Update for 15-11-21 16:00

tech/ATMega328P.wiki

@@ -236,4 +236,34 @@ NOTE to help with calculations, 0xFFFF is 65536 and 0xFF is 256
 | 256  | 62500   |
 | 1024 | 15625   |
 
+=== Analog to Digital ===
 
+The ATMega328P includes an [[ADC]] for reading analog inputs. The ADC, like
+everything else, is controlled via several control registers.
+
+The primary control register is the ADC Multiplexer Selection Register (ADMUX
+0x7C). Bit 7 and 6 select the reference voltage for the ADC. Any updates to
+this register do not take effect till after an ongoing ADC operations has
+completed. The internal voltage reference may not be used if a voltage is
+applied across the AREF pin. Below are valid values for bits 7 and 6
+
+| Bits (7-6) | Description |
+----------------------------
+| 00 | AREF, internal reference voltage turned off         |
+| 01 | AV,,cc,,, with an external capactior on AREF        |
+| 10 | Reserved (non functioning)                          |
+| 11 | Internal 1.1V reference used with capactior on AREF |
+
+Bit 5 of ADMUX changes the presentation of the ADC result, and when set to one
+(1) the value will be left adjusted, otherwise it will be right adjusted. This
+change happens instantly, regardless of the actions in the ADC
+
+Bits 3-0 set which ADC pin will be converted (NOTE these are the pins on PortC
+on the Arduino Nano). For values 0 through 7 (binary values in bits 3-0), will
+select the corresponding ADC pin. 0b1000 is a sepcial ADC, connected to the
+internal temperture sensor. 0b1110 (10d14) is the internal reference voltage of
+1.1V, and 0b1111 (10d15) is internal reference GND (0V). All non mentioned
+values are not used
+
+To control the ADC, the ADC control and Status Register A (ADCSRA 0x7A) and ADC
+Control and Status Register B (ADCSRB 0x7B) are used.