I hate crystals.

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0mega

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Well, after about 20 phone calls, i finally found a manufacturer in australia who will sell 40Mhz crystals.

So, i want to order some... only, the dude behind the desk asks me "do you want fundamental or 3rd harmonics".

So i'm like "What the f***?"

I need the crystal for a pic (18f452) .... what do i need?

CHeerZ JB
 

Fundamental - if you told him what it was for, I would have hoped he would have known?.
 
What do you mean? That it should be fundamental?

Anyway, he only manufactured crystals, i don't think he used them.
 
0mega said:
What do you mean? That it should be fundamental?
Click to expand...

A 'fundamental' crystal is one that oscillates at the specified frequency (in your case 40MHz), a 3rd harmonics (overtone) crystal oscillates naturally at only 1/3 of that (in your case 13.33MHz). The oscillator circuit has to be designed to force the crystal to vibrate at three times it's fundamental frequency.

Overtone crystals are usually used for high frequencies, it's difficult to built crystals with a high fundamental frequency.

Anyway, he only manufactured crystals, i don't think he used them.
Click to expand...

If he manufacturers crystals, he should be fully aware of their common uses - I'm sure he must get orders from lots of people wanting clock crystals.
 
It would be difficult to obtain 40Mhz crystals operating in fundamental mode. Even if you could, it might not oscillate at that mode.
Anyway, the PIC18F452 solves this problem by having a 4X clock multiplier circuit. This means you only need a 10Mhz crystal to operate at 40Mhz speed.
 
You serious? Well I never.... So I only need a 10Mhz Crystal then? Do i need to set flags on the chip or is it enabled by default?

Thanks,

John
 
Do i need to set flags on the chip or is it enabled by default?
Click to expand...

Set the CONFIG1H register fuses as follows to select "HS oscillator with PLL enabled/Clock frequency = (4 x FOSC)".

Code: 
 __CONFIG _CONFIG1H, _OSCS_OFF_1H&_HSPLL_OSC_1H
 
omega
in your case I would also say that you could use a pic that has an internal crystal which ones have the speed you need I don't know, its just that I know they make them such as the pic16f627.
 
psyco00 said:
omega
in your case I would also say that you could use a pic that has an internal crystal which ones have the speed you need I don't know, its just that I know they make them such as the pic16f627.
Click to expand...

Unfortunately it's not an internal crystal oscillator, only an internal RC oscillator - but they don't seem too bad to use.
 
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