_CCMRx register,

The PWM mode can be selected independently for each channel (one PWM per OCx output).

The OCxPE bit in the TIMx_CCMRx register is set to 1 to enable the corresponding preload register. Finally,

Setting the ARPE bit in the register enables auto-reload of the preload register (in up-counting or center-aligned mode).

Because the preload registers are transferred to the shadow registers only when an update event occurs, you must

All registers must be initialized by setting the UG bit in the TIMx_EGR register.

The OCx polarity can be programmed using the CCxP bit in the TIMx_CCER register. It can be set to either active high or

Set to active low. OCx output is enabled by setting the CCxE bit in the TIMx_CCER register to 1.

For more information, see the TIMx_CCERx register description.

In PWM mode (1 or 2), TIMx_CNT is always compared with TIMx_CCRx to determine whether

TIMx_CNT =< TIMx_CCRx。

Because the counter counts up, the timer can generate PWM in edge-aligned mode.

----------------------------------------------------------------------------------------------------------------------------------------------------------------

4. PWM edge-aligned mode

The following takes PWM mode 1 as an example. As long as TIMx_CNT < TIMx_CCRx, the PWM reference signal OCxREF is

If the comparison value in TIMx_CCRx is greater than the auto-reload value (in TIMx_ARR),

If the comparison value is 0, OCxRef remains at "0". Figure 183 shows an example of edge

Some PWM waveforms in edge-aligned mode (TIMx_ARR=8).

----------------------------------------------------------------------------------------------------------------------------------------------------------------

5. PWM steps - light brightness control:

       Check out the schematic for the LED:

        //①Find 4 pins according to the schematic diagram:

       PF9 can use TIM14_CH1, which means that channel 1 of timer 14 can be used to generate PWM output.

        TIM_TimeBaseInitTypeDef TIM_TimeBaseInitStruct; // Defined TIM attribute structure variable

      GPIO_InitTypeDef GPIO_InitStruct; //Define GPIO type variable

       TIM_OCInitTypeDef TIM_OCInitStruct; //Define variables for reuse functions

       ②// 1. Initialize clock: TIM14 and PF9

       RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_GPIOF, ENABLE);

       /* TIM3 clock enable */

     RCC_APB1PeriphClockCmd(RCC_APB1Periph_TIM14, ENABLE);

       // 2. Configure GPIO pins for multiplexing functions

       /* GPIOC Configuration: TIM14 CH1 (PF9) */

       GPIO_InitStruct.GPIO_Pin = GPIO_Pin_9; // Select pin PF9

       GPIO_InitStruct.GPIO_Mode = GPIO_Mode_AF; // Set to multiplexing function

       GPIO_InitStruct.GPIO_Speed ​​= GPIO_High_Speed; // Set the output speed to 100MHz

       GPIO_InitStruct.GPIO_OType = GPIO_OType_PP; // Set to push-pull output

       GPIO_InitStruct.GPIO_PuPd = GPIO_PuPd_UP; // Set to pull-up output

       GPIO_Init(GPIOC, &GPIO_InitStruct); // Installation parameters

       // 3. Connect the TIM and pin multiplexing functions: TIM14 and PF9

       GPIO_PinAFConfig(GPIOF, GPIO_PinSource9, GPIO_AF_TIM14);

       // 4. Configure the parameters of the TIM timer

       TIM_TimeBaseInitStruct.TIM_Period = 100-1; // Set the reload value ARR (control frequency)

       TIM_TimeBaseInitStruct.TIM_Prescaler = 8400-1; // Set the pre-scaling factor: period (times) 100Hz == 100us

       TIM_TimeBaseInitStruct.TIM_ClockDivision = TIM_CKD_DIV1; // Set the re-division value: TIM_CKD_DIV1 means no frequency division

       TIM_TimeBaseInitStruct.TIM_CounterMode = TIM_CounterMode_Up; // Set the counting mode

       TIM_TimeBaseInit(TIM14, &TIM_TimeBaseInitStruct);

       // 5. Configure multiplexing function: PWM

       TIM_OCInitStruct.TIM_OCMode = TIM_OCMode_PWM1; // Configure as PWM mode 1

       TIM_OCInitStruct.TIM_OutputState = TIM_OutputState_Enable; // Enable output

       //TIM_OCInitStruct.TIM_Pulse = CCR1_Val; // Initialize the configuration comparison value register

       TIM_OCInitStruct.TIM_OCPolarity = TIM_OCPolarity_High; //Configure as high level valid

       // 6.TIM14 channel 1 initialization

       TIM_OC1Init(TIM14, &TIM_OCInitStruct); // TIM14 channel 1 initialization

       // 7. Set the initial value of the automatic reload comparison value CCR1 to continuously generate PWM pulses

       TIM_OC1PreloadConfig(TIM14, TIM_OCPreload_Enable);

       // 8. Set the auto-reload value (ARR) to continuously generate PWM pulses

       TIM_ARRPreloadConfig(TIM14, ENABLE);

       /* 9. Enable timer 14 */

       TIM_Cmd(TIM14, ENABLE);

       // 10. Enable TIM1PWM output (advanced timer)

       //TIM_CtrlPWMOutputs(TIM1, ENABLE)

Set the comparison value function

void TIM_SetCompareX(TIM_TypeDef* TIMx, uint16_t Comparex);