Is the this constant incorrect?

Question

I am currently reading an application note from atmel, where i am currently trying to understand the code. The code generated a trapezoidal ramp,for controlling a stepper motor.

One thing I don't understand by the code, is a constant set to be

// Timer/Counter 1 running on 3,686MHz / 8 = 460,75kHz (2,17uS). (T1-FREQ 460750)
#define T1_FREQ 1382400

I am not sure i understand why the value is 1382400, rather than 460750, since the clock frequency prescaled by 8 is 460750... So why is it 1382400? Am I missing something?

.h file

/*This file has been prepared for Doxygen automatic documentation generation.*/
/*! \file *********************************************************************
 *
 * \brief Header file for speed_cntr.c.
 *
 * - File:               speed_cntr.h
 * - Compiler:           IAR EWAAVR 4.11A
 * - Supported devices:  All devices with a 16 bit timer can be used.
 *                       The example is written for ATmega48
 * - AppNote:            AVR446 - Linear speed control of stepper motor
 *
 * \author               Atmel Corporation: http://www.atmel.com \n
 *                       Support email: avr@atmel.com
 *
 * $Name: RELEASE_1_0 $
 * $Revision: 1.2 $
 * $RCSfile: speed_cntr.h,v $
 * $Date: 2006/05/08 12:25:58 $
 *****************************************************************************/

#ifndef SPEED_CNTR_H
#define SPEED_CNTR_H

/*! \brief Holding data used by timer interrupt for speed ramp calculation.
 *
 *  Contains data used by timer interrupt to calculate speed profile.
 *  Data is written to it by move(), when stepper motor is moving (timer
 *  interrupt running) data is read/updated when calculating a new step_delay
 */
typedef struct {
  //! What part of the speed ramp we are in.
  unsigned char run_state : 3;
  //! Direction stepper motor should move.
  unsigned char dir : 1;
  //! Peroid of next timer delay. At start this value set the accelration rate.
  unsigned int step_delay;
  //! What step_pos to start decelaration
  unsigned int decel_start;
  //! Sets deceleration rate.
  signed int decel_val;
  //! Minimum time delay (max speed)
  signed int min_delay;
  //! Counter used when accelerateing/decelerateing to calculate step_delay.
  signed int accel_count;
} speedRampData;

/*! \Brief Frequency of timer1 in [Hz].
 *
 * Modify this according to frequency used. Because of the prescaler setting,
 * the timer1 frequency is the clock frequency divided by 8.
 */
// Timer/Counter 1 running on 3,686MHz / 8 = 460,75kHz (2,17uS). (T1-FREQ 460750)
#define T1_FREQ 1382400

//! Number of (full)steps per round on stepper motor in use.
#define FSPR 200

#ifdef HALFSTEPS
  #define SPR (FSPR*2)
  #pragma message("[speed_cntr.c] *** Using Halfsteps ***")
#endif
#ifdef FULLSTEPS
  #define SPR FSPR
  #pragma message("[speed_cntr.c] *** Using Fullsteps ***")
#endif
#ifndef HALFSTEPS
  #ifndef FULLSTEPS
    #error FULLSTEPS/HALFSTEPS not defined!
  #endif
#endif

// Maths constants. To simplify maths when calculating in speed_cntr_Move().
#define ALPHA (2*3.14159/SPR)                    // 2*pi/spr
#define A_T_x100 ((long)(ALPHA*T1_FREQ*100))     // (ALPHA / T1_FREQ)*100
#define T1_FREQ_148 ((int)((T1_FREQ*0.676)/100)) // divided by 100 and scaled by 0.676
#define A_SQ (long)(ALPHA*2*10000000000)         // ALPHA*2*10000000000
#define A_x20000 (int)(ALPHA*20000)              // ALPHA*20000

// Speed ramp states
#define STOP  0
#define ACCEL 1
#define DECEL 2
#define RUN   3

void speed_cntr_Move(signed int step, unsigned int accel, unsigned int decel, unsigned int speed);
void speed_cntr_Init_Timer1(void);
static unsigned long sqrt(unsigned long v);
unsigned int min(unsigned int x, unsigned int y);

//! Global status flags
extern struct GLOBAL_FLAGS status;

#endif

... Fun fact,maybe something in to it? – leonard Dec 12 '16 at 14:25 — leonard, Dec 12 '16 at 14:25

Piglet · Accepted Answer · 2016-12-12T21:31:14.790

/*! \Brief Frequency of timer1 in [Hz].
 *

The comment tells you to modify T1_FREQ to your needs.

 * Modify this according to frequency used. Because of the prescaler setting,
 * the timer1 frequency is the clock frequency divided by 8.
 */

This line gives you an example for calculation. It's just an example. They could have used completely different numbers if the wanted to. They won't change a general example every time they use different prescalers or frequencies.

// Timer/Counter 1 running on 3,686MHz / 8 = 460,75kHz (2,17uS). (T1-FREQ 460750)

This line is the line you should change if needed. 1382400 is what they used in their code. If you need 460750 in your case, then replace the number...

#define T1_FREQ 1382400

Thanks. I really hoped that was the case. – leonard Dec 12 '16 at 21:50 — leonard, Dec 12 '16 at 21:50

Is the this constant incorrect?

1 Answers1