PWM inside out

1/*
2  Understanding PWM:
3  Requirement: 
4  any Arduino that contains  an ATmega328, even a barebone would do.
5  External hardware:
6
7  n nnn     ooo    n nnn     eee
8  nn   n   o   o   nn   n   e   e
9  n    n   o   o   n    n   eeeee
10  n    n   o   o   n    n   e
11  n    n    ooo    n    n    eee
12
13  PWM out: Pin-9 (controlled by TIMER-1)
14  Option: Connect anode of an LED to  pin-9,
15  cathode of LED to GND (don't forget the resistor).
16  Start the Serial  plotter and set the baud rate to 115200.
17  Since 1.6.7 the IDE supports drawing  of multiples graphs at once.
18  The area below the blue and the red graph will  always be the same.
19  The Serial Plotter replaces a dual beam oscilloscope.
20  You can also watch the 20 minutes video of Paul McWhorter
21  (https://www.youtube.com/watch?v=YfV-vYT3yfQ)
22  who is using a GW Instek GOS-620FG analogue oscilloscope
23  to demonstrate how  PWM works, or any other of the 7 million links
24  Google shows for PWM tutorial.
25  Side note: as soon as the pwm value exceeds 255 it goes back to 0.
26  No special  treatment is required.
27*/
28
29void setup() {
30  /*
31    all this stuff  is just to make sure the legend
32    next to the blue and red bar will show up  properly.
33    In case it shows some garbage just restart the plotter.
34  */
35  delay(700);
36  Serial.flush();
37  Serial.begin(115200);
38  delay(700);
39  Serial.flush();
40  Serial.println("PWM_signal mean_value");
41}
42
43byte  pwm;
44
45void loop() {
46  // Serial plotter always shows 500 samples:
47  const  int N = 500;
48  byte a[N];
49  analogWrite(9, pwm);
50  // to synchronize the  plotter TIMER1 has to
51  // start from zero to give a stable picture
52  TCNT1  = 0;
53  // read 500 samples fast:
54  for (int i = 0; i < N; i++) {
55    //  trick to read pin-9:
56    // sorry but this won't work: a[i] = digitalRead(9);
57    a[i] = PINB & 2; // result always 0 or 2
58    // with 21 you see exactly four  pulses in the plot
59    delayMicroseconds(21);
60  }
61  // calculate the mean  value:
62  float mean = 5.0 * pwm / 255;
63  String s = " " + String(mean);
64  // plot the samples (2 * 2.5 = 5 volts) and mean:
65  for (int i = 0; i < N;  i++) {
66    Serial.print(a[i] * 2.5);
67    Serial.println(s);
68  }
69  delay(400);
70  pwm++;
71}
72