[avr-gcc-list] ADC requires a pause for proper operation

[harshit suri]

Hello I am using the ATmega128. I have 3 sensors
connected to the ADC

PIN ADC0 a potentiometer .Vref used is AVCC = 5V
PIN ADC3 , ADC2 : differential signal from a servo
potentiometer giving feedback on its position . VREF
is internal 2.56Volts
PIN ADC5 : From a phototransistor giving the strength
of recieved signal Vref is AVCC = 5V
i noticed 2 problems
1)the effect of changing the photodiode sensor value
was changing the reading of the Potentiometer 
2)the value being sent back for the differential
signal from the servo sensor was nearly half of what
it should have been.It was wrong.
To try to correct this i did the following :
in the functions samplePot(), sampleServoPosition() ,
sampleRxSignalPower() :
I added a delay between setting up the ADC(choosing
ADC MUX and ADC REFS) and the actual start of ADC
conversion ADCSRA |= (BV(ADSC));...and this fixed the
problem. I have only been able to rationalize this by
thinking that their is a need to let Vref settle down
between the switching 2.56 and AVCC=5V .BUT this also
is wrong cause i tested by just sampling the sensors
using AVCC as Vref . I still got the coupling problem.

I also did some experiments and would also like to
report that page 235 of Atmega128 datasheet in the
section on "Changin Channel or reference Selection"
says that conversions should not be started within
125us of choosing the differential input for the first
conversion.
I actually found that the delay needed was 960us.I
used the following loop.I used a scope on PORTD pin to
measure the delay.
I measured this delay using :
PORTD ^= BV(1);
for (delay = 0 ; delay < 1230 ; delay++); 
PORTD ^= BV(1);
//perform single conversion
ADCSRA |= (BV(ADSC));

While the data sheet mentions that a delay is needed
while measuring differential signals. It does not
mention that u need a delay before measuring single
ended signals. However I found I needed a delay there
too. can anyone advise?

Here is the code that samples a sensor. i added the
for loop to add a delay in every such function

ADMUX |= (BV(REFS0) );
ADMUX &= ~(BV(REFS1) | BV(ADLAR) | BV(MUX4) | BV(MUX3)
| BV(MUX2) | BV(MUX1) | BV(MUX0));
long delay = 0;
for (delay = 0 ; delay < 20000 ; delay++); //I ADDED
THIS DELAY BEFORE THE STARTING OF CONVERSION
//perform a single conversion
ADCSRA |= (BV(ADSC));

I noticed that u need to add the delay only for the
first conversion. If u dont change the MUX AND Vref
then u dont need to add the delay again and again. But
if u change the channel then u need the delay again no
matter if its single ended or differential.


__________________________________
Do you Yahoo!?
Yahoo! Mail - More reliable, more storage, less spam
http://mail.yahoo.com_______________________________________________
avr-gcc-list mailing list
address@hidden
http://www.avr1.org/mailman/listinfo/avr-gcc-list