12F1840 ADCIN troubles, help needed :(

[CuriousOne]

Well, troubles continue.

I've built a following circuitry. 10K resistor goes to LATA.2 pin and goes then to gnd.
100k variable pot is connected one end to VCC, another to LATA.2
LED is connected to GND via 1k resistor from LATA.1

Wiring is 100% checked and corrected for shorts, misswire, etc.

The code is as follows:

OSCCON = %11110000  'SET INTOSC TO 32MHZ
TRISA=%00110000 'set PORt as input
'ANSELA=%00010101 'set PORTA.4 as analog
ADCON0=%00001111  'ENABLE AND CONFIGURE ADC
ADCON1=%11000011   'JUSTIFY
FVRCON=%11011111 'VREF ENABLE & set at 4.096v
WPUA=%00000000 'pull up disable
'CM1CON0=%00000000   'DISABLE COMPARATORS

DEFINE OSC 32
DEFINE ADC_BITS 10
DEFINE ADC_SAMPLEUS 50
DEFINE ADC_CLOCK 3
VOLT VAR WORD
LED VAR LATA.1

greater:
adcin 2,volt
high led
pause 1
low led
pause volt/10
goto greater

This code should do a simple thing - vary LED brightness according to pot setting. And sure it does, but the problem is that it works in very narrow range, because port sinks huge amount of current - up to 25mA via that pin. So say to achieve 2.5v on the pin, the pot resistance should be set somewhere to 40 ohms. I think this is not correct, right?

Also I found a strange thing - if I set TRISA=%00110010, which means to set LATA.2 as input, the code does not works at all, but it also stops consuming current on that pin. What can be the reason?

[mark_s]

Well, troubles continue.

Also I found a strange thing - if I set TRISA=%00110010, which means to set LATA.2 as input, the code does not works at all, but it also stops consuming current on that pin. What can be the reason?

I think you have set porta.1 to input and not porta.2

This sets PORTA.2 as input

%00110100

Not sure about your other problems

[HenrikOlsson]

Hi,
The input (basically) only sinks current when it's configured as an output and set low. When being used as an input for the ADC it obviously needs to be set to input - just as Mark has pointed out. Then it "sinks" current only when the input is connected to the S/H circuit which leads us to the next issue: Your 100k pot viloates the maximum recommended input impedance by an order of magnitude.

/Henrik.

[CuriousOne]

I have created a voltage divider, 100k and 10k resistors, ADC input connected in their middle point. The problem is that PIC shorts the input to GND, but still works as ADC. This has no relation to input impedance, it feels like same pin is both digital out and analog in at same time. Is this possible?

[HenrikOlsson]

Hi,
In your original code ( TRISA=%00110000 ) leaves PortA.2 as an output which is wrong.
In your post you contiune to say TRISA=%00110010, to set LATA.2 as input which again, as mark_s points out, is wrong, the above will STILL leave PortA.2 as an OUTPUT.

Your setting of ANSELA in the code is commented out which doesn't really have any effect on the ADC input since it defaults to analog but it WILL have an effect on the pins you want to use as digital.

The ADC DOES put a load on the node when it samples the input that's where the max input impedance comes from. Too high impedance and too short sampling time makes the voltage at the node droop when it's sampled.

/Henrik.

[pedja089]

HenrikOlsson,
If you use 10K and 100K for divider, impedance that PIC input see is 9,09K.

[CuriousOne]

Yes I have commented ANSELA because it had no effect anyways.

[CuriousOne]

And I know that ADC needs some current, but not 25mA! On the RA3 or LATA.3 how you call it, which is input only, ADC works just fine and does not hog current.