DT interrupt/pulse generator

[Adrian]

Thanks Skimask

The reload criteria seem to be right, as you also concur, but for the life of me I still can't understand why it is over 100 ticks out. I have measured the frequency both on a counter and on a scope so I think we can be pretty sure that's OK. I hope I can solve this rather than it just goes down as one of life's little mysteries.....!
Thanks

Adrian

[skimask]

Thanks Skimask

The reload criteria seem to be right, as you also concur, but for the life of me I still can't understand why it is over 100 ticks out. I have measured the frequency both on a counter and on a scope so I think we can be pretty sure that's OK. I hope I can solve this rather than it just goes down as one of life's little mysteries.....!
Thanks

Adrian

What do you mean by '100 ticks out'?
Do you mean that it's over 100 ticks off of what you calculated to be the reload value?
If that's the case, you might be forgetting about the overhead of the program itself (2 cycles to jump in, 2 cycles out, a couple of cycles to reload the timer itself, and so on)...
Or something different?
Maybe your 20Mhz is off?

[Adrian]

What do you mean by '100 ticks out'?
Do you mean that it's over 100 ticks off of what you calculated to be the reload value?
If that's the case, you might be forgetting about the overhead of the program itself (2 cycles to jump in, 2 cycles out, a couple of cycles to reload the timer itself, and so on)...
Or something different?
Maybe your 20Mhz is off?

Yes - that's what I meant. I imagined that the programme overhead would mop up 'some' 'ticks' but I was surprised when it ended up at 107 adrift. The oscillator frequency is a good point - I'll check that. At the end of the day I can adjust the programme to get the frequency as close as I need so I guess I shouldn't be too concerned

Thanks for your input

Adrian

[skimask]

I imagined that the programme overhead would mop up 'some' 'ticks' but I was surprised when it ended up at 107 adrift.

You can count that out yourself. Just look at the .lst file, run thru the instructions manually and count them (branches, goto's, returns, retfie, all take 2 cycles, check the datasheet).
If you know that number down to the exact cycle, you should be able to program in a specific frequency and check your oscillator frequency against that with a reasonable amount of accuracy.

[Darrel Taylor]

Adrian,

All latencies can be easily compensated for, by ADDING the reload value to the timer instead of just copying the value to the time.

At the time the interrupt occurs, the value of the timer is 0, and it continues counting while the chip is doing what ever it has to do to get to the Handler.

When it gets to the handler, and is ready to reload the timer, the timer already has an EXACT count of how long it took to get there. So when you ADD the const to the timer, it actually subtracts the time it took from the const and EVERYTHING is accounted for.

Take a look at this page ...
http://www.darreltaylor.com/DT_INTS-...rTemplate.html

That program has everything you need, but at a minimum, check out the ReloadTMR1 routine which shows an easy 8 instruction cycle adding reload.

hth,

[Adrian]

Darrel

Thank you very much. I have taken a look and 'instantly' it doesn't look so easy to understand, however it also appears to be very crafty and very useful. I am going to plug away at your code until it IS understood and then add this new-found knowledge to the ever increasing store of accumulated wisdom from all of you experts.

Many thanks again to you for enlightening me and of course to Skimask for his valuable input

Adrian

[Darrel Taylor]

I have taken a look and 'instantly' it doesn't look so easy to understand, ...

It does look a bit confusing at first, but it couldn't be easier.
A complete understanding of how it works will come later after getting it to work.

This is really the only section you need to digest.
Set Freq to 2000, Prescaler to 1, and T1CON to 0. Fill in the Main program and T1handler that gets called on each interrupt.

Done deal.

;--- Change these to match the desired interrupt frequency -------------------
;--- See http://DarrelTaylor.com/DT_INTS-14/TimerTemplate.html for more Info.
@Freq       = 2000                ; Frequency of Interrupts in Hz
@Prescaler  = 1                   ; Timers Prescaler setting
T1CON = $00                       ; $00 = Prescaler 1:1, TMR1 OFF
; $00=1:1, $10=1:2, $20=1:4, $30=1:8 --  Must match @Prescaler value

@ INT_ENABLE  TMR1_INT            ; enable Timer 1 interrupts
GOSUB StartTimer                  ; Start the Timer

;____Your Main Program goes here______________________________________________
Main:
  ;   ---- Your Main Program goes here ----
GOTO Main

;____This routine is Called on each TMR1 Interrupt____________________________
T1handler:
  ;   ---- Your interrupt routine goes here ----
  
@ INT_RETURN