版本 07d35ea971496b1d6f5e303194cb0ab5ab671972

Pulse-Width Modulation (PWM)

Introduction

Pulse-Width Modulation, 又稱pulse-duration modulation(PDM),其利用在頻率不變的狀態下, 改變工作週期大小, 使整體平均電壓值上升或下降, 藉此間歇性電壓及功率切換以節省能源及控制等效果.

.. image:: /PWM_intr.PNG - 因PWM需要透過Timer來實現, 後面會稍作簡介

Clock control

在學習Timer前, 先瞭解系統clock是如何產生的, 以及給Timer的clock值.
A part of clock tree (STM32F407xx Reference mamual - p. 576)

.. image:: /Timer_clock.png

HSE clock (High speed external clock signal)

我們使用Crystal resonator作為外部震盪, 其頻率為8Hz（由STM32f4 discovery user manual 可知）

APBx timer clocks 計算
- 參閱system_stm32f4xx.c
  - line 341 RCC->CR |= ((uint32_t)RCC_CR_HSEON);
    - enable HSE(High Speed External clock)
  - line 374 ~ 379 配置與啟動PLL
  - line 146 ~ 151, 可知
    - PLL_VCO = (HSE_VALUE or HSI_VALUE / PLL_M) * PLL_N
    - SYSCLK = PLL_VCO / PLL_P
    - 並定義 PLL_M = 8, PLL_N = 336, PLL_P = 2 而外部晶體頻率為8MHz
    - -> PLL_VCO = (8MHz / 8) * 336 = 336MHz
    - -> SYSCLK = 336MHz / 2 = 168MHz
  - line 365 RCC->CFGR |= RCC_CFGR_HPRE_DIV1;
    - 此行設定HCLK = SYSCLK / 1 = 168MHz
  - line 372 RCC->CFGR |= RCC_CFGR_PPRE1_DIV4;
    - 此行設定APB1 peripheral clocks為 HCLK / 4 = 42MHz
  - 而由上圖可知
    - APB1 timer clocks = APB1 peripheral clocks * 2 = 84MHz
    - 該clock亦為後面Timer章節提及的 CK_INT 之值

Timers

Basic timers 基本定時器(TIM6 和 TIM7)

具有 16-bit auto-reload upcounter driven by a programmable prescaler
用途: Synchronization circuit to trigger the DAC(內部連接至DAC)

Advanced-control timers 高級控制定時器(TIM1 和 TIM8)

具有 16-bit up, down, up/down auto-reload counter driven by a programmable prescaler which allowing dividing the counter clock frequency either by any factor between 1 and 65536(2^16).
用途: 最多有四個獨立通道可用於input capture, output compare, PWM generation(Edge and Center-aligned Mode) and one-pulse mode output.
與TIM2&TIM5之差異: 可以 Break input to put the timer’s output signals in reset state or in a known state

General-purpose timers 通用定時器(TIM9 to TIM14)

具有16-bit auto-reload up counter.
以及16-bit programmable prescaler used to divide the counter clock frequency by any factor between 1 and 65536.
用途: 最多有兩個獨立通道可用於 input capture, output compare, PWM generation(Edge and Center-aligned Mode) and one-pulse mode output.

General-purpose timers 通用定時器(TIM2 to TIM5) (STM32F407xx Reference mamual - p. 576)

具有16-bit (TIM3 & TIM4) or 32-bit (TIM2 & TIM5) up, down, up/dowm auto-reload counter.
以及16-bit programmable prescaler used to divide the counter clock frequency by any factor between 1 and 65536.
用途: 最多有四個獨立通道可用於 input capture, output compare, PWM generation(Edge and Center-aligned Mode) and one-pulse mode output.
Block diagram

.. image:: /BD of GP timer.png

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)。
Counter modes (STM32F407xx Reference mamual - p. 579)

Counting Mode

up counting mode

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TIM_TimeBaseInitStruct.TIM_CounterMode = TIM_CounterMode_Up;

.. image:: /Timer_upcounting.png

down counting mode

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TIM_TimeBaseInitStruct.TIM_CounterMode = TIM_CounterMode_Down;

.. image:: /countdown.png

center aligned mode

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TIM_TimeBaseInitStruct.TIM_CounterMode = TIM_CounterMode_CenterAligned1;

.. image:: /center_aligned.png

Timer Mode

Input capture mode

PWM input mode

Forced output mode

Output compare mode

PWM mode

PWM configuration
- 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
- 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。
- 輸出脈波週期 = (TIM_Period+1) * (TIM_Prescaler+1) * (TIM_ClockDivision+1) / TIMxCLK
- 以LED為例：

.. image:: /timer.png

TIM_Prescaler = 500-1, TIM_Period = 1680-1 , TIMxCLK=84MHz
輸出脈波週期 = (TIM_Period+1) * (TIM_Prescaler+1) / TIMxCLK
- ```
    = (1680-1+1) * (500-1+1) /84000000 = 0.01 s(100Hz)
```
0.01 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

RCC_Configuration

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RCC_AHB1PeriphClockCmd( RCC_AHB1Periph_GPIOD , ENABLE );//Enalbe AHB for GPIOD
RCC_APB1PeriphClockCmd( RCC_APB1Periph_TIM4, ENABLE );//Enable APB for TIM4

GPIO_Configuration

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GPIO_InitTypeDef GPIO_InitStructure;//Create GPIO_InitStructure 
GPIO_StructInit(&GPIO_InitStructure); // Reset GPIO_structure

