PIC16F1709 and ADC VREF

[Markdem]

Hi All,

Can someone throw a quick eye over this and have a look to try to see where I am going wrong?

CCS = 5.025

[PCM programmer]

AN3 is on pin RA4 in the 16F1709. First question is, are you using pin RA4
for the input voltage ? What is the voltage on pin RA4 ?

[Markdem]

Hi PCM,

Yes, learned my lesson well last time I mixed up the port and channel names

I set the input voltage at about 2 volts for testing.

If I don't use the vref the reading is ok, so I must be using the right pin on the hardware. It has something to do with how I am setting up the vref but I have never used a internal one before.

Thanks

[Ttelmah]

Very simple little fault. What is 'Vref'?. It is not the internal fixed voltage reference. It is the Vref pin.....
To connect to the internal 'fixed voltage reference' (FVR), you need:

setup_adc_ports(sAN3 | VSS_FVR);

If you look at the diagram of the ADC in the data sheet, you will see that they show the three sources for the ADC +ve reference, as Vdd, Vref/AN1, or FVR.

As one other comment, 65472 is the maximum reading you can get from the ADC.
Remember it is a 10bit ADC. 0 to 1023. It is running in 'left shifted' mode by default, so will read in steps of 64. 0, 64.... 65408, 65472.

Normally much easier to specify #device ADC=10 so it reads as 0 to 1023.

Because the Vref pin in unconnected, the internal diodes in the ADC multiplexer, will be pulling it up to just below the voltage on your input, and you will get a maximum reading all the time....

[Markdem]

Thanks Ttelmah,

That thought also went thought my head, but the device file only has two options for setup_adc_ports;

[Markdem]

After typing all that I think I know what is going on. Funny how talking about a problem makes you work it out..

AN1 is also the ISCP CLK pin. I have my pickit connected for debugging.
I think this is working OK, but the pickit is making AN1 high.

I will check this tonight, but I think this will be it.

Now I need another way to debug :(

Thanks

[Ttelmah]

The third option to select the FVR, was obviously 'post' your compiler.
It'd probably work if you just add it. The value is:

#define VSS_FVR 0x3000000 //| Range 0-FVR

Since the value is just or'ed into the register, it will probably work.

[Markdem]

Well my idea did not work, but yours did

I now see what is going on. I was reading the datasheet the wrong way around..
For some resign I can't fathom I thought the box labelled ADC was the voltage reference source...

This is the second stupid question I have posted here in a row

Thanks again.

[Ttelmah]

It's a 'nomenclature' problem.

The chips for years had Vref inputs. Then they added an internal Vref. Now problem is what can they call it?. They call it a Vref in the section about it, but then when using it for the ADC, call it the 'fixed voltage reference' (FVR). Now it sort of makes sense, to give it a different name, but would have made more sense I think if they had just called it the Internal voltage reference. Then the ADC settings could have had IVref as an option.
In this case the problem was made more confusing by CCS having 'left it out' on the older compilers....

There is really no such thing as a 'stupid question', provided you actually follow the answers. The ones that annoy are where the posters don't actually try anything for themselves, or give us good data. In your case, you 'came back' and said 'the device file only has two options for setup_adc_ports'. A quick look at an older compiler showed that CCS had failed to include it on these. Nice and easy to solve, but 'not obvious' unless you have been playing with CCS for years (too long actually...).

Other good thing is you probably now understand these options better than most.....

[Markdem]

Indeed, my understanding has gone up.
As soon as I looked at the fact that the register should be set '11' I knew what was going on. Sometimes I just need a little push in the right direction.

Might be time for a compile upgrade too.

Thanks again.