details about read_adc function

Hicham · Joined: 09 Feb 2010 Posts: 17

Hello,
I'm new to PIC-C, and I'm moving from assembly to C,
when i program AD conversion (pic 16F876) with assembly
i used to it in 2 steps
first : charging the capacitor with
bsf ADCON0,ADON
and waiting about 20us (for my settings)

second: after that I launch the conversion with
bsf ADCON0,GO
and the conversion will start
third: when the AD conversion end, I'll have an interrupt so I can read the converted result (ADRESH & ADRESL) and the ADON bit will go low

But in CCS, I can't figure out how this work in details.
Does "ADC_START_ONLY" do the same job as "bsf ADCON0,ADON" ?

I checked the help, but not much about it, so any help here ?

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

Hicham · Joined: 09 Feb 2010 Posts: 17

Thanks,
Your code is very simple, I need to do other stuff while waiting the AD conversion.

languer · Joined: 09 Jan 2004 Posts: 144 Location: USA

When I want to see what's behind the curtain I normally take a peek Smile

So in this case if you look at the LST file:

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

Here is a revised program that shows how to start and then read the adc.
The two read_adc() operations shown below that do a Start and Read Only
are the equivalent of this line:

Hicham · Joined: 09 Feb 2010 Posts: 17

# languer
thanks man,its really helpful. and i like the LST trick.i'll use it

#PCM programmer
Thanks, but you can't do anything wile waiting for the AD conversion ??
check the previews post by languer. it has lots information.

thanks all for your help.i really appreciated

Ttelmah · Guest

Some comments:
A search in the forum, will find examples posted in the past, of using the separate commands inside a timer interrupt, so that on the first 'pass', you select the channel, then on the next, trigger the conversion to start, then read the value on the next. You can either use three states like this, or immediately select the next channel after reading.
Now, the ADC interrupt, is a bit of a 'hare'. The _only_ time the ADC interrupt is worth using, is if you are triggering the ADC, using the special event trigger of the CTC. Otherwise, it takes longer to get into, and out of an interrupt, than to simply wait for the ADC to complete....
If the ADC is setup to it's optimum clock, it only takes 24uSec to complete a conversion on the older chips, and on some of the newer chips less than half this. In general you can't do a lot in this sort of time, so separating the operations, only really makes sense when using a timer to control everything, and performing the actual readings in a timer based 'thread'.

Best Wishes