Simplify initRandomSeed() to reduce code size

Use only an ADC reading from an unconnected pin shifted left 16 bits
then added to micros().

src/Arduboy2.cpp

@@ -197,25 +197,16 @@ int Arduboy2Base::cpuLoad()
 void Arduboy2Base::initRandomSeed()
 {
   power_adc_enable(); // ADC on
-  randomSeed(~rawADC(ADC_TEMP) * ~rawADC(ADC_VOLTAGE) * ~micros() + micros());
+
+  // do an ADC read from an unconnected input pin
+  ADCSRA |= _BV(ADSC); // start conversion (ADMUX has been pre-set in boot())
+  while (bit_is_set(ADCSRA, ADSC)) { } // wait for conversion complete
+
+  randomSeed(((unsigned long)ADC << 16) + micros());
+
   power_adc_disable(); // ADC off
 }
 
-uint16_t Arduboy2Base::rawADC(uint8_t adc_bits)
-{
-  ADMUX = adc_bits;
-  // we also need MUX5 for temperature check
-  if (adc_bits == ADC_TEMP) {
-    ADCSRB = _BV(MUX5);
-  }
-
-  delay(2); // Wait for ADMUX setting to settle
-  ADCSRA |= _BV(ADSC); // Start conversion
-  while (bit_is_set(ADCSRA,ADSC)); // measuring
-
-  return ADC;
-}
-
 /* Graphics */
 
 void Arduboy2Base::clear()

src/Arduboy2.h

@@ -82,11 +82,6 @@
 
 #define CLEAR_BUFFER true /**< Value to be passed to `display()` to clear the screen buffer. */
 
-// compare Vcc to 1.1 bandgap
-#define ADC_VOLTAGE (_BV(REFS0) | _BV(MUX4) | _BV(MUX3) | _BV(MUX2) | _BV(MUX1))
-// compare temperature to 2.5 internal reference and _BV(MUX5)
-#define ADC_TEMP (_BV(REFS0) | _BV(REFS1) | _BV(MUX2) | _BV(MUX1) | _BV(MUX0))
-
 
 /** \brief
  * A rectangle object for collision functions.
@@ -712,9 +707,6 @@ class Arduboy2Base : public Arduboy2Core
    */
   int cpuLoad();
 
-  // Useful for getting raw approximate voltage values.
-  uint16_t rawADC(uint8_t adc_bits);
-
   /** \brief
    * Test if the specified buttons are pressed.
    *

src/Arduboy2Core.cpp

@@ -80,6 +80,9 @@ void Arduboy2Core::boot()
   setCPUSpeed8MHz();
   #endif
 
+  // Select the ADC input here so a delay isn't required in initRandomSeed()
+  ADMUX = RAND_SEED_IN_ADMUX;
+
   bootPins();
   bootOLED();
   bootPowerSaving();
@@ -139,10 +142,12 @@ void Arduboy2Core::bootPins()
   // Port F INPUT_PULLUP or HIGH
   PORTF |= _BV(LEFT_BUTTON_BIT) | _BV(RIGHT_BUTTON_BIT) |
            _BV(UP_BUTTON_BIT) | _BV(DOWN_BUTTON_BIT);
-  // Port F INPUT or LOW (none)
+  // Port F INPUT or LOW
+  PORTF &= ~(_BV(RAND_SEED_IN_BIT));
   // Port F inputs
   DDRF &= ~(_BV(LEFT_BUTTON_BIT) | _BV(RIGHT_BUTTON_BIT) |
-            _BV(UP_BUTTON_BIT) | _BV(DOWN_BUTTON_BIT));
+            _BV(UP_BUTTON_BIT) | _BV(DOWN_BUTTON_BIT) |
+            _BV(RAND_SEED_IN_BIT));
   // Port F outputs (none)
 
 #elif defined(AB_DEVKIT)
@@ -175,9 +180,10 @@ void Arduboy2Core::bootPins()
 
   // Port F INPUT_PULLUP or HIGH
   PORTF |= _BV(A_BUTTON_BIT) | _BV(B_BUTTON_BIT);
-  // Port F INPUT or LOW (none)
+  // Port F INPUT or LOW
+  PORTF &= ~(_BV(RAND_SEED_IN_BIT));
   // Port F inputs
-  DDRF &= ~(_BV(A_BUTTON_BIT) | _BV(B_BUTTON_BIT));
+  DDRF &= ~(_BV(A_BUTTON_BIT) | _BV(B_BUTTON_BIT) | _BV(RAND_SEED_IN_BIT));
   // Port F outputs (none)
   // Speaker: Not set here. Controlled by audio class
 

src/Arduboy2Core.h

@@ -112,6 +112,12 @@
 #define SPEAKER_2_PORT PORTC
 #define SPEAKER_2_DDR DDRC
 #define SPEAKER_2_BIT PORTC7
+
+#define RAND_SEED_IN A4 // Open analog input used for noise by initRandomSeed()
+#define RAND_SEED_IN_PORTF
+#define RAND_SEED_IN_BIT PORTF1
+// Value for ADMUX to read the random seed pin: 2.56V reference, ADC1
+#define RAND_SEED_IN_ADMUX (_BV(REFS0) | _BV(REFS1) | _BV(MUX0))
 // -----------------------
 
 // ----- DevKit pins -----
@@ -186,6 +192,12 @@
 //
 // Reference: https://github.com/Arduboy/Arduboy/issues/108
 
+#define RAND_SEED_IN A4 // Open analog input used for noise by initRandomSeed()
+#define RAND_SEED_IN_PORTF
+#define RAND_SEED_IN_BIT PORTF1
+// Value for ADMUX to read the random seed pin: 2.56V reference, ADC1
+#define RAND_SEED_IN_ADMUX (_BV(REFS0) | _BV(REFS1) | _BV(MUX0))
+
 #endif
 // --------------------