File content as of revision 4:853634e0d433:

/*
** mBuino_PowerMeter
** 
** This program shows how to read the current voltage on the 3V3 (VCC) pin of mBuino using the blink of a LED.
**
** Since mBuino uses the VCC voltage as reference for analog reads, all readings are relative.
** To get an absolute reading an external reference voltage is needed.
** An easy way to get an external reference is by using a zener-diode with proper characteristics.
** Not having such a zener around made me look for alternatives. I found some on this page:
**   http://www.talkingelectronics.com/projects/200TrCcts/200TrCcts.html#52
** The most interesting method I found was to use the LEDs on mBuino
** See the code comments and this programs homepage for more information.
**   https://developer.mbed.org/users/maxint/code/mBuino_PowerMeter/ 
**
** 0.1.150801 First version made for mBuino on mbed.org by maxint on 1-aug-2015
**
** Feel free to use this code anyway you want, but please acknowledge my work by mentioning maxint as creator.
**
*/


#include "mbed.h"
#include "USBSerial.h"

// The USB virtual serial port is used for debugging. Note: driver required, see http://developer.mbed.org/handbook/USBSerial
USBSerial serialUSB(0x1f00, 0x2012, 0x0001, false); // Virtual serial port over USB, set connect_blocking to false to allow starting without PC
#define USBSERIAL_DEBUG 1

DigitalOut LED[] = {(LED1), (LED2), (LED3), (LED4), (LED5), (LED6), (LED7)};// declare 7 LEDs

float readVCC(PinName nAnalogIn=P0_14, PinName nLed=LED6, int nBlinkDelayMsec=0, float ftVrefInit=1.91)
{   // Unlike Arduino, mBuino v1.5 has no internal reference voltage for analog reads.
    // As all analog reads are relative to VCC, it would be nice to be able to tell the VCC
    // Additionally knowing the current VCC is critical for battery operated applications.
    
    // Please note that the voltage on the 3v3 pin is always lower than 3.3V as diodes D10 and D11
    // are between the actual VCC of the 3.3V regulator and that of the CR2032 battery.
    // Measured voltages are 2.85V when powered by USB or lower when powered by battery.
    // When mBuino has its LEDs on, or when an external device is using VCC, the voltage will be lower too.

    // The current VCC voltage can be determined by using analogIn to read a known reference voltage.
    // One way to get a reference voltage is by connecting a led via a resistor between VCC and GND.
    // The voltage over the led is (more or less) constant (about 1.7V, depending on the kind of LED).
    // This constant voltage can be measured by using a multimeter.
    // Measured voltages are e.g. 1,77V  for a yellow led, 1,66 for a red led and 1,73 for a green led (and 1K resistor).
    // See http://www.talkingelectronics.com/projects/200TrCcts/200TrCcts.html#52 for more methods.
    //
    // As mBuino already has LEDs we can use them as a reference by connecting P0_14 to the left leg of LED7 or LED6.
    // To avoid potential damage and to allow PWM on LED7 a 100R resistor can be used to make the connection.
    // Note: P0_14 is the tiny dot right above the 1 of the printed version number.
    // See https://developer.mbed.org/platforms/Outrageous-Circuits-mBuino/ for pin-outs and schematics
    //
    AnalogIn ana(nAnalogIn);      // TODO: can be made static for better speed?
    DigitalOut ledBlink(nLed);      // TODO: can be made static for better speed?
    float ftVoltage, ftVref;
    bool fLedStatus;
    
    fLedStatus=ledBlink;   // remember status of the led so we can set it back
    ledBlink=1;   // put a high voltage on LED7 so we can measure it as a reference voltage and use that to calculate the VCC
    if(nBlinkDelayMsec!=0)
        wait_ms(nBlinkDelayMsec);     // allow some time to reach full voltage level (when using no delay the reading can be a bit lower)
    ftVoltage=ana.read();
    ftVref=ftVrefInit;                       // assume reference voltage on ana14 to be 1.91V, e.g. by connecting is via a 100R resistor to the left leg of LED7
#ifdef USBSERIAL_DEBUG
    serialUSB.printf("Vref %1.2f / Vread %1.2f%% ~~> ", ftVref, ftVoltage*100);
#endif

    // Some silly trial and error compensations. Note: this compensation is not very precise!
    // TODO: improve the compensation to get more accurate readings. Now final readings may differ up to 0.10V from 
    // actual values, depending on the reference voltage, length of the blink, resistor and LED used, VCC-level and probably temperature.
    if(!fLedStatus && nBlinkDelayMsec<=10)
        ftVref+=(0.05-nBlinkDelayMsec/200.0); //compensate for lower voltage due to short delays.
    ftVref-=(0.04+(ftVoltage-0.66)/4);   // compensation: Vref is slightly lower on low voltages.

