A/D conversion problem with 18F2455

[abdy37]

Hi everybody. I am now working on 18F2455 because of its USB feature. But I got some troubles with converting.I use 4MHz crystal/resonator. I apply a sinusoidal signal with the amplitude 2,5 or 5 volts and a frequency 2kHz from function generator to PORTA.0. And want an output from PORTA.1. But it behaves strange. The negative parts of this sinusoidal is cut off. It only receives the positive parts. I monitored this event on the scobe. And guess what I got as an output. It is 5V DC. It doesn't change. Today I will try it one more time. Does anyone have any suggestions?
What am I doing wrong?

[T.Jackson]

The negative parts of this sinusoidal is cut off. It only receives the positive parts.

You need to offset it. For example, if peak to peak is 5V then you need to reference the center to 2.5V. This is effectively your 0V rail when compared to actual AC. AC isn't AC unless it swings below the 0V rail.

[abdy37]

You need to offset it. For example, if peak to peak is 5V then you need to reference the center to 2.5V. This is effectively your 0V rail when compared to actual AC. AC isn't AC unless it swings below the 0V rail.

Hi T.Jackson. I will try it out tomorrow. I think it is caused by configuring the VREF+ and VREF-. I configured them as VREF+=Vdd and VREF-=Vss. So I think this why the negative sinusoidals are truncated. But in spite of this I think the A/D module must work. I don't have a chance to change the offset voltage to 2,5 volts as you offered. But I think can figure this out with VREF+=AN2 and VREF-=AN3 by pulling AN2 to +2,5 volt and AN3 to -2,5 volt.
Here is the code I used:

W0 VAR WORD

TRISA.0 = 1 ' Set pin 0 of PORTA an input

TRISA.1 = 0 ' Set pin 1 of PORTA an output

PORTA.1 = 0 ' Clear pin 1 of PORTA

ADCON0 = %00000001 ' Configuring Analog Channel Select bits, A/D

' Conversion Status bit, A/D On bit

ADCON1 = %00001110 ' Configuring Voltage Reference Configuration bits,

' A/D Port Configuration Control bits

ADCON2 = %10010000 ' Configuring A/D Result Format Select bit (Right justified ),

' A/D Acquisition Time Select bits (4 TAD), A/D Conversion

DEFINE OSC 4

DEFINE ADC_BITS 10

DEFINE ADC_CLOCK 1

DEFINE ADC_SAMPLEUS 11

Conversion:

ADCON0.1 = 1 ' Set GO/(DONE)' bit. Wait the Automatic Acquisition Time and Start Conversion

PAUSEUS 50

IF ADCON0.1 = 0 THEN ADC_BIT_RECEIVING

IF ADCON0.1 = 1 THEN Conversion

ADC_BIT_RECEIVING:

ADCIN 0,W0

W0.0 = PORTA.1

PAUSEUS 10

W0.1 = PORTA.1

PAUSEUS 10

W0.2 = PORTA.1

PAUSEUS 10

W0.3 = PORTA.1

PAUSEUS 10

W0.4 = PORTA.1

PAUSEUS 10

W0.5 = PORTA.1

PAUSEUS 10

W0.6 = PORTA.1

PAUSEUS 10

W0.7 = PORTA.1

PAUSEUS 10

W0.8 = PORTA.1

PAUSEUS 10

W0.9 = PORTA.1

END

[skimask]

But I think can figure this out with VREF+=AN2 and VREF-=AN3 by pulling AN2 to +2,5 volt and AN3 to -2,5 volt.

Good luck with that AN3 theory and hopefully you've got a couple of extra 18F2455's laying around.
Reference the 18F2455 datasheet, Section 28.0 and Table 28-28, Parameter A22.

[T.Jackson]

Suggest some good reading material on OPAMPS - Operational Amplifiers. You need to vertically shift up that sine wave so that 0v is actually 2.5v

[skimask]

w0 var word:trisa.0=1:trisa.1=0:porta.1=0:adcon0=1:adcon1=14:adcon2=$90
DEFINE OSC 4
DEFINE ADC_BITS 10
DEFINE ADC_CLOCK 1
DEFINE ADC_SAMPLEUS 11
Conversion:
ADCON0.1 = 1:PAUSEUS 50 :IF ADCON0.1 = 0 THEN ADC_BIT_RECEIVING
IF ADCON0.1 = 1 THEN Conversion                        
ADC_BIT_RECEIVING:
adcin 0,w0:w0.0=porta.1:pausus 10:w0.1=porta.1:pausus 10:w0.2=porta.1:pausus 10
w0.3=porta.1:pausus 10:w0.4=porta.1:pausus 10:wo.5=porta.1:pausus 10
w0.6=porta.1:pausus 10:w0.7=porta.1:pausus 10:w0.8=porta.1:pauseus 10
w0.9=porta.1
END

What is this program supposed to do anyways?
Might help if we saw the whole thing...

[abdy37]

Hi Skimask. Thank you for concerning with my problem. Now I am working on a little project or the beginning of a big project. I will explain it shortly.
I am applying a sinusoidal signal with the offset voltage of 2,5 volts and an amplitude of 5 volts to PORTA.0 . Its frequency is 1khz. So, it varies between (0~5) volts. I want a digital output coming from PORTA.1 . But however, so far
I get 5V(DC) from PORTA.1 . By the way, I use 10 bit ADC and the converted
digital values I want to get serially every time when the conversion is over.
The detail is as follows: When the GO/DONE bit is set, the ADC starts. It gathers
the 10 bit sampled value and writes it into ADRESH(highest 2 bits) and ADRESL(least 8 bits). I did not use it. I used ADCIN 0,W0 which reads the values
from PORTA.0 and writes to W0. After that, if you look at the code you can see that I am trying to get those values with delay(PAUSEUS). I am using 18F2455
and could not find much relating A/D Conversion with it. Maybe you have some suggestions. Because you know it is nothing but the beginnig of the project
and you can not come over the problem. It makes me really crazy. I need working ADC.

[skimask]

I am applying a sinusoidal signal with the offset voltage of 2,5 volts and an amplitude of 5 volts to PORTA.0

1st problem - A/D inputs on a PIC don't work with negative voltages. See Post #4.

Its frequency is 1khz. So, it varies between (0~5) volts. I want a digital output coming from PORTA.1

Do you mean you want to output, on A.1, a serial output representation of the analog input on A.0?

But however, so far I get 5V(DC) from PORTA.1

That's good because I don't see anything in your program that says to change the output on PortA.1 other than it's initial setup.

By the way, I use 10 bit ADC and the converted digital values I want to get serially every time when the conversion is over.

And again, I don't see any serial output commands.

I used ADCIN 0,W0 which reads the values from PORTA.0 and writes to W0. After that, if you look at the code you can see that I am trying to get those values with delay(PAUSEUS).

And again, how do you plan to see this output? Serial output? LCD? There aren't any in the program.

I am using 18F2455 and could not find much relating A/D Conversion with it.

You're kidding me right? Section 21 of the 18F2455 has no information in it that you can use?

I need working ADC.

You've got a working ADC, what you need to do is to get it to work FOR you. I think what you need to do is play around with the PIC and PBP first. Get an LCD working, get a pushbutton or something working with it...then, hook up a POT across an A/D input pin, then display the value of the POT, doesn't have to be resistance, doesn't have to be voltage, doesn't have to be anything in particular, just display a number that corresponds with the position of the POT as you turn it. Then when you get that working, you can easily work on reading a sine wave.

And besides, even though the A/D on a '2455 is rated for 100ksps and the PIC can run at 48Mhz, you won't be able to get any useful DSP out it. Not enough horsepower under the hood.