版本 39ea17c62218ce018f2a8279e125b9314a21b9f0
Pulse-width modulation(PWM)
Introduction
又稱pulse-duration modulation(PDM),是將類比信號轉為脈波的一種技術。
為何需要PWM?雖然類比電壓可直接用來控制,但類比電路控制信號容易隨時間漂移,功耗大。
.. image:: /pwm.png
如圖所示,PWM 電路主要功能是將輸入電壓的振幅轉換成脈衝寬度。一般轉換後脈波的週期固定,脈波的占空比(duty cycle)會依類比信號的大小而改變,可用來控制燈泡亮度、馬達轉速等等,脈波寬度越大燈泡亮度越亮、馬達轉速越快。
Implementation
PWM需要透過Timer來實現,STM32的Timer可分為以下幾種
- 基本定時器(TIM6 和 TIM7)
- 高級控制定時器(TIM1 和 TIM8)
- 通用定時器(TIMx):具有測量輸入信號的脈衝長度、產生輸出波形和PWM的功能。
- clock來源 : 內部clock(CK_INT)、外部clock模式1(TIx)、外部clock模式2(ETR)、內部觸發輸入(ITRx)。
- CK_INT -> AHB Prescaler(/1、2、…、512) -> APB1 Prescaler(/1、2、4、8、16) or APB2 Prescaler(/1、2、4、8、16)。
- Time Base Configuration :
.. image:: /down mode.bmp
.. image:: /center aligned.jpg
- TIM_ClockDivision - 採樣頻率基準,當連續採樣到N個有效電平時,才當作一次有效電平。
- TIM_Prescaler - 將TIMxCLK除以(TIM_Prescaler+1)
- TIM_CounterMode - 選擇計數模式
- TIM_Period - TIMx_ARR,counter 週期
- 輸出脈波週期 = (TIM_Period+1) * (TIM_Prescaler+1) * (TIM_ClockDivision+1) / TIMxCLK
- PWM configuration
- TIM_OCMode - PWM模式
- PWM 1 Mode : 在向上計數,TIMx_CNT<TIMx_CCRx時,輸出為1,否則輸出為0;在向下計數,TIMx_CNT>TIMx_CCRx時,輸出為0,否則輸出為1。
- PWM 2 Mode : 在向上計數,TIMx_CNT<TIMx_CCRx時,輸出為0,否則輸出為1;在向下計數,TIMx_CNT>TIMx_CCRx時,輸出為1,否則輸出為0。
- TIM_Pulse - TIMx_CCRx,脈衝寬度
- TIM_OCPolarity - 設置輸出極性,例如:TIM_OCPolarity_High PWM輸出關閉時默認為低電位。
- TIM_OCMode - PWM模式
以LED為例:
.. image:: /timer.png
TIM_Prescaler = 499,TIM_ClockDivision=0,TIMxCLK=84MHz
輸出脈波週期 = (TIM_Period+1) * (TIM_Prescaler+1) * (TIM_ClockDivision+1) / TIMxCLK
= (1999+1) * (499+1) * (0+1)/84000000 ≒ 0.012 s
0.012 s 遠小於人眼視覺暫留的時間(0.1~0.4s),因此上圖的PWM 1 duty cycle(75%)>PWM 2 duty cycle(25%),PWM 1 Mode看起來會比PWM 2 Mode亮。
Code_section
//====set pin enable=====================================
RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_GPIOD | RCC_AHB1Periph_GPIOB | RCC_AHB1Periph_GPIOA, ENABLE ); //開啟Pin腳 D B E提供使用
RCC_APB1PeriphClockCmd( RCC_APB1Periph_TIM4 | RCC_APB1Periph_TIM3, ENABLE ); //開啟Timer
GPIO_PinAFConfig(GPIOD, GPIO_PinSource12, GPIO_AF_TIM4);
GPIO_PinAFConfig(GPIOD, GPIO_PinSource13, GPIO_AF_TIM4);
GPIO_PinAFConfig(GPIOD, GPIO_PinSource14, GPIO_AF_TIM4);
GPIO_PinAFConfig(GPIOD, GPIO_PinSource15, GPIO_AF_TIM4);
/* GPIOA Configuration: CH1 (PB4) and CH2 (PB5) */ //設定PB4 5為輸出模式 提供模擬中斷來源
GPIO_InitStructure2.GPIO_Pin = GPIO_Pin_4 | GPIO_Pin_5 ;
GPIO_InitStructure2.GPIO_Mode = GPIO_Mode_OUT; //輸出信號模式
GPIO_InitStructure2.GPIO_OType = GPIO_OType_PP;
GPIO_InitStructure2.GPIO_Speed = GPIO_Speed_100MHz;
GPIO_InitStructure2.GPIO_PuPd = GPIO_PuPd_UP;
GPIO_Init(GPIOB, &GPIO_InitStructure2);
/* GPIOA Configuration: (PA2) and (PA3) */ //設定PA2 3為輸入模式 提供接收中斷來源
GPIO_InitStructure2.GPIO_Pin = GPIO_Pin_2 | GPIO_Pin_3 ;