    // Calculate the VCC by dividing the Vref by the analog reading
    ftVoltage=ftVref/ftVoltage;

    ledBlink=fLedStatus;             // restore led7 to its original status
    return(ftVoltage);
}


void ledFlash(uint8_t nLed, float ftDelay=0.005)
{
    if(nLed>6) nLed=6;
    LED[nLed]=!LED[nLed];
    wait(ftDelay);
    LED[nLed]=!LED[nLed];
}

void SweepAllLeds(uint8_t nMaxLeds=7, bool fLeftToRight=true, float flDelay=0.1)
{   // light all leds up from left to rigth or vise-versa and then switch them off from reverse direction
    // leds on, left to right
    if(nMaxLeds>7) nMaxLeds=7;
    for(int n=0; n<nMaxLeds; n++)
    {
        LED[fLeftToRight?n:nMaxLeds-1-n] = 1; // turn on
        wait(flDelay); // delay
    }
    // leds off, right to left
    for(int n=0; n<7; n++)
    {
        LED[!fLeftToRight?n:nMaxLeds-1-n] = 0; // turn off
        wait(flDelay); // delay
    }
}


main()
{
    // Minimal voltage on 3v3 pin is about 1.8V. Below this mBuino may react weird or stop working.
    // Default reference for voltage on mBuino's LEDs is 1.91v, measurement pin is P0_14.
    // Press any key on USB-serial to switch to 1.85v and pin P0_15 for external (green) LED.
    float ftVmin=1.80, ftVled=1.91, ftVcc;
    PinName pinAnalog=P0_14, pinLed=LED6;
    Timer t;

    // small initial delay to show all leds are working and to allow USB connection
    SweepAllLeds();
    SweepAllLeds(7, false,0.02);
    SweepAllLeds();

    // flash a led while waiting for console response as we 
    t.start();
    while(!serialUSB.readable() && (t.read_ms() <= 3000))
    {
        serialUSB.printf("."); 
        ledFlash(0);
        wait(0.1);
    }

    // show status on console and give instructions
    serialUSB.printf("\r\n=== mBuino PowerMeter ====  \r\n");
    serialUSB.printf("Measuring LED pin %d [%1.2fV] using pin %d\r\n", pinLed, ftVled, pinAnalog);
    serialUSB.printf("Hit spacebar to change to LED pin 4 [1.85V] using pin 15.\r\n");

    // perform voltage readings, indicate voltage level and process serial console input
    while(true)
    {
        //ftVCC=readVCC();  // float readVCC(PinName nAnalogIn=P0_14, PinName nLed=LED6, int nBlinkDelayMsec=0, float ftVrefInit=1.91)
        ftVcc=readVCC(pinAnalog, pinLed, 0, ftVled);
        serialUSB.printf("VCC: %1.2fV\r\n", ftVcc);

        // indicate voltage level using all 7 LEDs on mBuino
        if(ftVcc>ftVmin)
        {
            uint8_t uLed=0;
            if(ftVcc>=1.90) uLed=1;
            if(ftVcc>=2.00) uLed=2;
            if(ftVcc>=2.20) uLed=3;
            if(ftVcc>=2.40) uLed=4;
            if(ftVcc>=2.60) uLed=5;
            if(ftVcc>=2.75) uLed=6;
            SweepAllLeds(uLed+1);
            ledFlash(uLed);
            wait(1);
        }
        else
        {   // battery is very low: show rapid flashing
            ledFlash(0);
            wait(0.25);
        }
        
        // process serial console input
        if(serialUSB.readable())
        {   // switch pins when is spacebar is hit
            if(serialUSB.getc()==' ')
            {
               pinLed=(pinLed==P0_4?LED6:P0_4);
               //ftVled=(ftVled!=1.85?1.85f:1.91f);   // hmm, weird bug; no conditional assignment on floats?
               if(ftVled!=1.91f) ftVled=1.91f; else ftVled=1.85f;
               pinAnalog=(pinAnalog==P0_15?P0_14:P0_15);
               serialUSB.printf("Switched to measuring LED pin %d [%1.2fV] using pin %d\r\n", pinLed, ftVled, pinAnalog);
            }
            else
            {
               serialUSB.printf("Hit spacebar to switch between pins (4/15 or LED6/14).\r\n");
            }
        }
    }    
}