I hate crystals. - Electronic Circuits Projects Diagrams Free

0mega · 16th March 2004, 05:27 AM

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

**Nigel Goodwin** · 16th March 2004, 05:56 AM

Quote:

Originally Posted by 0mega

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?.

0mega · 16th March 2004, 07:00 AM

What do you mean? That it should be fundamental?

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

**Nigel Goodwin** · 16th March 2004, 08:41 AM

Quote:

Originally Posted by 0mega

What do you mean? That it should be fundamental?

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.

Quote:

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

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.

motion · 16th March 2004, 09:58 AM

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.

0mega · 17th March 2004, 05:54 AM

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

motion · 17th March 2004, 01:58 PM

Quote:

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

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

psyco00 · 25th March 2004, 01:38 PM

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.

**Nigel Goodwin** · 25th March 2004, 06:41 PM

Quote:

Originally Posted by psyco00

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.

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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16th March 2004, 05:27 AM	(permalink)
0mega	I hate crystals. 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

16th March 2004, 07:00 AM	(permalink)
0mega	What do you mean? That it should be fundamental? Anyway, he only manufactured crystals, i don't think he used them.

16th March 2004, 09:58 AM	(permalink)
motion	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. __________________ "Having to do with Motion Control"

17th March 2004, 05:54 AM	(permalink)
0mega	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

25th March 2004, 01:38 PM	(permalink)
psyco00	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.