PWM Timing Issue

Question

I am going through a series of tutorials to get familiar with the Arduino, and I am having trouble understanding why something isn't working the way that I expected it to. The following two blocks of code do the same thing, but I tried it both ways to see if there was any change, and the results were essentially identical (which the exception of the delay(800)). The problem that I am experiencing is that the LED dims far slower than it brightens. In fact the 0-255 loop seems to be happening in about a second, whereas the 255-0 loop is taking closer to five seconds. There does not seem to be a reason for this in the code, so I am at a loss. Any help or an explanation would be greatly appreciated!

#define ledPin 9

void setup() {
  // put your setup code here, to run once:
  pinMode(ledPin, OUTPUT);
}

void loop() {
  // put your main code here, to run repeatedly:
  for(int a = 0; a <= 255; a++) {
    analogWrite(ledPin, a);
    delay(8);
  }
  for (int a = 255; a >= 0; a--) {
    analogWrite(ledPin, a);
    delay(8);
  }
}

And

//Controlling LED By PWM
//The LED lights up gradually,and then goes out gradually,repeatedly
//Email:support@sunfounder.com
//Website:www.sunfounder.com
//2015.5.7
/**************************************************************/
const int ledPin = 9; // the pin that the LED is attached to pin 9
void setup ()
{
pinMode(ledPin, OUTPUT); // declare pin 9 to be an output
}

void loop()
{
for (int a=0; a<=255;a++) //loop from 0 to 255
{
analogWrite(ledPin, a); // set the brightness of pin 9:
delay(8); //wait for 8 ms 
}
for (int a=255; a>=0;a--) //loop from 255 down to 0
{
analogWrite(ledPin, a); // set the brightness of pin 9:
delay(8); //wait for 8 ms 
}
delay(800); //wait for 800 ms 
}

mcb1 · Accepted Answer

Both loops should take just over 2 seconds to complete. 255 * 8 = 2040 mS = 2.04 secs LED's are not linear, and I suspect what you are seeing is the brightness increasing in the first loop until you reach what appears to be the maximum brightness, but the PWM is not 255. It then goes into the second loop and again you're not seeing anything until it reaches a much lower point. You could try adding a pause in-between the two loops and show the LED in the opposite state before the loop starts. //Controlling LED By PWM
//The LED lights up gradually,and then goes out gradually,repeatedly
//Email:support@sunfounder.com
//Website:www.sunfounder.com
//2015.5.7
/**************************************************************/
constint ledPin = 9; // the pin that the LED is attached to pin 9
void setup () 
{ 
pinMode(ledPin, OUTPUT); // declare pin 9 to be an output
} 
void loop() 
{ 
for (int a=0; a<=255;a++) //loop from 0 to 255
{ 
analogWrite(ledPin, a); // set the brightness of pin 9:
delay(8); //wait for 8 ms 
} 
delay(500); //wait for 500 ms 
analogWrite(ledPin, 0); //Turn OFF the led
delay(500); //wait for 500 ms to show the LED is off
for (int a=255; a>=0;a--) //loop from 255 down to 0
{ 
analogWrite(ledPin, a); // set the brightness of pin 9:
delay(8); //wait for 8 ms 
} 
delay(500); //wait for 500 ms 
analogWrite(ledPin, 255); //Turn ON the led
delay(500); //wait for 500 ms to show the LED is on
} Mark

beacon_dave · Answer

you could perhaps use the millis() function to help diagnose this more accurately https://www.arduino.cc/reference/en/language/functions/time/millis/ store the value returned by millis() to a variable at the start of the loop and subtract it from the value returned by millis() at the end of the loop, and send it to the serial console. this should give you a fairly accurate indication of the number of milliseconds the loop took to run. do this for both loops and compare the results.

PWM Timing Issue

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