GPIO_InitStructure2.GPIO_Mode = GPIO_Mode_IN;
GPIO_InitStructure2.GPIO_Speed = GPIO_Speed_100MHz;
GPIO_InitStructure2.GPIO_OType = GPIO_OType_PP;
GPIO_InitStructure2.GPIO_PuPd = GPIO_PuPd_DOWN ;
GPIO_Init(GPIOA, &GPIO_InitStructure2);
// Setup Blue & Green LED on STM32-Discovery Board to use PWM. / //PD與LED3456相連實作PWM
GPIO_InitStructure.GPIO_Pin = GPIO_Pin_12 | GPIO_Pin_13| GPIO_Pin_14| GPIO_Pin_15;
GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF; // Alt Function - Push Pull
GPIO_InitStructure.GPIO_OType = GPIO_OType_PP;
GPIO_InitStructure.GPIO_Speed = GPIO_Speed_100MHz;
GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_NOPULL;
GPIO_Init( GPIOD, &GPIO_InitStructure );
//======================================================
//====Interrupt set section==============================
//==清空中斷標誌==
EXTI_ClearITPendingBit(EXTI_Line2);
EXTI_ClearITPendingBit(EXTI_Line3);
//==選擇中斷PIN腳 A2 A3======
SYSCFG_EXTILineConfig(EXTI_PortSourceGPIOA, EXTI_PinSource2);
SYSCFG_EXTILineConfig(EXTI_PortSourceGPIOA, EXTI_PinSource3);
//==
EXTI_InitStructure.EXTI_Line = EXTI_Line2 ; //選擇中斷線路2
EXTI_InitStructure.EXTI_Mode = EXTI_Mode_Interrupt; //設置為中斷請求,非事件請求
EXTI_InitStructure.EXTI_Trigger = EXTI_Trigger_Falling //設置中斷觸發方式為 下降沿觸發
EXTI_InitStructure.EXTI_LineCmd = ENABLE; //外部中斷enable
EXTI_Init(&EXTI_InitStructure);
EXTI_GenerateSWInterrupt(EXTI_Line2);
EXTI_InitStructure.EXTI_Line = EXTI_Line3 ; //選擇中斷線路3
EXTI_InitStructure.EXTI_Mode = EXTI_Mode_Interrupt; //设設置為中斷請求,非事件請求
EXTI_InitStructure.EXTI_Trigger = EXTI_Trigger_Falling; //設置中斷觸發方式為 下降沿觸發
EXTI_InitStructure.EXTI_LineCmd = ENABLE; //外部中斷enable
EXTI_Init(&EXTI_InitStructure);
EXTI_GenerateSWInterrupt(EXTI_Line3);
//===給予中斷分組與優先權的設定===
NVIC_PriorityGroupConfig(NVIC_PriorityGroup_1); //選擇中斷分組2
NVIC_InitStructure.NVIC_IRQChannel = EXTI2_IRQn ; //選擇中斷通道2
NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 0; //搶占式中斷優先級設置為0
NVIC_InitStructure.NVIC_IRQChannelSubPriority = 0; //響應式中斷優先級設置為0
NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE; //enable中断
NVIC_Init(&NVIC_InitStructure);
//========================================================
//===Timer set section========================================
PrescalerValue = (uint16_t) ((SystemCoreClock /4) / 100000) - 1; //算脈衝周期的時間
//==TIMER 4 設定
TIM_TimeBaseStructInit( &TIM_TimeBaseInitStruct );
TIM_TimeBaseInitStruct.TIM_ClockDivision = TIM_CKD_DIV1;
TIM_TimeBaseInitStruct.TIM_CounterMode = TIM_CounterMode_Up; //上數模式
TIM_TimeBaseInitStruct.TIM_Period = 2000 - 1; // 0..2000 //周期
TIM_TimeBaseInitStruct.TIM_Prescaler = PrescalerValue;
TIM_TimeBaseInit( TIM4, &TIM_TimeBaseInitStruct );
//==TIMER 4 CCRx 暫存器設定
