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carl
Joined: 06 Feb 2008
Posts: 240
Location: Chester
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 Posted: Mon Apr 07, 2008 3:03 pm |
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Tell me if i'm wrong but the slowest interrupt time I can get with using Timer2 is:
setup_timer_2(T2_INTERNAL|T2_DIV_BY_16|255|16)
So for a 4MHz Crystal:
(1/(4M/4/16)) x 256 x 16 = 65.536ms
If this is correct then I cant use this technique to set my variable tick rate because I need the tick to vary between 125ms and 37.5seconds.
Carl |
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Ttelmah
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| Posted: Tue Apr 08, 2008 2:13 am |
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Of course you can.
Count.....!
Program the interrupt for 62.5mSec. Then do an update every 2 interrupts for the 'fastest' requirement in your range, up to every 600 interrupts for your slowest. Gives 62.5mSec 'steps' over the required range.
Without hardware changes, you are not going to get your slow clock, so you have to use tricks to get it. This is entirely your decision. If instead you fed the clock from (say) a 3.3KHz source, then the hardware could do what you want....
Best Wishes |
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carl
Joined: 06 Feb 2008
Posts: 240
Location: Chester
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| Posted: Tue Apr 08, 2008 5:17 am |
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Yep thats what I thought I'd have to do. the problem is though that the count value will be 300 different values. when a particular count value is reached it needs to do 'what it needs to do' and then reset the count back to zero. Can this be done easily for the different 300 count times??
I suppose its just a matter of including a bit of logic into the statements.
I like your comment Ttelmah:
| Quote: | | so you have to use tricks to get it |
Thankyou again
Carl |
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Ttelmah
Guest
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| Posted: Tue Apr 08, 2008 10:08 am |
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All you do, is count downwards.
| Code: |
int16 count_to_use=2;
void count_code (void) {
static int_16 count=1;
if (--count==0) {
count=count_to_use;
//perform required operation here
}
}
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Then when you want to change the count, just set 'count_to_use' to the new count value required. (to avoid glitches, disable the interrupt, and write the new value into the variable, then re-enable the interrupt - otherwise if the value is 'half updated' when the interrupt occurs, you can get an unexpected behaviour).
Best Wishes |
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carl
Joined: 06 Feb 2008
Posts: 240
Location: Chester
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| Posted: Tue Apr 08, 2008 10:35 am |
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Thanks Ttelmah,
you have been a great help.
I will give it a go and let you know
Carl |
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carl
Joined: 06 Feb 2008
Posts: 240
Location: Chester
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| Posted: Tue Apr 08, 2008 2:19 pm |
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Completed
It works!!!!!!!
The code below is compilable for a (1s to 300s) selectable stepup/down and stepdown/up 16bit output.
| Code: | #include <18F4525.h>
#fuses NOWDT,NOPROTECT,BROWNOUT,PUT,NOLVP,NOPBADEN
#use delay(clock=4000000)
void count_code (void);
long int stepDir3;
//long int stepDir4;
//float time_init();
long int z,outs2;
long int count_to_use;
static long int count=1;
#int_TIMER2
TIMER2_isr()
{
if (--count==0) {
count=count_to_use; //perform required operation here
if (stepDir3 == 1){ //Step Up
outs2 += 16383;
output_B(make8(outs2,0));
output_D(make8(outs2,1));
}
else if (stepDir3 == 0) { //Step Down
outs2 -= 16383;
output_B(make8(outs2,0));
output_D(make8(outs2,1)); }
if (outs2 == 0){ //Change to Step up next time
stepDir3 = 1; }
else if (outs2 >= 65532){ //Change to Step down next time
stepDir3 = 0;
}
}
}
void main() {
stepDir3 =1;
z=300;
count_to_use=(z*4); // enable all interupts
setup_timer_2(T2_DIV_BY_16,243,8);
enable_interrupts(INT_TIMER2);
enable_interrupts(global);
while (TRUE) ;
} |
Thats more or less it for my project - all modes are done - the time option is done - and it works - and i've got 3 weeks left to spare.
Thankyou so much everybody for your help (especially Ttelmah, Robert and of course PCM programmer) - I'll try not to bother you again - at least not untill my next project!
Best Regards
Carl |
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