Arduino PWM的探究

2016年1月4日：

http://blog.csdn.net/xuanyuanlei1020/article/details/49902779 //PWM的秘密翻译

1.检验是PWM输出是否会受delay函数的影响，结论为：不受影响，只有再次调用analogWrite的时候才会改变PWM的频率。

程序：

 int ledPin = 9;      // LED connected to digital pin 9
 int val = 0;         // variable to store the read value


 void setup()
 {
   pinMode(ledPin, OUTPUT);   // sets the pin as output
 }


 void loop()
 {
   analogWrite(ledPin,125);
   delay(5000);
 }

2.怎样改变PWM的频率：结论是，频率是可以改变的，遵循valerio_sperati的步骤即可。

http://forum.arduino.cc/index.php?topic=72092.0 //感谢valerio_sperati的分享

Till now I just understood that arduino 2560 uses 5 different timers: 

timer 0 for pin 13 and 4        //1000HZ by default(actually 976HZ)

timer 1 for pin 12 and 11       //500HZ by default(actually 489HZ)
timer 2 for pin 10 and 9        //500HZ by default(actually 489HZ)
timer 3 for pin 5 and 3 and 2   //500HZ by default(actually 489HZ)
timer 4 for pin 8 and 7 and 6   //500HZ by default(actually 489HZ)

Hi guys! After some day, I finally managed to change the PWM frequency of all PWM pins of my Arduino Mega 2560.
All the explanations are foundamentally here: 
http://usethearduino.blogspot.com/2008/11/changing-pwm-frequency-on-arduino.html 
In this brief article the author explains very clearly how to change some values in Arduino Diecimila registers, in order to enhance the PWM frequency.
Here I simply modified a little bit the author'code to make it match with Arduino 2560.
Here follows my brief instructions and descriptions!
1) Arduino 2560 has 12 pins supporting PWM. They are from 2 to 13 included.
2) the PWM default frequency is 490 Hz for all pins, with the exception of pin 13 and 4,
whose frequency is 980 Hz (I checked with an oscilloscope).
3) In order to change frequency on pin 'A', we have to change some value in the timer 
(or register), controlling pin 'A'. This is the list of timers in Arduino Mega 2560:
timer 0 (controls pin 13, 4);
timer 1 (controls pin 12, 11);
timer 2 (controls pin 10, 9);
timer 3 (controls pin 5, 3, 2);
timer 4 (controls pin 8, 7, 6);
As you can see, a given timer controls more than one pin (every change about a timer will affect all pins depending on it!).
4) You can access a timer simply changing in your code (tipically in the setup()), the value of variable TCCRnB, where 'n' is the number of register. So, if we want  to change the PWM frequency of pins 10 and 9,  we will have to act on TCCR2B .

5) The TCCRnB is a 8 bit number.  The first three bits (from right to left!) are called CS02, CS01, CS00, and they are the bits we have to change.
Those bits in fact represent an integer number (from 0 to 7) called 'prescaler' , that Arduino uses to generate the frequency for PWM.
6) First of all, we have to clear these three bits, i.e they must be all set to 0:
int myEraser = 7;             // this is 111 in binary and is used as an eraser
TCCR2B &= ~myEraser;   // this operation (AND plus NOT),  set the three bits in TCCR2B to 0
7) now that CS02, CS01, CS00  are clear, we write on them a new value:
int myPrescaler = 3;         // this could be a number in [1 , 6]. In this case, 3 corresponds in binary to 011.   
TCCR2B |= myPrescaler;  //this operation (OR), replaces the last three bits in TCCR2B with our new value 011
 now we have a new PWM frequency on pin 9 and 10!
I registered those values on all PWM pins, changing the value of prescaler (the only exception are pins 13 and 14, see later):
prescaler = 1 ---> PWM frequency is 31000 Hz
prescaler = 2 ---> PWM frequency is 4000 Hz
prescaler = 3 ---> PWM frequency is 490 Hz (default value)
prescaler = 4 ---> PWM frequency is 120 Hz
prescaler = 5 ---> PWM frequency is 30 Hz
prescaler = 6 ---> PWM frequency is <20 Hz
(prescalers equal t 0  or 7 are useless). 
Those prescaler values are good for all timers (TCCR1B, TCCR2B, TCCR3B, TCCR4B) except for timer 0 (TCCR0B). In this case the values are:
prescaler = 1 ---> PWM frequency is 62000 Hz
prescaler = 2 ---> PWM frequency is 7800 Hz
prescaler = 3 ---> PWM frequency is 980 Hz (default value)
prescaler = 4 ---> PWM frequency is 250 Hz
prescaler = 5 ---> PWM frequency is 60 Hz
prescaler = 6 ---> PWM frequency is <20 Hz
Note that timer 0 is the one on which rely all time functions in Arduino: i.e., if you change this timer, function like delay() or millis() will continue to work but at a different timescale (quicker or slower!!!)
Ok, I hope it will help! --这个很重要，timer 0 不要随便改变参数；
Cheers, Valerio  
试验程序：

 int ledPin = 9;      // LED connected to digital pin 9
 int val = 0;         // variable to store the read value

 void setup()
 {
   int myEraser = 7;             // this is 111 in binary and is used as an eraser
   TCCR2B &= ~myEraser;   // this operation (AND plus NOT),  set the three bits in TCCR2B to 0
   int myPrescaler = 1;         // this could be a number in [1 , 6]. In this case, 3 corresponds in binary to 011.
   TCCR2B |= myPrescaler;  //this operation (OR), replaces the last three bits in TCCR2B with our new value 011
 }

 void loop()
 {
   analogWrite(2,125);
   analogWrite(3,125);
   analogWrite(4,125);
   analogWrite(5,125);
   analogWrite(6,125);
   analogWrite(7,125);
   analogWrite(8,125);
   analogWrite(9,125);
   analogWrite(10,125);
   analogWrite(11,125);
   analogWrite(12,125);
   analogWrite(13,125);
   delay(5000);
 }