Add all analog references supported by the ATtinyX5 series

[mischnic]

Now it should possible to use the 2.56V voltage reference on the ATtinyX5's.

Should resolve #1739 , where the formula is derived from.

[sandeepmistry]

@mischnic thanks for submitting. The changes look good to me testing with the following sketch (based on the AnalogInput example):

int sensorPin = A0;    // select the input pin for the potentiometer
int ledPin = 13;      // select the pin for the LED
int sensorValue = 0;  // variable to store the value coming from the sensor

void setup() {
  // declare the ledPin as an OUTPUT:
  pinMode(ledPin, OUTPUT);

  analogReference(EXTERNAL); // added
}

void loop() {
  // read the value from the sensor:
  sensorValue = analogRead(sensorPin);
  // turn the ledPin on
  digitalWrite(ledPin, HIGH);
  // stop the program for <sensorValue> milliseconds:
  delay(sensorValue);
  // turn the ledPin off:
  digitalWrite(ledPin, LOW);
  // stop the program for for <sensorValue> milliseconds:
  delay(sensorValue);
}

Here are the sketch size differences:

Board	Before	After	Difference
Uno	1,042 bytes	1,054 bytes	12 bytes
ATtiny25	748 bytes	762 bytes	14 bytes
ATtiny45	752 bytes	766 bytes	14 bytes
ATtiny85	752 bytes	766 bytes	14 bytes

Without the additional call to analogReference(EXTERNAL); inside setup() there was no change in code size.

@cmaglie @facchinm @matthijskooijman any opinions on the extra code size? We could wrap the change in wiring_analog.c in an #ifdef to avoid the additional code size on non-ATtinyX5 boards like the Uno.

[facchinm]

I'd personally avoid to change the ADMUX line for all targets, since the same can be accomplished with saner INTERNAL2V56 and INTERNAL2V56_EXTCAP defines and an #ifdef guard.

Something like

 +#elif defined(__AVR_ATtiny25__) || defined(__AVR_ATtiny45__) || defined(__AVR_ATtiny85__)
 +  #define DEFAULT 0
 +  #define EXTERNAL 4
 +  #define INTERNAL1V1 8
 +  #define INTERNAL INTERNAL1V1
 +  #define INTERNAL2V56 9
 +  #define INTERNAL2V56_EXTCAP 13

and

#if defined(ADMUX)
if defined(__AVR_ATtiny25__) || defined(__AVR_ATtiny45__) || defined(__AVR_ATtiny85__)
ADMUX = (analog_reference << 4) | (pin & 0x07);
#else
ADMUX = (analog_reference << 6) | (pin & 0x07);
#endif

[sandeepmistry]

@facchinm that's a great suggestion!

@mischnic what do you think? Do you have time to update your pull request with the suggestion?

[mischnic]

@sandeepmistry Ok

[sandeepmistry]

@mischnic thank you!