GCBASIC / Discussion / Programmers and Chip Files Problems: Low voltage detection

Jose Houle - 2026-02-08

hello guys,
I need your help :)
I can't find a way to measure Vdd and compare it to a voltage reference generated by the PIC.

The main objective is to set off every output if a short circuit happen in a any output circuit.
So I need to detect when Vdd is dropping to low..

im using PIC18F24Q10

please help me
I'm frustrated on chatgpt :)
José

Last edit: Jose Houle 2026-02-08

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Hello,

I used internal voltage reference to read the battery voltage of my circuit (Short Circuit Probe).
If you read the Vdd voltage by using 2 resistors as voltage dividor, would that be work?
But maybe I miss something from your question.

Here the function I created (Sorry Anobium, I was using my own ADC routine and not the build in from GCBasic):

// Procedure to read AN3 analog values - 10 bits resolution for Battery voltage
// This Function is not portable like that to other microcontroller because of non using GCBASIC internal ADC commands

Function myBatteryReadAD10() As Word                                     // Port RA4 - AN3 is hardcoded inside the procedure, this code is not portable per default   
    FVRCON = 0b10000010                                                  // FVREN is activated and set to 2.048V (page 142)
    ADCON1 = 0b10010011                                                  // ADFM right justified, ADC conversion clock @Fosc/8, Vref- at Gnd, Vref+ at FVR 2.048V (page 151), voltage divider 0,32
    Set ANSELA.4 On                                                      // Define Port RA4 as Analog input (page 123)
    ADCON0 = 0b00001101                                                  // AN3 (RA4) actif, ADC is enabled (page 150)
    Wait 25 us                                                           // Wait 25µs to let charge capacitor (time could be increased!) - (see more page 154)
    // But FVRRDY is always ON on PIC16F1825 according note 1 page 142, anyway keep the instruction in the code for education and experience !
    Wait While FVRCON.FVRRDY OFF                                         // Wait until internal voltage reference is ready (previously cleared in the procedure by set FVRCON), 
    SET GO_NOT_DONE ON                                                   // Start the Analog Digital conversion
    Wait While GO_NOT_DONE ON                                            // We are waiting up conversion is completed, this is a blocking routine waiting on the flag 
    SET ADCON0.ADON OFF                                                  // Deactivate the ADC 
    myBatteryReadAD10 = ADRESL                                           // Copy LSB into low part variable to send back from function ADC value is 10 bits length (page 149)
    myBatteryReadAD10_H = ADRESH                                         // Copy MSB into high part variable to send back from function ADC value is 10 bits length (page 149)
    FVRCON = 0b00000010                                                  // FVREN is deactivated (page 142)   
End Function

Anobium - 2026-02-08

:-)

😄
1

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Fabrice Engel - 2026-02-08

btw, probably you noticed, the PIC used in my circuit is a PIC16F1825

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Jose Houle - 2026-02-09

Hello Fabrice
Thank you so much for your reply !!

I can't wait to try it later today !!

Thanks !!!

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Fabrice Engel - 2026-02-10

Hi Jose,

I cross my fingers that you will find the way to test Vdd. If you are looking for more content round the ADC management and how I used it, you can have a look in the full source code of my project.
The source code is documented line by line, so I guess you will be able to better understand how I used it myself and how I was able shutoff the circuitry ordered from the microcontroller.

The full project page is on Elektor here:
https://www.elektormagazine.com/labs/short-circuit-probe

Or on PCBWay here:
https://www.pcbway.com/project/shareproject/Short_Circuit_Probe_f5e16641.html

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Roger Jönsson - 2026-02-18

(I guess):
You can set the PIC18F24Q10 ADC to use VDD as reference, then use the ADC to measure one of the internal voltage reference sources?

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Play with this.

#chip 18f24q10
#option Explicit


    DIM ADCValue AS WORD
    DIM ReadVDDmV AS WORD

    FVRInitialize ( FVR_off )
    ' --------------------------------------------------------
    ' Enable FVR at 1.024V
    ' FVRCON:
    ' bit7 FVREN = 1 (enable)
    ' bit6 FVRRDY = read-only (wait until 1)
    ' bit1-0 ADFVR = 01 (1.024V)
    ' --------------------------------------------------------
    FVRInitialize ( FVR_1x )

    ' Wait for FVR to stabilise
    wait while FVRIsOutputReady = false

    ' Read ADC
    ADCValue = READAD10( AN0 )

    ' --------------------------------------------------------
    ' Compute VDD in millivolts:
    ' VDDmV = (1023 * 1024) / ADCValue
    ' 1023 * 1024 = 1,047,552 fits in LONG
    ' --------------------------------------------------------

    ReadVDDmV = ( 1023 * 1024 ) / ADCValue

Low voltage detection

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Low voltage detection