pic16f877a ADC & External sensors

kel · Joined: 17 Oct 2005 Posts: 68 Location: Brisbane

Guys what is wrong with following code?
I've a temperature sensor connected to PortA A0 and a potentiometer connected to A1.first I read the A0 AdC and delay for 20us and read ADC A1.
The problem is the value reading from the sensor is the same as the value reading from the potentiometer.If i avry the potentiometer, the values in both readin g vary.This is not suppose to be true as the two readings are different.
What is the problem?

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

The reason no one has replied is because, in part, your code doesn't
even match your description.

Here you say:

kel · Joined: 17 Oct 2005 Posts: 68 Location: Brisbane

Thanks you thank you...I deleted the right interrupt subroutine and place the code wrongly on the uart interrupt subroutine!! I will be careful next time..
I have added the suggested correct steps and will test it shorlty and i will also let you of the progress...
This is what i had originally..

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

My advise for you is to do things in the most simple way possible.
Don't use the A/D isr. Don't use ADC_READ_ONLY, etc.
All those things are for intermediate-to-advanced CCS programmers.
Just use plain old read_adc().

Do all your A/D reading this way:

kel · Joined: 17 Oct 2005 Posts: 68 Location: Brisbane

Thank you programmer the code work now when i make the following changes:
void read_adc_vals(void){
static int32 avgvalz;
static int16 adcvalz;
static int16 adcarr[10];
signed int8 cnt;
static float angul_deg;

set_adc_channel(0); //set channel to channel0
delay_us(20);
/*this loops are used to give smoother temperature*/
for(cnt=0; cnt<10; cnt++)
{

kel · Joined: 17 Oct 2005 Posts: 68 Location: Brisbane

I used the functions read_adc(ADC_START_ONLY) & read_adc(ADC_READ_ONLY) IN THE INTERRUPTS coz i was advised to do so by one of the forum users!! Razz

http://www.ccsinfo.com/forum/viewtopic.php?t=25834

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

They didn't realize what they were doing.

Ttelmah · Guest

In fact if you read the thread, it was suggested that if all he wanted to do, was read the ADC, then it was much better to switch to just using the default behaviour. The problem was that he arrived with a set of code, doing things in about as complex way as possible (using ADC interrupts), which seemed to suggest that he wanted to read the ADC, very rapidly, and synchronously to a clock event. Hence a whole load of code versions were suggested doing this. Then gradually it became more and more clear that this was not the target.
This is now the fourth thread, he has run for this code, and in each, he has got other people to do a little more, but does not actually seem to solve things much himself...

Best Wishes

kel · Joined: 17 Oct 2005 Posts: 68 Location: Brisbane

You are right,i've not tried long enough to seek for help.I'm a beginner and will try next time hard enough and will seek for help if i fail completely.This way, i will be able to learn understand and help others too.
My previous suggest that i should have someone actually posting the actual codes for read_adc(),set_up_adc(),setup_adc_channel(),adc_interrupt etc for me to understand them on how they are setup...
thanks again,you have always help me with my codes.
cheers mate.
Rolling Eyes

Ttelmah · Guest

There is a problem here. CCS, try in many ways to 'remove' you from the hardware. So you can (for example), just make the read_adc call, and have no idea about what is really going on below the skin. Realistically the best place for this data, is the chip's data sheet, and the application notes from MicroChip. The data sheet, really is essential reading. If you look at a daa sheet, and select the section on the ADC (I have just opened the one for the 18Fxx2 for example), you find a section giving the control 'flow' needed to handle the ADC.
Now if you look at this, you can then get an idea of what the various C instructions actually 'do'.
It starts by saying:

'configure analog pins, voltage reference, and digital I/O'. This is the 'SETUP_ADC_PORTS' instruction.
Then it goes on to say 'select ADC channel' - the 'set_ADC_CHANNEL' instruction.
Then it says 'select ADC conversion clock' - 'setup_adc'
Then it says 'turn on the ADC module'. This is automatically done for you by the compiler as part of the setup_adc instruction.

Now it then moves on to 'waiting for the acquisition time'(this is explained slightly latter in the section, including details of how to calculate it for a given hardware - this is the stage, most commonly forgotten!.

Then it says 'start the conversion'.

Next section, then says 'wait for the conversion to complete'

Then 'read the result'.

Now the read_adc function, automatically performs all three of these sections for you, _unless you specifically 'ask' it to do them seperately_. As for why you'd want to do them seperately?. This avoids having to sit waiting for the conversion to complete. So you can trigger a start, and then go away, and do something else, then read the value, without any delays.
Now the 'logic' of the function in this regard, only really becomes obvious when you read the data sheet.
However the simple use of the adc, without this complexity, is best seen by looking in the examples. EX_ADMM10.C, shows a simple example of reading an ADC, letting CCS do all the work, and averaging the results.

Best Wishes