[Resolved] BUG 16F1824 External Vref+ not working.

[medelec35]

Attached is a very basic flowchart to test external Vref+
If using EB006 then connect:
POT (AN3) to A6
Vref (A1)Voltage to B6
LCD Port C (C0 to C5)
All working as expected with FC5 and below
Not working with FC6 and FC7

Martin

[medelec35]

Looks like bits 0 & 1 of

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ADCON1: A/D CONTROL REGISTER 1 

not being assigned correctly.

I can get it working by modifying PIC_CAL_ADC.c
to:

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if (Vref = 1){											//assign VREF functionality
			ADCON1 = ADCON1 |  0x02;
    }	

But that does not sort out FVR
I'm guessing Vref = 1 is the value of selection?
So if Vref = 1 then external Vref is selected.
Vref = 0 then internal Vref is selected.
Vref = 2 then FVR is selected?
I could be totally wrong?

So this will probably not work:

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if (Vref = 1){											//assign VREF functionality
			ADCON1 = ADCON1 |  0x02;					          //Select Vref+
		}
		else
		if (Vref = 2){											//assign VREF functionality
			ADCON1 = ADCON1 | (Vref & 0x03);			//Mod to allow FVR as VREF
			FVRCON=FVRCON | 0xC1; }

Out of curiosity, how can I modify what code does depending on selection?
For example if VDD do a, or Vref+ do b or FVR is selected do c etc.
As it does nor appear to be Vref = 0, Vref = 1 or Vref = 2.

Martin

Edit: Going down the wrong lines since if use

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if (Vref = 1)

Then external Vref is used on all ADC components even if VDD is selected.
So had to use if

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 (Vref != 0)

so only ADC component that has Vref+ will use external Vref.

[LeighM]

Hi Martin,
Thanks for your work on this, please try the attached.
Thanks,
Leigh

ps. btw, some of your problems are down to that "easy to do in C" typo

if (Vref = 1)

which will set Vref to 1 and always be true, should be (but I'm guessing you know already!)...

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if (Vref == 1)

[medelec35]

Hi Leigh,
Thanks for the update.
All three selections are working like a charm.

ps. btw, some of your problems are down to that "easy to do in C" typo

Ah yes I was assigning rather than comparing.

Thank you.

Martin

[jgu1]

Hi Both!

This new cal file, is this for FC 6 and FC7 Please

Br

Jorgen

[medelec35]

Hi Jorgen,

This new cal file, is this for FC 6 and FC7 Please

No it can't be for both V6 and V7 since V6 uses Boost C compiler (uses lower case register names) and V7 uses XC8 compiler (uses upper case register names).
So I have attached an unofficial V6 version that's copied from LeighM Fix.

Martin

[jgu1]

Thank you Martin

Jorgen

[medelec35]

Your welcome Jorgen.
I did say based but have changed it to copied as it was just a direct copy of Leighs code.
So Leigh should have all the credit for that one.

Martin

[jgu1]

Hi Martin!

I have played a little with your example with Vref. In test (simulation) before and after update. I see the same result before and after update. Maybe it only work in real world.

I see when I turn the pot from 0 to max. in display 0 - 1023 ok Int. But as I understand by using the Vref. in your example "Vref 250 Then I should see in the display 1023 when the pot is half "speed" (ajd. to 2.5V) but it is still 1023 by max after. Hope you understand my question. Or do I misunderstand concerning the use of Vref.

Br

Jorgen

[medelec35]

Hi Jorgen,
You will only see a difference with hardware, but you are getting the concept correct.
If you look at the datasheeet for 16F1825 you will see that ADCON1 register controls ADPREF (Analogue to Digital Positive REFerence voltage) bit.
On page 149 shows a switch controlled by ADPREF.
As its internal to 16F1825 the simulator won't show the difference between VDD and +REF unless you are using GetVoltage or GetString.
In those cases the VREF+ within properties will take effect.
If using VREF+ then you need to set a fixed voltage at the VREF+ pin by using two resistors as and example.
So VREF+ may not be +2.5V if two resistors are not identical values.

Martin

[jgu1]

Hi Martin!

Thank you for your explanation, Then I understand it correct. I Will test it in Real " world " this weekend. I have never used Vref before.

Thank's

Br Jorgen

[medelec35]

Hi Jorgen,
At the time of writing, the following only applies to:

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12F1822, 16F1823, 16F1824, 16F1825, 16F1828, 16F1829

You have three Vref options to choose from:
VDD - This uses the microcontroller supply voltage as a reference.
The resolution for Byte (8 bit) = 5/256 = 19.53mV (so for every increase in voltage by 19.53mV, ADC value increases by 1)
The resolution for Int (10 bit) = 5/1024 = 4.88mV

VRef+ - This uses the microcontroller external pin for the voltage reference.
So if two resistors connected set the voltage on external pin to 2.5V.
The resolution for Byte (8 bit) = 2.5/256 = 9.77mV
The resolution for Int (10 bit) = 2.5/1024 = 2.44mV

FVR (Fixed Reference Voltage).
This uses an accurate internal voltage.
The default for Flowcode has set this voltage to 1.024V
The resolution for Byte (8 bit) = 1.024/256 = 4mV
The resolution for Int (10 bit) = 1.024/1024 = 1mV
It's the 1mV resolution which makes this setting very useful indeed!
For Example see this post.
Note: By changing values of

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FVRCON

within

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PIC_CAL_ADC.c

FVR can be

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1.024V, 2.048V & 4.096V

Martin

[jgu1]

Hi again Martin!

Again thank´s for your great and big explanation. I will order a 16F18xx (have no in my stock) and make a test.

Best regard Jorgen

[medelec35]

No Problem.
18f1825 for example are cheap chips with loads of rom and functions.

By the way a small typo.
i posted

The resolution for Byte (10 bit)

When it should of stated
The resolution for Int.
Now corrected text.