Multiplication

PROMOD · Joined: 01 Feb 2018 Posts: 42

In channel 0 voltage = 5v
In channel 1 voltage = 2v

Mike Walne · Joined: 19 Feb 2004 Posts: 1785 Location: Boston Spa UK

I don't understand what you're trying to do!

To me 5V * 2V = 10V^2 (I.e 10 volt squared.)

What answers are you getting with your hardware?

Show us a short complete code which illustrates the problem.

Mike

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

Post your variable declarations for y, k, a.

newguy · Joined: 24 Jun 2004 Posts: 1900

What are "k", "a", and "y" defined to be? If they are defined as "int" and you're dealing with a 16- or 18-series PIC, then "int" = "int8" = "byte". Define your variables as being "unsigned int16" and see what happens.

temtronic · Posted: Wed Mar 28, 2018 5:11 pm

We also don't know which PIC, how the ADC is setup (Vref comes to mind as well as clock speed) and what your answer to y=k*a is, other than you say it's not 10V.
There also could be HUGE errors if you're using floating point math, or some other part of the program could be affecting the 'y' result BEFORE it is transmitted to you. Now IF the 'y' answer is going to a PC, perhaps the PC program that is receiving the data is misinterpreting the data, or the transmission between the unknown PIC and the PC is bad.
We can probably think of 100 different and each possibly correct reasons for the 'error', but we NEED to see your entire program to locate it for you.

Jay

PROMOD · Joined: 01 Feb 2018 Posts: 42

I wold like measure the instantaneous power at t1 for the above voltage and current wave. For this I write the following code.

Mike Walne · Joined: 19 Feb 2004 Posts: 1785 Location: Boston Spa UK

No, it's not OK.

First, the Pic can't handle negative inputs, so you need to offset your analogue inputs to roughly half your reference voltage.
Second, your making it hard work by using floats throughout, use integers and either use the decimal adjusted print or convert to floats at the end.
Third, I showed a solution to this problem recently on this forum. Follow this link:-
https://www.ccsinfo.com/forum/viewtopic.php?t=56115&start=0&postdays=0&postorder=asc&highlight=

Mike

temtronic · Posted: Thu Mar 29, 2018 5:09 am

In addition to Mike's comments...
this...
setup_adc(adc_clock_internal);
is WRONG

Please consult table 20-2 ,in the ADC section of the datasheet as to why it is wrong.
As a sidenote, I'd like to know HOW you decided 'internal' was a valid option, as we see it posted here a lot. If it's from some Mcrochip document, please post a link to it,as it is in error.

Also ,as a general comment, you should add comments at the end of each line to tell others what the code is doing. While some code may be obvious,most isn't , like why /204 in the ISR, why energy/50 in main?
You could have 'bad math' where /202 is correct, not /204 but we can't tell you as we don't know the reason for those numbers. While it may be obvious to you now, come Fall, you'll be wondering too !

Jay

Ttelmah · Joined: 11 Mar 2010 Posts: 19215

Unfortunately Jay, a couple of the old CCS examples use it.
These were ones just trying to show 'operation' without worrying about accuracy, and they didn't want to have to work out the correct divisor for different clock rates.

Then it gets posted here a lot.

It's also OK on some of the newest PIC's (particularly PIC24's).

So it keeps getting used. I think it's going to have to evolve into a 'sticky', but people don't seem to read these either.... Sad

PROMOD · Joined: 01 Feb 2018 Posts: 42

temtronic · Posted: Fri Mar 30, 2018 12:34 pm

Which adc clock you use, has to be determined by you, the programmer and the designer of the product. If you look at the ADC section of the datasheet, table 20-2 shows a selection of adc clock speeds. Given the 40MHz of the PIC clock, you have 2 choices. Which one YOU decide has to be based upon how YOU use the ADC and what sort of analog input you've designed. Chapter 20.3 tells about the 'math', as well there's a chart or two.
Generally speaking, it's best to 'buffer' the signal going to the PIC's ADC section. That can be as simple as a single channel 'rail to rail' input AND output opamp though I always use 4 channel opamps. Similar price and greater PCB options...like the client decides they NEED another sensor input !
Unlike digital where there are 3 states and easy to understand, proper analog design and PCB layout can take decades to master (if THAT is possible).
The good new for you is there's ONLY 2 clock choices, select one, run 10,000 samples and then run the other, run 10,000 more examples and decide which is best for you.

Jay