Wrong # of cycles in delay_cycles...?

[smashHIT]

So, I'm trying to produce a 38,5kHz square wave on my PIC for an IR diod. I'm running a 16F874A in 4MHz, wich means that the internal clock is 1MHz. I have figured out that I need a pulse period of ~26us to achive 38,5kHz. Wich means that i can set a PIN to be high for 13us, then low for 13us and so on... right?
Well, i tried this code: (sample)

[PCM programmer]

Any instruction that does a jump takes two cycles.
Look at it again.

[smashHIT]

Yes, forgot about about that NOP due to the jump. Thanks!

[Ttelmah]

Remember also that the 'output_high', and 'output_low', each take a cycle, and the while loop involves both a test, and a jump. You'd probably need the first delay to be twelve cycles (to allow for the output instruction), and the second to be 9 (guessing slightly here).

Best Wishes

[SherpaDoug]

How long a distance are you trying to measure? What resolution and accuracy do you need? What is the medium, air, water, vacuum?
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The search for better is endless. Instead simply find very good and get the job done.

[DragonPIC]

Looks like you could accomplish what you are doing mor precisely using PWM.
_________________
-Matt

[smashHIT]

My mind is set on a miniature "radar" unit, but with IR-diodes. The medium will be air, and the maximum rage will be arund 40cm (with 5 different zones). It's just fur fun!
The diodes sit ontop of a servo, that spins 180 degrees, back and fourth.

Okey, so you say I should use PWM... how do I manage to get the dividers and stuff right for a 38,5kHz pulse?

I red that it was recomended to use another IC, like 555 or so, to get the modulation pulse. And then control that signal (on and off), with the PIC +some logic. Anyone familiar with that?

[DragonPIC]

setup_ccp1(CCP_PWM);
setup_timer_2(T2_DIV_BY_1,26,1); //(1/4MHz)*4*1*26 = 26us = 38.5Khz
SET_PWM_DUTY(52); //13us/(1*(1/4MHz))

should set things up for you. You will have to use RC2 though.
_________________
-Matt