Added comments to indicate how activity is detected on the TWI and SPI

Author: Roy, Elizabeth

cores/arduino/wiring_analog.c

@@ -70,7 +70,9 @@ int analogRead(uint8_t pin)
 {
 	if(pin > NUM_ANALOG_INPUTS) return NOT_A_PIN;
 
-	/* Check if TWI is operating on double bonded pin */
+	/* Check if TWI is operating on double bonded pin (Master Enable is high 
+		in both Master and Slave mode for bus error detection, so this can 
+		indicate an active state for Wire) */
 	if(((pin == PIN_A4) || (pin == PIN_A5)) && (TWI0.MCTRLA & TWI_ENABLE_bm)) return 0;
 
 	uint8_t low, high;
@@ -120,7 +122,8 @@ void analogWrite(uint8_t pin, int val)
 	uint8_t bit_pos  = digitalPinToBitPosition(pin);
 	if(bit_pos == NOT_A_PIN) return;
 
-	/* Special check for SPI_SS double bonded pin -- no action if SPI is active */
+	/* Special check for SPI_SS double bonded pin -- no action if SPI is active 
+		(Using Slave Select Disable as indicator of SPI activity) */
 	if((pin == 10) && (SPI0.CTRLB & SPI_SSD_bm)) return;
 
 	// We need to make sure the PWM output is enabled for those pins