add notes on PWM

examples/AS5600_pwm_test/AS5600_pwm_test.ino

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+//
+//    FILE: AS5600_pwm_test.ino
+//  AUTHOR: Rob Tillaart
+// PURPOSE: demo
+//     URL: https://github.com/RobTillaart/AS5600
+//          https://forum.arduino.cc/t/questions-using-the-as5600-in-pwm-mode/1266957/3
+//
+//  monitor the PWM output to determine the angle
+//  not tested with hardware yet
+
+int PWMpin = 7;
+
+long fullPeriod = 0;
+
+
+float measureAngle()
+{
+  // wait for LOW
+  while (digitalRead(PWMpin) == HIGH);
+  // wait for HIGH
+  while (digitalRead(PWMpin) == LOW);
+  uint32_t rise = micros();
+
+  // wait for LOW
+  while (digitalRead(PWMpin) == HIGH);
+  uint32_t highPeriod = micros() - rise;
+
+  // wait for HIGH
+  while (digitalRead(PWMpin) == LOW);
+  fullPeriod = micros() - rise;
+
+  float bitTime = fullPeriod / 4351.0;
+  float dataPeriod = highPeriod - 128 * bitTime;
+  float angle = 360.0 * dataPeriod / (4095 * bitTime);
+  return angle;
+}
+
+
+//float testMath(uint32_t fullPeriod, uint32_t highPeriod)
+//{
+//  float bitTime = fullPeriod / 4351.0;
+//  float dataPeriod = highPeriod - 128 * bitTime;
+//  float angle = 360.0 * dataPeriod / (4095 * bitTime);
+//  return angle;
+//}
+
+
+void setup()
+{
+  Serial.begin(115200);
+  Serial.println(__FILE__);
+
+  pinMode(PWMpin, INPUT_PULLUP);
+
+//  //  test code
+//  for (int pwm = 0; pwm < 4096; pwm++)
+//  {
+//    Serial.println(testMath(4351, 128 + pwm), 2);
+//  }
+}
+
+
+void loop()
+{
+  float angle = measureAngle();
+  Serial.println(angle, 1);  //  print with 1 decimal
+
+  delay(1000);
+}
+
+
+//  -- END OF FILE --

notes_on_PWM.md

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+
+## notes on PWM 
+
+#### Description
+
+If you do not know the PWM frequency you can determine the angle
+with PWM in the following way.
+
+In one period there are 4351 bits (128 + 4095 + 128)
+These come typically in one pulse like
+
+```
+00001111111111111111111111111111111111100000000
+    HEADER          DATA               POSTLOW
+```
+
+#### Step 1
+
+Determine the duration of one cycle by measuring the time between two RISING edges.
+Lets call this time FULLSCALE == 4351 bits.
+
+Then the time for one bit BITTIME = round(FULLSCALE / 4351.0);
+
+#### Step 2
+
+Measure the duration of the HIGHPERIOD == HEADER + DATA
+
+We know the HEADER is 128 bits and FULLSCALE = 4351 bits.
+
+DATAPERIOD = HIGHPERIOD - 128 \* BITTIME
+
+ANGLE = 360 \* DATAPERIOD / (4095 \* BITTIME)
+
+Code: AS5600_pwm_test.ino
+
+#### Related
+
+https://github.com/RobTillaart/AS5600
+https://forum.arduino.cc/t/questions-using-the-as5600-in-pwm-mode/1266957/3
+
+