AVR-Chat

You might lookup how to sleep while the conversion is being done,
this reduces noise. The ADC interrupt will wake you up
when it has completed.

Regards,
   JJ

On Tuesday, May 11, 2004, at 12:49 PM, Craig Limber wrote:

>
> Hi there;
>
> I am having a wee bit of trouble with a noisy analog to digital  
> conversion
> setup and was wondering if anyone out there could suggest some things
> to try to clean it up.
>
> The processor is a Mega32 running at 8 MHz with a crystal.  Power is
> regulated 5 volts from a battery.  The circuit has a very generous
> supply of bypass and filter capacitors.  A 'scope on the power and ADC
> lines doesn't show any noise that it can detect.  I am using the LC
> circuit the manual recommends for the AVCC and it's voltage measures
> the same as that found on the chip's VCC lines.  I have a .1uf disk cap
> going from AREF to ground but nothing else since I am using the
> Internal voltage reference.  All other aspects of the circuit work as I
> expect.  No motors or other inductive loads are currently in use or
> hooked up.  I am using avr-libc and gcc.
>
> The sensors I am trying to read are Sharp GP2D120 4-30cm infrared
> range sensors.  I have four.  If I read just one on any channel the
> signal is clean and consistent.  As soon as I try to multiplex them I
> run into trouble.  The values keep jumping about plus or minus about
> half a volt around the reading I expect to get.  It is so wild that
> even a routine to average out the results isn't good enough.  The
> sensors are powered all the time.  I switch channels once every 100
> milliseconds and don't try to get the first reading until 500
> milliseconds after poweron.  The sensors themselves are getting a good,
> well filtered 5 volt power from the same source that powers the rest of
> the circuit.
>
> I am using the interrupts to handle conversion.
>
> Here is my setup code:
>
> // global variables
> uint8_t curchannel = 0, // the current channel I want to read
>         doingadc = 0,   // don't start new conversion before previous  
> done
>         adchannel[4];   // where to store the results
>
>   ADMUX = _BV(ADLAR)     // left justify result; 8 bits good enough
>         | _BV(REFS0)     // AREF has external capacitor
>         | _BV(REFS1);    // use internal 2.56 volt reference
>   ADCSRA = _BV(ADEN)     // enable ADC doodad
>          | _BV(ADIE);    // enable ADC interrupt
>
> // using minimum prescaler of 2
>
> So, once every 100 milliseconds I do this:
>
> if (!doingadc)
> {
>   if (curchannel & 0x01) sbi(ADMUX, MUX0); else cbi(ADMUX, MUX0);
>   if (curchannel & 0x02) sbi(ADMUX, MUX1); else cbi(ADMUX, MUX1);
>   if (curchannel & 0x04) sbi(ADMUX, MUX2); else cbi(ADMUX, MUX2);
>   doingadc = 1;
>   sbi(ADCSRA, ADSC);  // start conversion
>   curchannel++;       // prepare for next one
>   if (curchannel == 4)
>     curchannel = 0;
> }
> else
>   error("started new conversion before previous one was done");
>
> Here is my signal routine:
>
> SIGNAL(SIG_ADC)
> {
>   adchannel[curchannel] = ADCH;
>   doingadc = 0;
> }
>
> If I insert a statement to set curchannel to a fixed value instead
> of incrementing it then it works.  As soon as I try to read from
> more than one channel after another the noise comes in.  The pins
> in use have no other purposes and are not used as outputs or have
> the internal pullup resistors activated.  I have low-pass filters
> attached to the ADC input pins that I can enable/disable with jumpers.
> I tried going to a direct connection as well but that didn't make any
> difference that I could see.  I tried shielding and/or twisting the
> cables to the sensors but that didn't help either.
>
> The doingadc variable is just a way to test that I am not trying to
> start a new conversion before the last one was finished.   I tried
> using the ADC sleep mode to help with noise but no joy there.
>
> So, I am stumped and scratching my head.  Any suggestion on how I can
> clean it up?
>
> Craig
>
>
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