according to the atmega1280 specification, the internal reference voltage can be selected at either 1.1v or 2.56v. (not just 2.56v, as the wiki suggests)
The analogReference(uint8_t mode) function simply sets a variable that is (later) used by analogRead(uint8_t pin) to set bits 6 and 7 (REFS1:0) of the ADMUX register. (These details can be observed in .../arduino/hardware/cores/arduino/wiring_analog.c) The REFS1:0 bits determine the source of the reference voltage as follows:
REFS1
REFS0
Voltage reference selection
0
0
AREF, internal Vref off
0
1
AVCC
1
0
Internal 1.1v Voltage reference
1
1
Internal 2.56v Voltage reference
.../arduino/hardware/cores/arduino/wiring.h only makes the following defines: #define INTERNAL 3 #define DEFAULT 1 #define EXTERNAL 0
as you can see - there is no definition for the internal 1.1vref.
Adding the following to wiring.h will allow you to select an internal 1.1v voltage reference: #if defined(AVR_ATmega1280) #define INTERNAL_1V1 2 #define INTERNAL_2V56 3 //added for completeness #endif
selecting the analog reference is as simple as:
analogReference(INTERNAL_1V1);
or
analogReference(INTERNAL_2V56);
of course, INTERNAL_1V1 is just a macro... to use the internal 1.1v reference without modifying any of the arduino code, simply use the following:
analogReference(2)
I have tested this and can easily switch between an internal reference voltage of 1.1v or 2.56v. I hope this comes in handy!
I do NOT know how this will behave on other hardware - hence the '#if defined(AVR_ATmega1280)' - consult your hardware manual!
Thanks,
i have also use this in the 168 and 328, (the 1.1Volt i mean)
could you add this also in those?
Thanks
Pat Gadget
Montreal
1.1v is the only internal voltage reference on the 168/328. Nothing to add!
(I finally checked the 168 and 328 documentation. Setting bits 6 and 7 of ADMUX register [which is all the analogReference(...) does] to a value of 2 is "Reserved." In short, don't do it.)
The 1280 has either 2v56 or 1v1. The above modification simply makes a define that isn't present for the 1280.
Is there any way to allow INTERNAL to still be valid on mega bords? If someone has a sketch for another board that uses analogReference(INTERNAL) it will nolonger compile.
