About PIC16f877A ADC - bad readings

[Jean_Paul]

Hi guys

I wrote a code to read using the adc of the pic16f877a three external analog inputs. I connect to AN0, AN1 and AN3 and then send the data to the serial port. when I connect the AN3 input to ground 0v, the AN0 decrease the value to the same value. I dont know what is happening.

My code is:

#include "E:\pictest\ac2877a.h"
#include <stdio.h>
#include <string.h>


void main()
{
int8 value1, value2, value3;



setup_adc_ports(ALL_ANALOG);
setup_adc(ADC_CLOCK_INTERNAL);
setup_psp(PSP_DISABLED);
setup_spi(FALSE);
setup_counters(RTCC_INTERNAL,RTCC_DIV_1);
setup_timer_1(T1_INTERNAL|T1_DIV_BY_1);
setup_timer_2(T2_DISABLED,0,1);
setup_ccp1(CCP_OFF);
setup_comparator(NC_NC_NC_NC);
setup_vref(FALSE);
do {
delay_ms(5);

putc(0xAA);

set_adc_channel(0);
value1 = read_adc();
putc(value1);

set_adc_channel(1);
value2 = read_adc();
putc(value2);

set_adc_channel(3);
value3 = read_adc();
putc(value3);





} while (TRUE);
}

[Guest]

J-P

You must wait between switching between the ADC input channel and sampling. Plenty of posts here about the importance of this delay.


HTH,
Bill

[dyeatman]

Example of the delay needed:

set_adc_channel(1);
delay_us(20);
value2 = read_adc();

[Joshua Lai]

How to calculate the time needed for ADC dalay and the input current?

[valemike]

In the Microchip databook, there's an exhaustive explanation of the linear approximation method that Microchip uses, along with a bunch of formulas.

However, I don't understand the formulas. So what the earlier post that tells you to wait 20 us might be fine. I usually wait 1 millisecond after changing the adc channel.

-Mike

[Joshua Lai]

I tried using setup_adc(ADC_CLOCK_INTERNAL), then tried setup_adc(TC_DIV_32);

I know with 20Mhz frequency, Tosc32 is 1.6us, using RC oscillator is 4us.

Why I get random A/D value when using setup_adc(ADC_CLOCK_INTERNAL)?

What actually use internal RC oscillator means? Why need to use SLEEP?

[Ttelmah]

If you have access to a Spice modelling package, you can use the figures given for the equivalent internal capacitance, internal resistance in the multiplexer and the feeds, source impedance, and the required accuracy, to give a very accurate estimate of the charging time needed. Generally the results agree quite closely to those from the simple capacitor calculation used in the data sheet. The current flow, varies massively, with the rate of change of the input signal.
The point about 'sleep', is noise. The best way of reading the ADC, is always to stop as many noise sources in the chip as possible. With the internal RC, because the sample timing becomes asynchronous to the main clock, this becomes even more important.
Provided your signal source meets the suggestions on impedance, a delay of between 10, and 20uSec is adequate. this varies with chip versions (which have different capacitances and internal reistances, and impedance recommendations), with the 16F877, needing about 20uSec, using a 10K source impedance, and the 18F452, needing 10uSec, using a 2.5K source impedance.

Best Wishes