DT interrupt/pulse generator

[Adrian]

I have used Darrel's interrupts to generate a 1KHz pulse stream. As I'm using TOGGLE the interrupts needs to be at a 2KHz rate. Using PicMultiCalc with 7 instructions and 20MHz oscillator this gives Timer reload as 63043. Using this I get 959 Hz. I have to load 63150 to get about 1,000Hz. 107 ticks seems a long way out to me. Can someone please tell me why the great discrepancy and/or what I'm doing wrong. Many thanks
Adrian

[skimask]

I have used Darrel's interrupts to generate a 1KHz pulse stream. As I'm using TOGGLE the interrupts needs to be at a 2KHz rate. Using PicMultiCalc with 7 instructions and 20MHz oscillator this gives Timer reload as 63043. Using this I get 959 Hz. I have to load 63150 to get about 1,000Hz. 107 ticks seems a long way out to me. Can someone please tell me why the great discrepancy and/or what I'm doing wrong. Many thanks
Adrian

Looks about right to me...
20Mhz oscillator = 5Mhz instruction rate/Timer increment rate
5,000,000 / ( 65536-63043) = 2005 Hz
5,000,000 / ( 65536-63150) = 2095 Hz

Neither number above compensates for the 7 instructions, but the idea is the same.
You've got the numbers right...or at least it looks like it.

[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.