[sdiy] understanding direct AVR ADC...multiple channels

[Ove Ridé]

On 6 March 2012 01:29, dan snazelle <subjectivity at hotmail.com> wrote:
> however, I have never been able to find any good articles that show how to sample with the ADC directly WHEN YOU HAVE FOUR INPUTS.
>
> In other words, lets say I have two audio sources, two CV potentiometers.
>
> I have never understood if I am supposed to take the ADC code and quadruple it? and if so, doesnt that mean that every step will have to wait for every other step?

The chip only has a single ADC, which is shared between all channels
via a multiplexer. So yes, you can only read one channel at a time.
You may want to switch between the two audio ADC channels every loop
cycle, and read the pot data only every xth cycle of the loop.

REPEAT 16 TIMES{
  Read audio 1
  Read audio 2
}

Read potentiometer 1

REPEAT 16 TIMES{
  Read audio 1
  Read audio 2
}

Read potentiometer 2

Choose a good value for how often you want to read the potsdepending
on desired time resolution. This will give you some very small (one
sample) hiccups in the sound at that regular interval, but better than
not being able to read the potentiometer at all, right?,

> Will this STILL be quicker?

If I understand your question correctly, the answer is that the
conversion will take the same time, but I believe you can use the CPU
for other things while the conversion is being performed. The Arduino
analogRead fucntion on the other hand will stay in a loop polling for
the conversion to be finished. So, rolling your own code will more
efficient for that reason. (This is how I believe things are, anyway.
I'm reading the datasheet and making gueses how the Arduino coed works
internally. I may be somewhat off.)

> Also...advice...should I just ditch the upper 2 bits and deal directly with the 8 bit register of the ADC?

Almost. Ditch the lower 2 bits. Ditching the top 2 bits would create a
four repetitions of 0-255 over full voltage range.
Tip: There's a flag called ADLAR (bit 5 of the ADMUX reigster) which
controls the distribution of bits between ADCH and ADCL.
ADLR = 0 pushes the bits to the bottom, so ADCL contains the lowest 8
bits and ADCH the highest 2.
ADLR = 1 pushes the bits to the top so ADCH contains the highest 8
bits and ADCL contains the lowest two.
If you want to read everything as a 16 bit value, use mode 0. If you
want to use only 8 bits, use mode 1 and only read ADCH to get your 8
bits.

> and should I use free running? or triggered?

Good question. I would probably personally at least begin by
triggering the ADC manually from your code so you have control over
things until you learn how things work.

> There are only a few pages I can find that tread on this territory and none of them are SDIY oriented.
>
> ANY HELP, links, code, etc book recommendations, etc would be much appreciated!
>
> I have gotten ONE CHANNEL working

Ah! Use the lower 4 (or really 3) bits of the ADMUX register to select
the input. I think the Arduino pretty much puts the ADC channel
selection here. This is also where you have the aformentioned ADLAR
bit, as well as voltage reference selection bits.

I don't know if you need this advice, but another thing you need to
consider for audio use is biasing and levels. The ADC will read values
between 0 and 5V whereas audio signals might be bipolar, and perhaps
stronger than 5V peak to peak. Simple fix: Connect the ADC pin to
ground and +5V through a resistor 4.7k or 10k resistor to each place.
This is to center voltage level. Then connected the audio source to
that point through a 4.7 uF capacitor. This should make the bipolar
audio signal at the input swing around 2.5 V rather than 0 V, so you
can reda the full range with the ADC. There are mroe elaborate things
you can do I suppose, but that's better than clipping away the
negative half of the waveform.

Potentiometers can just be conected straight to the ADC, with the
clockwise terminal to the positive voltage used by the Atmega and the
CCW terminal to Gnd. (I.e. no special treament/biasing needed.)

Above all, read the datasheet!

-- 
/Ove

Blog: <http://blog.gg8.se/>

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