Forum: µC & Digital Electronics Servo Control ATMega2560

von Dave W. (bluevacuum)

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I'm new to microcontroller programming and am trying to get a Mega2560 
running at 16Mhz to move a servo back and forth.  I'm trying to 
controller the servo by letting the timer do all the work and for some 
reason I am having problems. I am curious as to whether or not this can 
be done without using interrupt handeling. My understanding is that when 
I set the Port to output, OC1A should be updated automatically.  The 
idea is to clear OC1A when upcounting and set it again when 
downcounting.  By changing the value of OCR1A, I should be able to 
change the pulse width. Unfortunately, it doesn't work in the simulator, 
which doesn't suprise me since the winavr simulator seems to have a 
number of bugs in it, but it doesn't work on my 2560 properly either. 
I'd appreciate any help.

#ifndef F_CPU
#warning "F_CPU war noch nicht definiert!"
#define F_CPU 16000000UL
#include <util/delay.h>

void main (void) {
  //Set testFactor to 1 for real life or to a higher value
  //to speed up the simulation
  int testFactor=1; //ICR1 and OCR1A are divided by this value

  //Initialize PORTB

  PORTB=0b00100000;  //start with OC1A high
  DDRB= 0xFF;  //set OC1A to output (among others)

  //Initialize Timer/Counter 1
  //System Clock as source
  //Mode 8: Phase and Freq. Correct PWM top=ICR1
  //OC1A output: Non-inverted
  //OC1B output: currently disabled
  //OC1C output: currently disabled

  OCR1A=1000/testFactor; //set 1ms pulse  1000=1ms  2000=2ms
  TCCR1A=(1<<COM1A1);//COM1A1 Clear OCnA when match counting up,Set on 
  TCCR1B=(1<<WGM13)|(1<<CS11);// Phase and Freq correct ICR1=Top

while (1)

      OCR1A = 1000; //leave servo at min rotation
      OCR1A = 2000; //leave serve at max rotation



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von Oliver (Guest)

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>Unfortunately, it doesn't work in the simulator,
>which doesn't suprise me since the winavr simulator seems to have a
>number of bugs in it

If you refer to simulavr, I would not call it "bugs". simulavr does not 
simulate any of the peripheral units at all.

Beside that, I do not see any real problems in your code. If the 2560 is 
running at 16MHz, it should produce a suitable output.

Did you check, if the clock frequency is actually 16MHz? External 
crystal in place? Fuses programmed accordingly?


von Dave Waite (Guest)

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Hi Oliver,

thank you very much for having a look.  I've been looking at the fuse 
bits and they have the following values

BODLEVEL disabled
OCDEN unchecked
JTAGEN unchecked
SPIEN checked
WDTON unchecked
EESAVE unchecked
BOOTSZ Flash size = 4096 words start address=$1F000
BOOTRST checked
CKDIV8 unchecked
CKOUT unchecked
SUT_CKSEL Ext. Crystal Osc. 8.0- MHz; Start -up time: 16K CK + 65ms

I would assume (although assumptions are bad) that this is configured to 
run from the external crystal running at 16Mhz, but am uncertain due to 
the 8.0 MHz listed in the SUT_CKSEL.  Also, since I am using Timer 1, do 
you know if I need to set EXCLK in the ASSR register to get it to use 
the 16MHz crystal instead of the internal clock?

As far as the simulator, it is the built in software AVR Simulator that 
comes with AVR Studio.  It shows WGM mode 0x02 running the software, 
which is obviously wrong-a "feature" as we used to call it.  Playing 
around manually with WGM10, WGM11, WGM12, and WGM13, I am unable to 
access WGMs higher than 0x03.  I've read in other threads about this 
problem with other targets under AVRStudio.

Once again, thanks, and I will post later to let you know if changing 
the clock speed helps.


von Dave W. (bluevacuum)

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I have solved the problem.  The trafo connected to the servos had a 
defective voltage divider and overloaded the servos.  The original 
program does work.  Thanks for the help.


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