Two questions about 12F75 A/D converters

Welshman · Guest

I am having a problem with my 12F675 chips' A/D converters. I'm not trying to do anything particularly complicated, essentially just read in a value. I wrote some test code to make sure the A/D converter was working, which I include below:
<code>
#include <12f675.h>
#use delay (clock = 20000000)
#define LED PIN_A2
void initialize ();
long volt_max;
main ()
{
 initialize ();
 while (true)
 {
 volt_max = read_adc ();
 if (volt_max < 250) led_blink (1);
 if ((volt_max >= 250) & (volt_max < 500)) led_blink (2);
 if ((volt_max >= 500) & (volt_max < 750)) led_blink (3);
 if (volt_max >= 750) led_blink (4);
 delay_ms (2000);
 }
}
void initialize ()
{
 setup_adc (ADC_CLOCK_DIV_2);
 setup_adc_ports (AN1_ANALOG);
 set_adc_channel (1);
 return;
}
</code>
Note: the led_blink(i) function simply blinks an LED i number of times, and I know for sure it works.

First, I am not using a separate voltage reference, just Vdd. When I run this program, which should blink an LED a number of times directly dependent on the voltage on the A/D converter, it will only blink once for voltages of 1V to 4.9V, and then from 4.9V to 5V it blinks twice. For my code, if the LED blinks once, there should be a voltage of 0V to 1.25V on the A/D converter, but that is not the case. If it blinks twice, there should be a voltage of 1.25V to 2.5V on the A/D converter, which is also not happening. The most befuddling aspect of this problem is that if I change <code>volt_max</code> to an <code>int</code> instead of a <code>long</code>, and then adjust the test values in the code accordingly (i.e., 250-500-750 converts to 64-128-192), everything works fine. I thought maybe (a) the A/D converter is not doing full 10-bit conversion, or (b) the format of the output is not being set or read correctly by the C compiler, but it's probably something else. I have tried manually setting the ADFM bit to both states in the ADCON register after the A/D converter setup code, but neither way works. As such, I'm rather out of ideas. 
My second question is, if I am running the 12F675 off the internal crystal, can I use the option <code>setup_adc (ADC_CLOCK_INTERNAL);</code>? The datasheet for the 12F675 gives the impression that the A/D converter has its own oscillator, but I have not been able to verify if both will work at the same time because I have been testing my chip with an external crystal to eliminate a possible source of error. When I build my final device, however, I will not have enough pins to use an external crystal.

Well, thanks a lot in advance to any help anyone can give.

Regards,
Chris
___________________________
This message was ported from CCS's old forum
Original Post ID: 12109

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

:=I am having a problem with my 12F675 chips' A/D converters. I'm not trying to do anything particularly complicated, essentially just read in a value. I wrote some test code to make sure the A/D converter was working, which I include below:
:=
:=#include <12f675.h>

To get 10-bit A/D resolution, you need to insert the
following line, at this point:

#device adc = 10

:=#use delay (clock = 20000000)
:=#define LED PIN_A2
:=void initialize ();
:=long volt_max;
:=main ()
:={
:= initialize ();
:= while (true)
:= {
:= volt_max = read_adc ();
:= if (volt_max < 250) led_blink (1);

These tests should use the Logical AND operator,
which is "&&".

:= if ((volt_max >= 250) & (volt_max < 500)) led_blink (2);
:= if ((volt_max >= 500) & (volt_max < 750)) led_blink (3);
:= if (volt_max >= 750) led_blink (4);
:= delay_ms (2000);
:= }
:=}
:=void initialize ()
:={

If you look at the 12F675 data sheet, in Table 7-1, it gives
the allowable A/D clock divisors for a 20 MHz crystal.
These divisors are 32, 64, or the internal A/D clock.
You shouldn't use a divisor of 2.
The data sheet can be downloaded here:
<a href="http://www.microchip.com/1000/pline/picmicro/category/flash/index.htm" TARGET="_blank"> <a href="http://www.microchip.com/1000/pline/picmicro/category/flash/index.htm" TARGET="_blank">http://www.microchip.com/1000/pline/picmicro/category/flash/index.htm</a></a>

:= setup_adc (ADC_CLOCK_DIV_2);
:= setup_adc_ports (AN1_ANALOG);
:= set_adc_channel (1);
:= return;
:=}
___________________________
This message was ported from CCS's old forum
Original Post ID: 12110

Welshman · Guest

Thanks a million. My code worked the first time I made the changes.

Regards,
Chris
___________________________
This message was ported from CCS's old forum
Original Post ID: 12130