GPIO_PinAFConfig(GPIOD, GPIO_PinSource12, GPIO_AF_TIM4); // set GPIOD_Pin12 to AF_TIM4
GPIO_PinAFConfig(GPIOD, GPIO_PinSource13, GPIO_AF_TIM4); // set GPIOD_Pin13 to AF_TIM4
GPIO_PinAFConfig(GPIOD, GPIO_PinSource14, GPIO_AF_TIM4); // set GPIOD_Pin14 to AF_TIM4
GPIO_PinAFConfig(GPIOD, GPIO_PinSource15, GPIO_AF_TIM4); // set GPIOD_Pin15 to AF_TIM4
  

// Setup Blue & Green LED on STM32-Discovery Board to use PWM.
GPIO_InitStructure.GPIO_Pin =  GPIO_Pin_12 | GPIO_Pin_13| GPIO_Pin_14| GPIO_Pin_15; //PD12->LED3 PD13->LED4 PD14->LED5 PD15->LED6
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 );

TIM_Configuration

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TIM_TimeBaseInitTypeDef TIM_TimeBaseInitStruct;//Create TIM Time Base Init structure 
TIM_OCInitTypeDef TIM_OCInitStruct;//Create TIM Output Compare Init structure

// Let PWM frequency equal 100Hz. ( 84MHz / 1680 /500 = 100Hz )
// Let period equal 1600. Therefore, timer runs from zero to 1600. 
TIM_TimeBaseStructInit( &TIM_TimeBaseInitStruct );//reset TIM_TimeBaseStructInit
TIM_TimeBaseInitStruct.TIM_Period = 1680 - 1;   
TIM_TimeBaseInitStruct.TIM_Prescaler = 500 - 1; 
TIM_TimeBaseInitStruct.TIM_CounterMode = TIM_CounterMode_Up;    
TIM_TimeBaseInit( TIM4, &TIM_TimeBaseInitStruct );

TIM_OCStructInit( &TIM_OCInitStruct );reset TIM_OCStructInit
TIM_OCInitStruct.TIM_OutputState = TIM_OutputState_Enable;
TIM_OCInitStruct.TIM_OCMode = TIM_OCMode_PWM1;


// TIM4_CCR register (16 bits). Value can range from zero to 65535.
// TIM_Pulse = Compare Capture register(CCR) = 1680 ( duty_cycle = 100%)
TIM_OCInitStruct.TIM_Pulse = 1680; //(0=Always Off, >1680 =Always On)

TIM_OC1Init( TIM4, &TIM_OCInitStruct ); // set TIM_OCInitStruce to TIM4_channel1
TIM_OC2Init( TIM4, &TIM_OCInitStruct ); // set TIM_OCInitStruce to TIM4_channel2
TIM_OC3Init( TIM4, &TIM_OCInitStruct ); // set TIM_OCInitStruce to TIM4_channel3
TIM_OC4Init( TIM4, &TIM_OCInitStruct ); // set TIM_OCInitStruce to TIM4_channel4

TIM_Cmd( TIM4, ENABLE ); //Enables  TIM4 peripheral

PWM control

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while(1) // Do not exit { if(brightness + n <= 0) who_run = (who_run + 1) % 4;

if (((brightness + n) >= 3000) || ((brightness + n) <= 0))
  n = -n; // if  brightness maximum/maximum change direction   
brightness += n;

//Light LEDs in turn
 switch(who_run){
     case 0:
         TIM4->CCR1 = brightness - 1; // set brightness
         break;
     case 1:
         TIM4->CCR2 = brightness - 1; // set brightness
         break;
     case 2:
         TIM4->CCR3 = brightness - 1; // set brightness
         break;
     case 3:
         TIM4->CCR4 = brightness - 1; // set brightness
         break;
 }
for(i=0;i<10000;i++);  // delay

}

return(0); // System will implode }

Demo video

http://youtu.be/RHzLguvuOKY
http://youtu.be/guG6MdLIwfI (LED 3 4 5 6依序亮-暗轉換)
http://youtu.be/ygELIXJFHM8
http://youtu.be/jJfebbbMDPw (透過外接 BUTTON 調整 LED亮度)
http://youtu.be/bFRWfXI_k5Q (透過PWM使MOTOR達到快慢快慢變化)

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。

例如:使用9V電池來給一燈泡供電，連接電池跟燈泡時間為50ms，斷開電池和燈泡時間為50ms。1秒鐘(1000ms)過後，會重複此過程10次，燈泡將會連接到一個4.5V電池(9V電池的50%)上一樣。
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.
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。
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?

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
改成中央對齊計數方式會怎麼樣?

.. image:: /中央對齊.png Reference =============

http://blog.csdn.net/scarlettsp/article/details/6656588
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。
http://blog.sina.com.cn/s/blog_76c545390100ovfj.html

P.155 8.3.3 計數器模式

範例程式

https://github.com/PJayChen/STM32f4_discovery_TIM_PWM_Output

依序點亮LED
https://gitcafe.com/embedded2012/P-coolod/blob/master/Lab-6/discoveryF4/TIM_PWM_Output/main.c

此程式為依序更改 LED 3 4 5 6(利用PD12 13 14 15 作PWM應用)亮度由暗漸漸轉亮再由亮漸漸轉暗
https://gitcafe.com/embedded2012/P-coolod/blob/master/Lab-6/discoveryF4/GPIO_test/main.c

此程式碼利用PB4 5作Output pin持續輸出高態電壓,PA2 3作Input Pin接收來在output pin的電壓輸入,並設定負緣觸發中斷,中斷觸發以改變duty cycle佔空比,電壓高低態變化則利用外接button,當按下button則電壓高->低 ,PA2 3接收到變化便會觸發中斷,透過改變Timer來做duty cycle佔空比的改變,以達到LED亮暗改變.(以下圖說明)

.. image:: /embedded/PWM-GPIO_test_Explain_figure.png