//******************************************************************************
// F2013 periodically measures voltage produced by the LED, then drives
// the LED with a PWM signal proportional in some way to the light level
//
// MSP430F2013
// -----------------
// /|\| XIN|-
// | | |
// --|RST XOUT|-
// | |
// | P1.0|-->RESISTOR-->LED-->GROUND
//
//******************************************************************************
#include <msp430x20x3.h>

#define PIN_0 (0x01 << 0)
#define PIN_1 (0x01 << 1)
#define OUTPUT_PIN (PIN_0)
#define A0_PIN PIN_0

// Turn reference on; note that this takes substantial power
void SD16ReferenceOn ()
{
  int i;
  SD16CTL = SD16REFON+SD16SSEL0; // 1.2V ref, SMCLK,(don't)enable output of reference
  for (i = 0; i < 0x3600; i++); // Delay for 1.2V ref startup
}

void SD16ReferenceOff()
{
  SD16CTL = 0; // Reference off
}

//
// Measure the LED voltage; this is proportionate to the ambient
// light level. Units are arbitrary.
//
int doSD16Conversion (int nAvgCycles, int refAlwaysOn)
{
  int i;
  unsigned long results = 0;
  // Make the LED pin an input, connect to ADC
  P1DIR &= ~(A0_PIN);
  P1SEL |= (A0_PIN);
  P1REN &= ~(A0_PIN);
  // Configure the ADC
  SD16CTL = SD16REFON+SD16SSEL0; // 1.2V ref, SMCLK,(don't)enable output of reference
  SD16CCTL0 |= SD16SNGL+SD16UNI; // Single conv, Unipolar
  SD16INCTL0 = SD16INCH_0 + SD16GAIN_1; // Change gain here if desired
  if (refAlwaysOn == 0)
  {
    for (i = 0; i < 0x3600; i++); // Delay for 1.2V ref startup
  }
  
  for (i = 0; i < nAvgCycles; i++)
  {
    SD16CCTL0 |= SD16SC; // Set bit to start conversion
    while ((SD16CCTL0 & SD16IFG)==0); // Poll interrupt flag for CH2
    results += SD16MEM0; // Save CH2 results (clears IFG)
  }
  results /= nAvgCycles;
  if (refAlwaysOn == 0)
  {
    SD16CTL = 0; // Reference off
  }
  SD16AE &= ~0x01; // disable input A0. IMPORTANT! if not cleared, can't enable output on this pin.
  return (unsigned int)results;
}
  
//
// Set hardware to so that the LED can be driven high
// by changing only the port pin direction; this
// prepares the hardware for the software-generated PWM
//
void ledInit ()
{
  P1DIR &= ~(PIN_0);
  P1SEL &= ~(PIN_0);
  P1OUT |= (PIN_0);
  P1REN = 0;
}

// If ON_WHEN_DARK is defined, the LED will get brighter as
// the light level decreases. Comment the definition out
// for the opposite behavior. This function returns 0 if
// the LED is off in order to enable power-saving in the
// main loop.

#define ON_WHEN_DARK
int doPWM (unsigned int nLightLevel)
{
  #define PWM_LEVELS 64
  #define DARK_OFFSET 2700 // this may need adjustment for different systems
  unsigned int i, j;
  unsigned int level;
  static unsigned int nMinLevel = DARK_OFFSET;
  static unsigned int nMaxLevel = 0;
  // Keep track of the minimum light level
  if (nLightLevel < nMinLevel)
  {
    nMinLevel = nLightLevel;
  }
  // Also keep track of the maximum level
  if (nLightLevel > nMaxLevel)
  {
    nMaxLevel = nLightLevel;
  }
  
  int nMaxLevel2 = (nMaxLevel - nMinLevel)/PWM_LEVELS;
  if (nMaxLevel2 <= 0)
  {
    nMaxLevel2 = 1;
  }
  
  if (nLightLevel < nMinLevel)
  {
    level = 0;
  }
  else
  {
    level = (nLightLevel - nMinLevel)/nMaxLevel2;
  }
  
  ledInit();
  // Do PWM of light level
  for (j = 0; j < 200; j++)
  {
    for (i = 0; i < PWM_LEVELS; i++)
    {
    #ifdef ON_WHEN_DARK
    if (i < level)
    {
      P1DIR &= ~(PIN_0);
    }
    else
    {
      P1DIR |= PIN_0;
    }
    #else
    if (i < level)
    {
      P1DIR |= PIN_0;
    }
    else
    {
      P1DIR &= ~PIN_0;
    }
    #endif
    }
  }
  return (level < PWM_LEVELS);
}

void main(void)
{
  unsigned int nLightLevel;
  //
  // Initialize th processor to use the internal
  // low-frequency oscillator, and set a timer for 250ms
  // period for the processor to go into low-power state
  // to save power.
  //
  BCSCTL3 |= LFXT1S_2; // Enable VLO for LFXT1
  WDTCTL = WDT_ADLY_250;
  IE1 |= WDTIE;

  while (1)
  {
    // Measure the light level using the LED
    nLightLevel = doSD16Conversion (1, 4);
    // Do PWM for a few hundred ms using the latest light level
    if (doPWM (nLightLevel) == 0)
    {
    // If light is off, sleep to save power
    // Turn off reference - this will otherwise draw several hundred uA
    SD16ReferenceOff();
    _BIS_SR(LPM3_bits + GIE); // Low power for 250ms
    SD16ReferenceOn();
    }
  }
}

#pragma vector=WDT_VECTOR
__interrupt void watchdog_timer (void)
{
  _BIC_SR_IRQ (LPM3_bits);
}