week ten Header Img

Week 13: Getting something out of the Fab-Pic

Hello again...
This weeks task: make your own board output something...

Let me short think about some output-ideas:

  • LED or seven-segment-display -> Look at week 8
  • Use the UART to bring some livesigns out -> Look at week 11
  • Use the I2C to fire some display -> Look at week 11

  • Okay, we will use a stepper motor. Always a nice feature if you want to make a robot or any moving device.

    Some theory about the standard stepper-interface:
    The most interfaces uses 3 signals:

  • Motor enable
  • Motor direction (CW or CCW)
  • Step

  • There are a lot of ready-to-use stepper-driver out there, for example the polulu boards.
    Classical self-made drivers uses the L297 and L298 ICs. A little bit outdated, but still good and easy to use.

    For making this assingment a little more interessting i will use an old 5.25" floppy drive. This old drives have a stepper motor for the read/write-head that is not controlled by the drive itself, but on the external floppy-controller. Therefore the drive has the enable, direction and step signal routed out to the connector.

    Book Img

    In my bookshelf i found an old book from 1994, with the connector-description from the old drives:

    Book Img

    Hooking the floppy and the Fab-Pic together:

    Book Img

    Book Img

    Writing some code, and this comes out (turn your volume up for the nice old floppy-sound):

    Ok, there was a little more work in it ;)

    For generating the step-pulses i use a timer of the pic, timer 1.
    This timer has a 16bit value, and after some calculating i get the values for the pre- and postscaler. All other configs are good described in the datasheet:

    T1CON = 0b00000011; //Config Timer2
    IPR1bits.TMR1IP = 1; //Setting Timer1 to high-prio
    PIE1bits.TMR1IE = 0; //Disable Timer1 Interrupt until drive to middle
    PIR1bits.TMR1IF = 0; //rest TMR1-Interrupt flag
    RCONbits.IPEN = 1; //enable High/Low-Prio
    ei(); // master switch to enable interrupt
    INTCONbits.GIE_GIEH = 1; //enable all Low-Prio-Interrupts
    INTCONbits.PEIE_GIEL = 1; //enable all Low-Prio-Interrupts

    Everytime the timer overflows an interrupt is generated. Due to the fact that our PICs just knows 2 interrupt-priorities we have to check on every interrupt which interrupt-source has generated the interrupt.

    After the check we can execute some code, and reset the interrupt.
    Because we want different frequencys we preload the timer-value on every interrupt:

    void interrupt trigger_interrupt(void) //High priority interrupts
      if (PIR1bits.TMR1IF == 1)
       if (motor_step == 1) {motor_step = 0;}
      if (motor_direction == 0) {motor_direction = 1;}
      else {motor_direction = 0;}
       motor_step = 1;
      TMR1H = high;
      TMR1L = low;
      PIR1bits.TMR1IF = 0;

    The calculation of the preload-value is made by the subroutine "play".
    The main program just calls play one time with a wished frequency, and after this nothing needs to be done anymore.

    void play(int frequency)
    //Using Timer1 for PWM-Generating
    //Timer is configuered as 16-Bit, running on FOSC/4 = 8MHz, 65536 counts max
    //For one periode pf sound (1Hz) two timer-overflow are needed (one low, one high)
    //TMR1H is the high register
    //TMR1L is the low register
    long temp;
    if (frequency <=62) frequency = 62;
    if (frequency <=62) frequency = 62;

    temp = -((8000000/frequency/2)-65536);
    high = temp / 255;
    low = temp - (high * 255);

    drive_enable = 0; //Switch drive on }

    If you want to stop the output we just stop the motor by killing the enable signal:

    void stop()
      drive_enable = 1; //Switch drive off

    Another possibilty is to switch of the timer-interrupt:

    PIE1bits.TMR1IE = 0; //Disable Timer1 Interrupt

    In a music-title the breaks are very important. Therefore a little sleep-subroutine is used:

    //function to generate delays of ms without max limit
    void sleep (int delay)
    int i;
    for(i=0; i<=delay; i++)

    The main code consists just out of



    i need some notes

    okay, a little excurse:
    A lot of music files you can find in the web are in MIDI-format.
    This format is designed in a time, when memory was very expensive. And it is desigend by musicans, for musicans.
    Because of this the time-delays are not in seconds but in quarter-notes and metronome-ticks.
    And there are no frequencys but notes. Yes notes. Like C1 or D3.
    Due to the MTM-Week (still have to write the docu for presentation next day) i will not explain the whole format to you. If you are interessded, google it. There are a lot of good explanations out there. I used these to write a php-script.
    Here on github the server can´t execute PHP, so you have to execute it on your PC or server.
    For some time you can also use the srcipt on my own server:


    I´ve zipped the sourcecode for the PIC and the PHP-Script for you,
    download it here:


    So long, have fun and save the princess!