Counting instruction cycles

[GregEigsti]

Sorry for the simple question; I just want to verify my thinking.

I am working on a timing critical project (IR learning/playback) and want to further tune my code so that I get/play IR bursts with as accurate as possible timing. I am using an 18F2550 clocked at 4mhz (external crystal) and would like to 'calculate' the offset between when my ISR is called and when I stuff away timer values (there are a few lines of management code between the two). I glanced through Microchip's PDF for this chip and see that an instruction takes 4 clock cycles but because of pipelining it is reduced to 1 cycle. I just want to make sure that I am reading my .lst file correctly.

I have to admit complete ignorance as to what the internal speed of my PIC clock is (as applies to timer1). At one time I had this figured out but have lost that knowledge... USB CDC and bootloader are working for this project so I know that I have one of the compatible USB internal clock speeds (though I am confused by the 18F2550 doc at this point). A couple years ago I found a great thread on this forum about the different fuses values as they pertain to USB clock speeds - but I cannot find it now. If anyone is familiar with this thread I'd love to see it!

My question is this... If I want to divine the timer1 value when my ext_isr is called is it a simple matter of each instruction line (between the top of ext_isr and my call to get_timer1()) counting for 1 cycle? If so, by my calculations, I would need to subtract 31 cycles from the return value of get_timer1().

[PCM programmer]

With a 4 MHz oscillator, it's a 1 MHz instruction clock. Each instruction
cycle takes 1 usec. Most instructions execute in 1 instruction clock
but some take 2 or 3. Look at this table in your PIC data sheet. Look
in the "Cycles" column to see how many instruction cycles each
instruction takes:

[GregEigsti]

Thanks for the great info! I had seen the oscillator clock vs. instruction clock info before but it did not sink in. Now reading the excellent thread below to learn more.

Timers - what you should know

Thanks!
Greg