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Altera_Forum's avatar
Altera_Forum
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21 years ago

Nios II Timer

I have the need for a 10 mS periodic timer interrupt for my system. I used the timer in the SOPC builder and selected a time base of 10 mS. To keep the size low, I opted for the simple periodic timer. After loading the new config file and writing a simple program to toggle an LED in the timer IRQ, every thing went well. I used the alarm function. My first attempt was to pass the alarm start function nticks per second so I can see the LED blink. Worked perfect! However, I tried to increase the speed, but there was no change in the IRQ frequency http://forum.niosforum.com/work2/style_emoticons/<#EMO_DIR#>/huh.gif . No matter what I set the ntick parameter to, I always got a 1 second IRQ. Do I need to opt for the fully functional timer in the SOPC? Or is there something that I am doing wrong? Please advise...

Rick

11 Replies

  • Altera_Forum's avatar
    Altera_Forum
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    21 years ago

    You will need to enable the "writeable period" checkbox in the timer properties sheet. The simple periodic timer does not have this enabled, so you can&#39;t change the period. Secting the "full featured" version would do this by default.

    Steve.
  • Altera_Forum's avatar
    Altera_Forum
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    21 years ago

    Thanks for the tip! Just wondering why the IRQ is 1 second when I set the simple timer it to 10mS.

  • Altera_Forum's avatar
    Altera_Forum
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    21 years ago

    Hi rppolicy.

    Here are my thoughts...

    When you call the alarm_start function, the first call to alarm_callback will go occur in nticks. After the alarm_callback function gets called, the return value of the alarm_callback funtion becomes nticks.

    If you started with a sample timer, the alarm_callback function will look something like this: 

    alt_u32 my_alarm_callback (void* context)
{
   /* 
   * This return value is used to determine when the call back function will 
   * be called again...  In this case, once every second
   */
  return alt_ticks_per_second();  
}

    where the return value (100) is the number of ticks until 1 second expires, which then calls your alarm_callback funtion again (once every second).

    I believe that if you return 1, you will get an interrupt in "one tick" which should be your 10 ms.

    Hopefully this helps.

    tjd
  • Altera_Forum's avatar
    Altera_Forum
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    21 years ago

    Hello,

    In most of the example designs, included with Nios II, a 1ms timer is included for clock ticks. If you look at how this is implemented (hardware in SOPC Builder, software in the ./components/altera_avalon_timer). Focus on the 
    ./components/altera_avalon_timer/altera_avalon_timer_sc.c
    file.

    This timer generates an interrupt every 1ms. At first glance, it looks like it&#39;s doing exactly what you want, only at 1ms intervals.

    Regards,

    - slacker
  • Altera_Forum's avatar
    Altera_Forum
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    21 years ago

    Thanks again for the help. Didn&#39;t realize the return value of the callback setup the next alarm. Got it working perfectly!

    Rick
  • Altera_Forum's avatar
    Altera_Forum
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    11 years ago

    hello, I have one question over here.

    Did the callback function called exactly after 10ms after the call of alt_start_alarm.

    I have faced a delay in calling the callback for the first time, after that it worked well. Do you have any idea about this ?

    configured delay is 4 sec. first time callback called after 5 sec, after that callback called in every 4 sec. what could be the problem ?
  • Altera_Forum's avatar
    Altera_Forum
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    11 years ago

    No. we are not using cash for data master in nios.

    we are using nios II/e version
  • Altera_Forum's avatar
    Altera_Forum
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    11 years ago

    Personally, for an embedded system, I'd not use a timer interrupt but just read a 'time counter' register at appropriate times.

    It is likely that the timer interrupt just sets a flag that the main processing loop checks - so it is just as easy for the processing loop to check the time itself.

    Getting rid of the interrupt removes all the ISR entry/exit costs and all the places where the interrupt would have to be disabled.
  • Altera_Forum's avatar
    Altera_Forum
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    11 years ago

    but the problem what I faced is the delay is not accurate for the first time, after that it works fine.

    alt_start_alarm(for 4 sec) ...............5secs................ callback called .........4sec.........callback called.............4sec...........callback called

    why it take 5 sec for the first time. Is it because the function alt_start_alarm take 1 sec extra