Hi, my Nerdkits's just arrived several days ago. I've gone through all exercises in the Nerdkits guide and now I'm working on the Making music with MCU project. I have question on the play_tone function in the source code provided. Actually at first I cannot understand it so I try to write the code in the way I understand (no switch):

   // musicbox1.c
// for NerdKits with ATtiny26L
// mrobbins@mit.edu

// F_CPU defined for delay.c
#define F_CPU 8000000UL // 8MHz

#include <avr/io.h>
#include <avr/interrupt.h>
#include <avr/pgmspace.h>
#include <util/delay.h>
#include <inttypes.h>
#include <stdlib.h>

#include "../libnerdkits/delay.h"
#include "../libnerdkits/lcd.h"
#include "../libnerdkits/uart.h"

// PIN DEFINITIONS:
//
// PC4 -- piezo buzzer

void play_tone(uint16_t delay, uint8_t duration) {

  uint16_t cycles = 500*duration/delay;

while(cycles > 0) {

  PORTC |= (1<<PC4);
  delay_us(delay);
  PORTC &= ~(1<<PC4);
  delay_us(delay);
  cycles--;

 }
}

#define D5 851
#define E5 758
#define Fsh5 675
#define G5 637
#define A5 568
#define B5 506
#define C6 477
#define D6 425
#define DUR 400

int main() {

    OSCCAL = 176;

    DDRC |= (1<<PC4);

    while(1) {

        play_tone(D5, DUR);
        play_tone(E5, DUR);
        play_tone(D5, DUR);
        play_tone(G5, DUR);
        play_tone(Fsh5, 2*DUR);

        play_tone(D5, DUR);
        play_tone(E5, DUR);
        play_tone(D5, DUR);
        play_tone(A5, DUR);
        play_tone(G5, 2*DUR);

        delay_ms(500);

    }
    return 0;
}

where the duration is in millisecond, which is just directly comes from: duration = 2xdelayxcycles (I also change (duration)DUR to 400)

But it turns out to have a strange behavior, it plays each note very fast and when I increase the duration sometimes it becomes even faster. After that I just copy the play_tone function from the source code provided and it works fine..

void play_tone(uint16_t delay, uint8_t duration) {
  // delay is half-period in microseconds
  // duration is in 10ms increments

  // example: 440Hz --> delay=1136

  // duration = 2*delay * cycles (all in same units)
  // cycles = 10000 * duration / delay / 2
  // cycles = 100 * duration / (delay/50)
  uint16_t tmp = 100 * duration;
  uint16_t delaysm = delay / 50;
  uint16_t cycles = tmp / delaysm; 
}

(duration:DUR=40)

I'm wondering if someone could explain me why we have to write the code this way. Mathematically, it's the same so I think it should be due to programming reason..

Hi meeser,

I think your factor should be 5000, not 500 if you want to do it that way. I think it will probably work.

Humberto

Thank you very much for your help, I thought I did try that but now I've tried it again and it works now... May be I made some other misatkes that time. Both codes work the same now! Thank you

meeser

I did not understand the math you did from lines 23-33. Can you explain it?

srry, the math from the original code.