Ok, I have lifted pin 62 on the Atmega128, so AREF is no longer connected to AVcc.  I would like to use the interal 2.56V reference, so I need to set REFS1 and REFS0 to 11--how do I go about doing this?


On 4/24/06, Jonathan Stewart <aggie.stewart@gmail.com> wrote:

I have an analog accelerometer that i need to take readings from with
the ADC.  Vcc on the accelerometer is 3V and the it tells acceleration
+- 1.5g. so when i move in one direction and the ADC max out I lose my
perspective of the force that were there.  The added space of going from
2.56V to 3V as a reference would help.  If i plug a voltage on the V pin
for an ADC channel will i then take readings on that scale? i.e if 3V
were used the ADC would make values from 0 to 3V or would I short the board?


> Date: Mon, 24 Apr 2006 07:09:10 -0700
> From: "Dave Hylands" < dhylands@gmail.com>
> To: gumstix-users@lists.sourceforge.net
> Subject: Re: [Gumstix-users] ADC Vref
> Reply-To: gumstix-users@lists.sourceforge.net
> Hi Jonathan,
> On 4/23/06, Jonathan Stewart <aggie.stewart@gmail.com> wrote:
>> > I want to control the ADC Vref.  How do I go about that?  The Atmel data
>> > sheet refers to a register that can be changed that does this but I do no=
> t
>> > understand how to change that register and if I change it once will that
>> > change presist through later restarts.
> Figure 108 (on page 232) of the datasheet shows what's required.
> Unfortunately, in order to be able to switch the ADC reference, the
> AREF pin needs to be unconnected. On the robostix its currently
> connected to AVcc.
> If you lift pin 62 (the trace connecting 62 to 64 is under the
> ATMega128 chip - so the only way to disconnect 62 is to lift the pin),
> then you can use the REFS0 and REFS1 bits in the ADMUX register to
> control AREF.
> This register (and in fact all of the registers) will get reset to
> their default value upon reset.
> The only choices that you have (using the switch) are to use AVcc or
> 2.56v and these choices would only be available if the AREF pin is
> unconnected.
> Why are you trying to change AREF?
> --
> Dave Hylands
> Vancouver, BC, Canada
> http://www.DaveHylands.com/

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