Estimating VDD using FVR by routing it to the ADC with no external circuitry

[trastikata]

Here's an example of measuring the VDD level internally on devices with FVR (Fixed Voltage Reference) without using any of the PIC pins or external circuitry. Here's how it works:

- Tie internally VREF to VDD
- Enable FVR at level, lower than VDD i.e. 1.024v, 2.048v ...
- Measure the FVR as referenced to VDD by selecting CHS<1111>
- Inversely calculate VDD as function of measured FVR, knowing that FVR is fixed to 1.024, 2.048v ...



And some code example:

Code Select Expand

Device = 18F14K50
Declare Xtal = 12

Declare Float_Display_Type = Large

Dim wADC As Word
Dim BatteryVoltage As Float
       
Main:
    ANSEL = 0 : ANSELH = 0  'All pins as digital
    ADCON1 = 0              'VREF tied to VDD and VSS
    ADCON2.7 = 1            'ADC data is right justified
    REFCON0 = %10100000     'FVR enabled at 2.048v, routed to CHS<1111>
   
    While 1 = 1
        Print At 1, 1, Dec VDD()
        DelayMS 500
    Wend
   
Proc VDD(), Float
    ADCON0.0 = 1            'ADC on
        wADC = ADIn 15      'Measure FVR channel as referenced to VDD
    ADCON0.0 = 0            'ADC off
   
    Result = 2097.152 / wADC    'Calculate VDD in function of FVR
EndProc


[Giuseppe]

This results in a stable reference voltage for the ADC

[charliecoutas]

Neat!

[Yves]

Hello Trasticata,

This would be a way of checking the output of the voltage regulator going below its optimum voltage when the battery powered instrument is used.

Regards,

Yves

[towlerg]

Although in the past the FVR has been describes as "shite", I found it perfectly adequate to measure battery voltage in 0.1 steps.

[towlerg]

Hey I just found a cool thing (well actually another cool thing) about this board. It warns you if somebody has posted whilst you were carefully crafting your missive.

[tumbleweed]

Although in the past the FVR has been describes as "shite", I found it perfectly adequate to measure battery voltage in 0.1 steps.

In some of the older chips the FVR accuracy is pretty poor... in the 10% range.
In most newer chips it's a bit better than that... 4-6%

Those specs are across the full temp range (ie -40 to +125), which is where most of the change in output occurs.
If you're working around room temp they're pretty good.