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 = 1000-1; // 0 .. 1000 (0=Always Off, 1000=Always On)
//==TIMER channel init
TIM_OC1Init( TIM4, &TIM_OCInitStruct ); // Channel 1 LED
TIM_OC2Init( TIM4, &TIM_OCInitStruct ); // Channel 2 LED
TIM_OC3Init( TIM4, &TIM_OCInitStruct ); // Channel 3 LED
TIM_OC4Init( TIM4, &TIM_OCInitStruct ); // Channel 4 LED
TIM_Cmd( TIM4, ENABLE );
TIM4->CCR1 = 500-1;
TIM4->CCR2 = 500-1;
TIM4->CCR3 = 500-1;
TIM4->CCR4 = 500-1;
//================================================================
//====中斷subroutine===============================================
void EXTI2_IRQHandler(void) //加長脈波頻寬的中斷subroutine
{
if(EXTI_GetITStatus(EXTI_Line2) != 0)
{ EXTI_ClearITPendingBit(EXTI_Line2);
if (brightness <= 1999 - 100)
brightness +=100;
TIM4->CCR1 = brightness;
TIM4->CCR2 = brightness;
TIM4->CCR3 = brightness;
TIM4->CCR4 = brightness;
EXTI_ClearITPendingBit(EXTI_Line2);
EXTI_ClearITPendingBit(EXTI_Line3);
} }
void EXTI3_IRQHandler(void) //減少脈波頻寬的中斷subroutine {
if(EXTI_GetITStatus(EXTI_Line3) != 0)
{ EXTI_ClearITPendingBit(EXTI_Line3);
if (brightness >= 199)
brightness -=100;
TIM4->CCR1 = brightness;
TIM4->CCR2 = brightness;
TIM4->CCR3 = brightness;
TIM4->CCR4 = brightness;
EXTI_ClearITPendingBit(EXTI_Line2);
EXTI_ClearITPendingBit(EXTI_Line3);
}
}
//============================================================
Demo
http://youtu.be/RHzLguvuOKY
http://youtu.be/guG6MdLIwfI
http://youtu.be/jJfebbbMDPw
補充
PWM較省電?
因為一般類比電壓要降低電壓輸出需靠增加電阻,源頭輸出電壓“持續”都為同一電壓,不過利用電阻改變最後輸出電壓,而PWM他靠的是一段時間內輸出的頻率來模擬類比電壓,“不需要持續的輸出”,故不會降電浪費在電阻上,即可達到省電效果。
overflow 和 UEV UIF的關係
UEV occurs everytime an overflow occurs, and UIF is a flag triggered when overflow occurs , if you don’t clean the flag, it will remain triggered. Reference
向上計數和向下計數的差別
timer 和 RTC的差別
The system timer is what makes everything in the computer run at the same speed. For example, your CPU won’t look for a fresh batch of data while the Memory is still trying to feed it the last batch.
The Real Time Clock tells the system what day/time it is.
- TIM_OCMode (timer output compare mode)
.. image:: /TIM_OCMode.png
- PWM先高後低和先低後高 在步進馬達上表現有什麼差別?
- 改成中央對齊計數方式會怎麼樣?
- Nyquist-Shannon Sampling Theorem 如果信號是帶限(某個頻率以上及以下~0)的,並且採樣頻率高於信號帶寬的一倍,那麼,原來的連續信號可以從採樣樣本中完全重建出來。
============= - http://zh.wikipedia.org/zh-tw/%E8%84%88%E8%A1%9D%E5%AF%AC%E5%BA%A6%E8%AA%BF%E8%AE%8A - http://ppt.cc/nIAF - http://www.google.com.tw/url?sa=t&rct=j&q=pwm&source=web&cd=10&cad=rja&ved=0CFMQFjAJ&url=http%3A%2F%2Fwww.vr.ncue.edu.tw%2Fesa%2Fa1001%2FPWM.pdf&ei=S3OoUO6pOafcmAWWiIGQDw&usg=AFQjCNE_7pw6paGQzH7kSwkwD2witqcC-A - http://hi.baidu.com/snic_k/item/0f045e3288e6683d2e20c42b - 陳志旺(2012),STM32 嵌入式微控制器快速上手,中國:電子工業出版社。