topic Re: ADC strange behaviour in LPC Microcontrollers

ADC strange behaviour

lpcware — Wed, 15 Jun 2016 19:45:46 GMT

Content originally posted in LPCWare by Genesy on Fri Nov 15 10:21:05 MST 2013
Hi,
as the title, I have a strange behaviour with ADC.
The program is this:

static uint32_t ADCChannelAcquisition(uint8_t channel){
ADCRead( channel );
while ( !ADCIntDone );
ADCIntDone = 0;
return ADCValue[channel];
}

int main(void) {

SystemInit();
ADCInit( ADC_CLK );

int i;
uint32_t val=0;
uint32_t acquisition[10];

uint32_t count=0;

//for (i=0; i<100000; i++){
//count++;
//}

for (i=0; i<10; i++){
acquisition = ADCChannelAcquisition(0);
}

for (i=0; i<10; i++){
val += acquisition;
}

uint32_t valRif = (uint32_t)(((float)val  / 10.0) + 0.5);


return 0 ;
}

The behaviour is different if I insert a breakpoint in the first acquisition as you can see in the attached images.
In the first case the values are higher than in the second case. What is the reason?
What values are reliable?

I tried to insert a delay before making ADC readings, but nothing has changed.

Thank you very much

Re: ADC strange behaviour

lpcware — Wed, 15 Jun 2016 19:45:47 GMT

Content originally posted in LPCWare by MarcVonWindscooting on Sat Nov 16 06:26:50 MST 2013
Just a guess: before Init the port pin is GPIO with pullup.
The you switch to ADC input and start sampling within a few us. Pretty short time!
Can your excess reading be the removed pullup-current + low-pass behaviour?

Re: ADC strange behaviour

lpcware — Wed, 15 Jun 2016 19:45:47 GMT

Content originally posted in LPCWare by Genesy on Mon Nov 18 01:51:54 MST 2013
Yes, I also thought that it could be a matter of timing and I inserted a short delay but wasn't enough.
Than it should be this late?

Re: ADC strange behaviour

lpcware — Wed, 15 Jun 2016 19:45:48 GMT

Content originally posted in LPCWare by MarcVonWindscooting on Mon Nov 18 12:11:47 MST 2013
Have you tried with a long delay in the seconds range (like your breakpoint does)?
100*1000 loops isn't that much nowadays ;-)

Also, are you sure, the loop is executes as expected? I believe so, too. But to be 100% sure, declare count 'volatile'.
What is the input circiut to your ADC? Voltage divider?

Re: ADC strange behaviour

lpcware — Wed, 15 Jun 2016 19:45:48 GMT

Content originally posted in LPCWare by Genesy on Tue Nov 19 01:37:01 MST 2013
Ok, I will try with your suggestions.
The ADC input is a voltage divider from a hall effect sensor.

Thank you very much Marc.