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Futterama
Joined: 17 Oct 2005
Posts: 98
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| Strange ADC behavior on PIC16F88 |
 Posted: Wed Nov 02, 2005 6:59 am |
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Hello forum,
I'm using the ADC on PIC16F88 and it works fine. But, I don't get the same reading every time even though the voltage on the ADC input is exactly the same.
This is my code:
.h file:
| Code: | #include <16F88.h>
#device adc=10
#fuses NOWDT,INTRC_IO, NOPUT, NOMCLR, BROWNOUT, NOLVP, NOCPD, NOWRT, NODEBUG, NOPROTECT, NOFCMEN, NOIESO
#use delay(clock=4000000)
#use rs232(baud=9600,parity=N,xmit=PIN_B3,rcv=PIN_B5,bits=8,invert) |
.c file:
| Code: | #include "myfile.h"
void main()
{
int16 i;
setup_adc_ports(sAN0|VSS_VDD);
setup_adc(ADC_CLOCK_DIV_8);
setup_spi(FALSE);
setup_timer_0(RTCC_INTERNAL|RTCC_DIV_1);
setup_timer_1(T1_DISABLED);
setup_timer_2(T2_DISABLED,0,1);
setup_comparator(NC_NC_NC_NC);
setup_vref(FALSE);
output_low(PIN_B3);
delay_ms(2);
while(1)
{
i = read_adc();
printf("%lu", i);
putc(13);
putc(10);
delay_ms(10000);
}
} |
The input of the ADC is connected to a resistor divider. The output from RS232 is like this:
886
877
886
877
886
877
... you get the point.
So, what the heck is going on? It's the same problem at different "update rates" (10000ms or 1000ms).
Hope you can help.
Regards,
Futterama |
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SherpaDoug
Joined: 07 Sep 2003
Posts: 1640
Location: Cape Cod Mass USA
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| Posted: Wed Nov 02, 2005 7:22 am |
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Try putting a cap, say 0.1 uF, from the A/D input to the A/D ground pin. Your resistor divider may have too high an impedance.
_________________
The search for better is endless. Instead simply find very good and get the job done. |
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Futterama
Joined: 17 Oct 2005
Posts: 98
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| Posted: Wed Nov 02, 2005 7:48 am |
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| SherpaDoug wrote: | | Try putting a cap, say 0.1 uF, from the A/D input to the A/D ground pin. Your resistor divider may have too high an impedance. |
The capacitor doesn't help.
I've changed the resistor divider to a 1k pot, and that doesn't help either.
I've changed to another PIC16F88, that doesn't help, so the PIC itself is fine.
It's always the same numbers coming out, even when changing PIC device.
I'm lost. What good is the ADC if it's faulty? |
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asmallri
Joined: 12 Aug 2004
Posts: 1634
Location: Perth, Australia
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| Posted: Wed Nov 02, 2005 8:47 am |
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You forgot to select the channel you want converted.
_________________
Regards, Andrew
http://www.brushelectronics.com/software
Home of Ethernet, SD card and Encrypted Serial Bootloaders for PICs!! |
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gs
Joined: 22 Aug 2005
Posts: 30
Location: Ioannina - Greece
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| Posted: Wed Nov 02, 2005 12:22 pm |
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measure the value 10 times in 1.5 ms intervals, Sum the results and divide by 10. This will give you a more stable value.
I use the following code in one of my projects. I also use RC filter (1K, 100nF) at the analog inputs
| Code: | int16 mesi_timi()
{
int16 value,sum;
int i;
sum=0;
for (i=1; i<=10; i++)
{
value=Read_ADC();
sum=sum+value;
delay_us(1500);
}
value=sum/10;
return value;
} |
_________________
www.hlektronika.gr |
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Futterama
Joined: 17 Oct 2005
Posts: 98
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| Posted: Thu Nov 03, 2005 12:21 am |
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| asmallri wrote: | | You forgot to select the channel you want converted. |
That's right. But I guess since I use port 0 it doesn't matter, cause setting the port to 0 doesn't help.
| gs wrote: | measure the value 10 times in 1.5 ms intervals, Sum the results and divide by 10. This will give you a more stable value.
I use the following code in one of my projects. I also use RC filter (1K, 100nF) at the analog inputs
| Code: | int16 mesi_timi()
{
int16 value,sum;
int i;
sum=0;
for (i=1; i<=10; i++)
{
value=Read_ADC();
sum=sum+value;
delay_us(1500);
}
value=sum/10;
return value;
} |
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Well, yes, but I'll also get a very stable reading by only reading every second measurement (since they are the same).
As you can see, it can't be analog noise since the values are the same two repeating regardless of the convertion rate. |
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Futterama
Joined: 17 Oct 2005
Posts: 98
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| Posted: Thu Nov 03, 2005 12:49 am |
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DOH! I found the reason for my error.
My supply IC can deliver 40mA. I had a LED blinking every time a conversion was sent (I cut this part out of my posted code). The LED caused the voltage to drop a bit every time it was on. That's why I got these strange readings! 
Thank you all - gs, I'll use your posted code to get stable readings, though I'll probably read 100 times instead of 10 
Regards,
Futterama |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Thu Nov 03, 2005 1:17 am |
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Also, the capacitor does make a difference. Just now I was testing
your problem on a PicDem2-Plus, with a 16F88 and PCM vs. 3.235,
and I got this output:
Notice that it bounces around a little bit.
| Quote: |
888
883
889
888
884
887
884
883
883
887
884 |
Then I added a 0.1 uF (i.e., 100 nF) capacitor close to the RA0 pin
and the GND pin on the 16F88. Then I got this output:
| Quote: |
886
885
885
886
886
886
886
886 |
It's much more stable. |
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