a/d reading jumping all over the place

[username]

I have a 16f877A set up with all analog on port A, when I read a port i get values that are jumping all over the place.

main() {
setup_adc_ports(ALL_ANALOG); setup_adc(ADC_CLOCK_DIV_32); set_adc_channel(0); delay_us(10);
rms_grid = read_adc();
delay_us(10);
set_adc_channel(1);
delay_us(10);
rms_gen = read_adc();
delay_us(10);

.......

the values seem almost random eventhough they are tied to a constant voltage for testing. i put in a loop and printf statements to monitor the values and its just all over the place

thanks.

can someone give me some idea to settle the values i am reading

[Guest]

what have you connected to the pin, is it just floating??

[username]

actually what i have it connected to is a simple voltage divider. i am wanting to monitor the level of a 12v battery, and yes they have a common ground. So since the voltage is from a battery it is a static value but the a/d is giving random values.

I will try a new pic when i get one. I have used them before and got a fairly stable reading, since i am using 'all analog' the reference should be vcc so it should give me a stable reading.........

[PCM programmer]

What resistor values are you using in your voltage divider ?
There could be a problem if they are too large.

[username]

they are fairly high, i was trying to minimize the current in the divider, i dont have the cct here and cant remember the values used off hand.

i will reduce their value and give it a try.

thanks

[Haplo]

According to the device datasheet, the maximum recommended impedance for analog sources is 10 kΩ.

[kable]

hummmm I wonder if the problem is that the voltage is being sampled from a voltage divider and its loading it down. I wonder if I should throw in a buffer. Shouldnt the a/d have a high impedance and not load the input?

ohh well

[tywais]

I just finished designing a data acquisition system using a 16F877 and had the same problem before I realized I forgot to set the ADFN bit for right justified ADCON1 = %10000000 else your bottom 6 bits are all zero and you will be getting the 4 lsb's at the top of the resulting word.