Микроконтроллеры AVR

#include <mega328p.h>


#define   AS2           5
#define   CS20          0
#define   TCR2UB        0
#define   OCR2UB        2
#define   TCN2UB        4


#define ASYNC_TIMER                        AS2
#define NO_PRESCALING                      CS20
#define ASYNC_TIMER_CONTROL_REGISTER       TCCR2B
#define ASYNC_TIMER_CONTROL_UPDATE_BUSY    TCR2UB
#define OUTPUT_COMPARE_UPDATE_BUSY         OCR2UB
#define TIMER_UPDATE_BUSY                  TCN2UB
#define TIMER                              TCNT2
#define OSCCAL_RESOLUTION                  8
#define LOOP_CYCLES                        7



/*! Modify CALIBRATION_FREQUENCY to desired calibration frequency
 */
#define CALIBRATION_FREQUENCY 4000000

/*! Frequency of the external oscillator. A 32kHz crystal is recommended
 */
#define XTAL_FREQUENCY 32768
#define EXTERNAL_TICKS 100               // ticks on XTAL. Modify to increase/decrease accuracy

/*! \brief Fixed calibration values and macros
 *
 * These values are fixed and used by all calibration methods. Not to be modified.
 *
 */
#define RUNNING 0
#define FINISHED 1
#define DEFAULT_OSCCAL_MASK        0x00  // Lower half and
#define DEFAULT_OSCCAL_MASK_HIGH   0x80  // upper half for devices with splitted OSCCAL register


#define DEFAULT_OSCCAL_HIGH ((1 << (OSCCAL_RESOLUTION - 1)) | DEFAULT_OSCCAL_MASK_HIGH)
#define INITIAL_STEP         (1 << (OSCCAL_RESOLUTION - 2))
#define DEFAULT_OSCCAL      ((1 << (OSCCAL_RESOLUTION - 1)) | DEFAULT_OSCCAL_MASK)

// **** Functions implemented as macros to avoid function calls
#define PREPARE_CALIBRATION() \
calStep = INITIAL_STEP; \
calibration = RUNNING;

#define COMPUTE_COUNT_VALUE() \
countVal = ((EXTERNAL_TICKS*CALIBRATION_FREQUENCY)/(XTAL_FREQUENCY*LOOP_CYCLES));

// Set up timer to be ASYNCHRONOUS from the CPU clock with a second EXTERNAL 32,768kHz CRYSTAL driving it. No prescaling on asynchronous timer.
#define SETUP_ASYNC_TIMER() \
ASSR |= (1<<ASYNC_TIMER); \
ASYNC_TIMER_CONTROL_REGISTER = (1<<NO_PRESCALING);


// Absolute value macro.
#define ABS(var) (((var) < 0) ? -(var) : (var));


#define NOP() #asm("nop");


//! The binary search step size
unsigned char calStep;
//! The desired counter value
unsigned int countVal;
//! Calibration status
unsigned int calibration;



/*
* This function increments a counter for a given ammount of ticks on
* on the external watch crystal.
*
*/
unsigned int Counter(void){
  unsigned int cnt;

  cnt = 0;                                                      // Reset counter
  TIMER = 0x00;                                                 // Reset async timer/counter
  while (ASSR & ((1<<OUTPUT_COMPARE_UPDATE_BUSY)|(1<<TIMER_UPDATE_BUSY)|(1<<ASYNC_TIMER_CONTROL_UPDATE_BUSY))); // Wait until async timer is updated  (Async Status reg. busy flags).
  do{                                                           // cnt++: Increment counter - the add immediate to word (ADIW) takes 2 cycles of code.
    cnt++;                                                      // Devices with async TCNT in I/0 space use 1 cycle reading, 2 for devices with async TCNT in extended I/O space
  } while (TIMER < EXTERNAL_TICKS);                             // CPI takes 1 cycle, BRCS takes 2 cycles, resulting in: 2+1(or 2)+1+2=6(or 7) CPU cycles
  return cnt;                                                   // NB! Different compilers may give different CPU cycles!
}                  


/*! \brief Initializes the calibration.
*
* Computes the count value needed to compare the desired internal oscillator
* speed with the external watch crystal, and sets up the asynchronous timer.
*
*/  
void CalibrationInit(void){

  COMPUTE_COUNT_VALUE();                                        // Computes countVal for use in the calibration
  OSCCAL = DEFAULT_OSCCAL;
  NOP();

  SETUP_ASYNC_TIMER();                                          // Asynchronous timer setup
}

void CalibrateInternalRc(void){
  unsigned int count;
  unsigned char cycles = 0x80;

  do{
    count = Counter();
    if (count > countVal)
      OSCCAL--;                                                 // If count is more than count value corresponding to the given frequency:
    NOP();                                                      // - decrease speed
    if (count < countVal)
      OSCCAL++;
    NOP();                                                      // If count is less: - increase speed
    if (count == countVal)
      cycles=1;			
  } while(--cycles);                                            // Calibrate using 128(0x80) calibration cycles

}

void calibrate_rc(void)
{
  CalibrationInit();                                            // Initiates calibration
  PREPARE_CALIBRATION();                                        // Sets initial stepsize and sets calibration state to "running"
  CalibrateInternalRc();                                        // Calibrates to selected frequency
}