Delay question

[therian]

wait 833kHz is in range of medium AM broadcast 300-3000kHz and 30-300kHz is longwave AM broadcast but as i know 3-30kHz is enough for submarine communication. so what wrong with using low frequency, huge antenna ?

[HenrikOlsson]

Therian,
First all, I've stated 250nS in my previos posts. That is of course wrong and should be 200nS. NOT 250.

With that being said you could go to a PIC capable of 40MHz, then a single cycle instruction (like NOP) is executed in 100nS instead of 200.

But then again if you use software to generate the carrier frequency you'd be hard pressed to have any instruction cycles left to be able modulate it. I really believe you should look at a dedicated TX/RX chipset or module.

Anyway sorry for the confusion. One instruction cylcle is 200nS at 20Mhz, nothing else.

/Henrik Olsson.

[therian]

im really thankful for you reply.
I know about cheap and good modules out there, but dont anyone try to make some from scratch, it easy to fing FM bugs circuits in google but searching for rf module give really few results

[paul borgmeier]

Not to muddy the waters because it sounds like you have decided on another route, but for completeness ...

Start:
  PortB.1 = 1     'Set pin high        
@ NOP             'Wait....Two NOP's are neede if we want 50% dutycycle
@ NOP             'since the GOTO takes two instruction cylcles.
  PortB.1 = 0     'Set pin low
Goto Start        'Do it again. The GOTO takes the same time as two NOP's

at 4 MHz, 3 instructions on and 3 instruction off = 6 instructions per cycle, therefore f=1000000/6=166666.667 Hz (twice that noted above)

Interestingly enough the above code does 3 instructions on and 4 instructions off on 16F devices and 3 on 3 off on 18F (at least on an 18F877A and 18F452, respectively). To combat this on the 16F's, you could do something like this in ASM

ASM
here
    bsf PORTB,1
    NOP
    NOP
    bcf PORTB, 1
    goto here
ENDASM

Good Luck

[HenrikOlsson]

Hi Paul,
Thanks for filling in and for correcting me. My head seems to be completely messed up today - can't seem to get one thing right.....

How come it differs between 16F and 18F parts? I haven't looked at the generated code but would've thought that the compiler generated just what you wrote in the 'pure asm' example.

Anyway, I'm off to bed now. Not doing much good anyway.

Thanks
/Henrik Olsson.

[paul borgmeier]

How come it differs between 16F and 18F parts?

The 16F code inserts a "clrf PCLATH" command before the goto - the 18F uses the bra command (instead of goto) and does not - why PBP inserts the clrf PCLATH, I would guess because "Start:" is in page 0 where PCLATH = 0 (EDIT: and does not check to see if we are already in page 0, which we are)

[HenrikOlsson]

Hi Paul,
New day, fresh head...
You said:

at 4 MHz, 3 instructions on and 3 instruction off = 6 instructions per cycle, therefore f=1000000/6=166666.667 Hz (twice that noted above)

That is of course correct. I based my calculation on a 20Mhz so 5000000/6 = 833kHz which is what I stated in my post which is also correct (but not half of 166kHz).

Thanks for the 16F vs 18F explanation. I guess it would take up more code to check which bank we're in before each goto instead of just setting the correct one?

Thanks!
/Henrik Olsson.