Adds support to user-supplied PWM frequency

code/parasite/lib/parasite/pwm.cpp

@@ -14,20 +14,21 @@ namespace {
 
 // No scaling. The PWM counter will increase with a frequency of 16MHz.
 constexpr unsigned long kPWMFrequencyPrescale = PWM_PRESCALER_PRESCALER_DIV_1;
-// In conjunction with the PWM clock frequency, kPWMMaxCounter defines the
-// frequency of the square wave.
-constexpr int kPWMMaxCounter = 32;
-// Since we want a duty cycle of 0.5, we flip the PVM output when the counter
-// reaches kPWMMaxCounter / 2/;
-constexpr int kPWMFlipCount = kPWMMaxCounter;
-
 }  // namespace
 
-void SetupSquareWave(int pin_number) {
+void SetupSquareWave(double frequency, int pin_number) {
+  // In conjunction with the PWM clock frequency, kPWMMaxCounter defines the
+  // frequency of the square wave.
+  const int max_count = 16e6 / frequency;
+
+  // Since we want a duty cycle of 0.5, we flip the PVM output when the counter
+  // reaches kPWMMaxCounter / 2/;
+  const int flip_at_count = max_count / 2;
+
   HwPWM0.addPin(pin_number);
   HwPWM0.setClockDiv(PWM_PRESCALER_PRESCALER_DIV_1);
-  HwPWM0.setMaxValue(kPWMMaxCounter);
-  HwPWM0.writePin(pin_number, kPWMFlipCount);
+  HwPWM0.setMaxValue(max_count);
+  HwPWM0.writePin(pin_number, flip_at_count);
   HwPWM0.begin();
 }
 

code/parasite/lib/parasite/pwm.h

@@ -6,9 +6,9 @@ namespace parasite {
 // This is a simple, single-channel PWM-based square wave generator.
 // This only ever works for a single pin. If you call this function
 // twice with different pin numbers, nothing good will come out of it.
-// I am particularly proud of how well I resisted making this "generic"
-// and "reusable".
-void SetupSquareWave(int pin_number);
+// I am particularly proud of how well I resisted making this generic
+// and reusable.
+void SetupSquareWave(double frequency, int pin_number);
 
 }  // namespace parasite
 #endif  // _PARASITE_PWM_H_

code/parasite/src/main.cpp

@@ -5,12 +5,13 @@
 constexpr int kLED1Pin = 17;
 constexpr int kLED2Pin = 18;
 constexpr int kPWMPin = 19;
+constexpr double kPWMFrequency = 500000;
 
 void setup() {
   Serial.begin(9600);
   pinMode(kLED1Pin, OUTPUT);
   pinMode(kLED2Pin, OUTPUT);
-  parasite::SetupSquareWave(kPWMPin);
+  parasite::SetupSquareWave(kPWMFrequency, kPWMPin);
 }
 
 void loop() {