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EX_STWT1.C questions

 
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art



Joined: 21 May 2015
Posts: 181

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EX_STWT1.C questions
PostPosted: Fri Jul 14, 2017 11:38 pm     Reply with quote

Hi,

I'm trying to understand EX_STWT1.C example.

1) I would like to know why the value for INTS_PER_SECOND is 19 ?

Code:

#define INTS_PER_SECOND 19         // (20000000/(4*4*65536))


2) What does it mean by (4*4*65536) ? What does 4*4 means ?
3) If I change the clock=48000000 , Is the INTS_PER_SECOND = (48000000/(4*4*65536)) =45.77 ?

4) Timer 1 overflow value is it fixed to 65535 ? Can we change it to other value ?

Code:

#INT_TIMER1                        // This function is called every time
void clock_isr() {                 // timer 1 overflows (65535->0), which is
                                   // approximately 19 times per second for
    if(--int_count==0) {           // this program.
      ++seconds;
      int_count = INTS_PER_SECOND;
    }
}

Ttelmah



Joined: 11 Mar 2010
Posts: 18602

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PostPosted: Sat Jul 15, 2017 1:09 am     Reply with quote

4*4 is 4 times 4.....

If you read the PIC data sheet, you will find that on the standard PIC's, the 'timer' is fed from Fosc/4 (master oscillator/4). There is then a programmable 'prescaler'. This is set to /4 (T1_DIV_BY_4). The timer then counts 65536.

No, you can't change the overflow value on these timers. There are things you can do, like set the timer 'forwards', but they all introduce slight errors. Timer2, has a programmable overflow, larger prescalers, and a postscaler, so if you are not using the PWM, this is a great way of having a timer at a 'human nice' interval.

Look in the code library. There is an accurate RTC there, that shows how you can still have an accurate time with timers counting with odd values like 45.776* per second. Yes, this is how often the interrupt would tick with a 48Mhz master clock.

<http://www.ccsinfo.com/forum/viewtopic.php?t=26177>
art



Joined: 21 May 2015
Posts: 181

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PostPosted: Sat Jul 15, 2017 4:34 am     Reply with quote

Dear Ttelmah

Thanks for the explanation.
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