From 92ba50c7b226c951ba440badf638801f12f3a9ff Mon Sep 17 00:00:00 2001
From: "David A. Mellis" <d.mellis@arduino.cc>
Date: Fri, 14 Dec 2007 04:21:59 +0000
Subject: [PATCH] Adding analogReference() function - needs testing on an
 ATmega8 (but works on the ATmega 168).

---
 cores/arduino/wiring.c        |  4 ----
 cores/arduino/wiring.h        |  5 +++++
 cores/arduino/wiring_analog.c | 16 ++++++++++++++--
 3 files changed, 19 insertions(+), 6 deletions(-)

diff --git a/cores/arduino/wiring.c b/cores/arduino/wiring.c
index 5dda822..ae8fcdd 100755
--- a/cores/arduino/wiring.c
+++ b/cores/arduino/wiring.c
@@ -170,10 +170,6 @@ void init()
 	sbi(TCCR2, WGM20);
 #endif
 
-	// set a2d reference to AVCC (5 volts)
-	cbi(ADMUX, REFS1);
-	sbi(ADMUX, REFS0);
-
 	// set a2d prescale factor to 128
 	// 16 MHz / 128 = 125 KHz, inside the desired 50-200 KHz range.
 	// XXX: this will not work properly for other clock speeds, and
diff --git a/cores/arduino/wiring.h b/cores/arduino/wiring.h
index e0c5315..df43e6c 100755
--- a/cores/arduino/wiring.h
+++ b/cores/arduino/wiring.h
@@ -57,6 +57,10 @@ extern "C"{
 #define FALLING 2
 #define RISING 3
 
+#define INTERNAL 3
+#define DEFAULT 1
+#define EXTERNAL 0
+
 // undefine stdlib's abs if encountered
 #ifdef abs
 #undef abs
@@ -93,6 +97,7 @@ void pinMode(uint8_t, uint8_t);
 void digitalWrite(uint8_t, uint8_t);
 int digitalRead(uint8_t);
 int analogRead(uint8_t);
+void analogReference(uint8_t mode);
 void analogWrite(uint8_t, int);
 
 void beginSerial(long);
diff --git a/cores/arduino/wiring_analog.c b/cores/arduino/wiring_analog.c
index 0ab32eb..30c5642 100755
--- a/cores/arduino/wiring_analog.c
+++ b/cores/arduino/wiring_analog.c
@@ -25,12 +25,24 @@
 #include "wiring_private.h"
 #include "pins_arduino.h"
 
+uint8_t analog_reference = DEFAULT;
+
+void analogReference(uint8_t mode)
+{
+	// can't actually set the register here because the default setting
+	// will connect AVCC and the AREF pin, which would cause a short if
+	// there's something connected to AREF.
+	analog_reference = mode;
+}
+
 int analogRead(uint8_t pin)
 {
 	uint8_t low, high, ch = analogInPinToBit(pin);
 
-	// the low 4 bits of ADMUX select the ADC channel
-	ADMUX = (ADMUX & (unsigned int) 0xf0) | (ch & (unsigned int) 0x0f);
+	// set the analog reference (high two bits of ADMUX) and select the
+	// channel (low 4 bits).  this also sets ADLAR (left-adjust result)
+	// to 0 (the default).
+	ADMUX = (analog_reference << 6) | (pin & 0x0f);
 
 	// without a delay, we seem to read from the wrong channel
 	//delay(1);