版本 5425f1c42eb1c1811429ca8d5df6ffe729a7a354
Changes from 5425f1c42eb1c1811429ca8d5df6ffe729a7a354 to 2a03605cdbc44f5a36a36a9a01f737fad758d0ef
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title: Pulse-width modulation(PWM)
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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:: /up mode.bmp
-----------
.. image:: /down mode.bmp
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.. 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。
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
=============
1. 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。
2. 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.
3. 向上計數和向下計數的差別
以四位元向上計數器為例，由四個JK正反器串接而成，輸出DCBA由0000依據二進位的變化至1111。下一個脈波來時，又回到0000。依此類推故此計數器可由0計數到15。而向下計數器與向上計數器不同點是下一級的CLK接於前一級的Q，輸出DCBA由1111向下計數到0000，待下一個時脈輸入時又變為1111，再依序變化。
4. 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.
5. TIM_OCMode (timer output compare mode)
.. image:: /TIM_OCMode.png
6. PWM先高後低和先低後高 在步進馬達上表現有什麼差別?
在馬達剛要啟動時，因為先高後低是在duty cycle的前半段先供電，而先低後高則是到duty cycle的後半段才供電，在馬達的表現上 先高後低的啟動速度會比先低後高還要快一點
7. 改成中央對齊計數方式會怎麼樣?
.. image:: /中央對齊.png
Reference
=============
- 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。
