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art

Joined: 21 May 2015
Posts: 181

EX_STWT1.C questions
Posted: Fri Jul 14, 2017 11:38 pm

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: 19289

 Posted: Sat Jul 15, 2017 1:09 am 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.
art

Joined: 21 May 2015
Posts: 181

 Posted: Sat Jul 15, 2017 4:34 am Dear Ttelmah Thanks for the explanation.
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