Pic Micro Pascal compiler - Tips - How to measure Vdd without any pin!

Welcome!

Welcome PIC fans!
(to Pascal language fans too!)

Documents

Unfold Documentation

Unfold How to support PMP

Unfold Manuals

Unfold Today's favourites

Technical

Unfold Tips

Downloads

All the categories

The last 5 downloads

PICmicro® Mid-Range MCU Family Reference Manual

PMP version 1.4.9

PICmicro® 18C MCU Family Reference Manual

MPASM / MPLINK / MPLIB user manual

MPLAB version 8.50

The last 5 most downloaded

PMP version 1.4.8

PMP V1.4 User Manual

MPLAB version 8.36

PicKit 2 Windows software - version 2.61.00

MPLAB version 8.30

Google

Tips - How to measure Vdd without any pin!

If we want to know what is the Vdd voltage to say, elaborating a "low battery" condition, the normal way would be to measure it with a resistive divider on an analog input pin. This uses two resistors and a pin.

Fortunately, on most devices, there is the possibility to measure the internal VRef, and if we use Vdd as the analog range, the result is reverse proportional to Vdd:

function Low_Bat : boolean; // Check battery

    { Note: Battery sense uses VP6 reference so we always measure 0.6 V,
            so a/d value is depending to VDD with the law: n = 0.6 * Max / Vdd,
            so the a/d value is reverse proportional to Vdd;
            so we can compute Vdd = 0.6 * Max / n;

            Power is from a 3.6 V lithium battery.
            We fix a low bat value to 3.1 V.
    }

const
    Max_Ana = 1023;                                        // 10 Bits analog max value
    VP6_Ref = 600;                                         // 0.6 V = 600 mV
    VP6_Ref_Error = - 25;                                  // -25 mV error on vref (manual adjustement for fine tuning)
    Low_Bat_Voltage = 3100;                                // 3.1 V = 3100 mV

    // Low battery trigger value in analog raw binary value
    Low_Bat_Trigger = (VP6_Ref + VP6_Ref_Error) * Max_Ana div Low_Bat_Voltage;

var
    V : word;
begin
    { A/D init: may be done once at program start-up }
    ANSEL := 0;                   { Set selected channels mask }
    ANSELH := 0;                  { Set selected channels mask }
    ADCON1.ADCS := 5;             { Set clock selection to FOsc/16 }
    ADCON0 := [ADFM, ADON]; { Configure for 10 bits right justified, turn on the A2D module }
    VRCON.VP6EN := true;          { Activate A/D VP6 reference for Vdd measurement }

    // Normally we have to wait a while here for settle time...

    ADCON0.CHS := 13;             { Select VP6 channel }
    delay(5); { Wait 5 µS for A2D amp to settle and capacitor to charge }

    ADCON0.GO := true;            { Start conversion }
    while ADCON0.GO_DONE do;

    V := (word(ADRESH) shl 8) or ADRESL;
    ADCON0.ADON := false; { Turn off AD to save power }

    Low_Bat := V >= Low_Bat_Trigger;
end;

Creation date : 2009.06.12 0:07 PM
Last update : 2009.06.12 0:10 PM
Category : Tips
Page read 1951 times

Print the article