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Example: PWM on STM32-Discovery Blue LED

Question asked by JimK on Feb 12, 2011
Latest reply on Dec 13, 2015 by Clive One
Hey,

I just spent a bunch of time sorting out how to get the darn PWM output on the blue LED on the STM32-Discovery board.  After a lot of time pouring over the datasheets and forum posts I was finally able to work out the following code.  Seems to work very well.

    RCC_APB2PeriphClockCmd(    RCC_APB2Periph_GPIOA | RCC_APB2Periph_GPIOB |
            RCC_APB2Periph_GPIOC | RCC_APB2Periph_GPIOD | RCC_APB2Periph_GPIOE |
            RCC_APB2Periph_AFIO, ENABLE );
 
    RCC_APB1PeriphClockCmd( RCC_APB1Periph_TIM3, ENABLE );
 
    // Set the Vector Table base address at 0x08000000.
    NVIC_SetVectorTable( NVIC_VectTab_FLASH, 0x0 );
 
    NVIC_PriorityGroupConfig( NVIC_PriorityGroup_4 );
 
    // Configure HCLK clock as SysTick clock source.
    SysTick_CLKSourceConfig( SysTick_CLKSource_HCLK );


    // Setup Blue LED on STM32-Discovery Board to use PWM.
    GPIO_InitStructure.GPIO_Pin =  GPIO_Pin_8;
    GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF_PP;            // Alt Function - Push Pull
    GPIO_Init( GPIOC, &GPIO_InitStructure );
    GPIO_PinRemapConfig( GPIO_FullRemap_TIM3, ENABLE );        // Map TIM3_CH3 to GPIOC.Pin8
 
    // Let PWM frequency equal 100Hz.
    // Let period equal 1000. Therefore, timer runs from zero to 1000. Gives 0.1Hz resolution.
    // Solving for prescaler gives 240.
    TIM_TimeBaseInitTypeDef TIM_TimeBaseInitStruct;
    TIM_TimeBaseStructInit( &TIM_TimeBaseInitStruct );
    TIM_TimeBaseInitStruct.TIM_ClockDivision = TIM_CKD_DIV4;
    TIM_TimeBaseInitStruct.TIM_Period = 1000;
    TIM_TimeBaseInitStruct.TIM_Prescaler = 240;
    TIM_TimeBaseInit( TIM3, &TIM_TimeBaseInitStruct );
 
    TIM_OCInitTypeDef TIM_OCInitStruct;
    TIM_OCStructInit( &TIM_OCInitStruct );
    TIM_OCInitStruct.TIM_OutputState = TIM_OutputState_Enable;
    TIM_OCInitStruct.TIM_OCMode = TIM_OCMode_PWM1;
    // Initial duty cycle equals 0%. Value can range from zero to 1000.
    TIM_OCInitStruct.TIM_Pulse = 0;
    TIM_OC3Init( TIM3, &TIM_OCInitStruct );
    TIM_Cmd( TIM3, ENABLE );

Also, below is a useful macro that sets the PWM duty cycle once the output is running.

// Set duty cycle when using TIM3 as a PWM output.
#define SetTIM3Duty( val )    TIM3->CCR3 = val

It's neat to see the blue LED ramping and fading smoothly.

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