skyrock

Hello everybody,

As with the little crystal problem i've had last few days, i finally found out why it didnt work but I dont understand it..

It happened that the crystal I used cannot be grounded in order to work. I can only make it work like the circuit in the attachment. Is it normal?

I'm using Nigel's main board and Led board for testing using PIC16f628

http://www.winpicprog.co.uk/pic_tutorial_main_board.htm

I attached the 2 legs from the crystal to RA6 & 7, and then my board turns on the LED on portb blinking at 1 second interval. But this is without any capacitor and grounding... (SPOOKY..) Does anybody knows why?

EDIT: it doesnt work already after i tried it again. would it because there's charge left inside the crystal? It's just plain wierd..

Attached Images

	crystal.JPG (7.4 KB, 30 views)
	IMG061.jpg (490.3 KB, 25 views)

Nigel Goodwin

Connect the capacitors, they are an essential part of the oscillator circuit - without them it 'may' work some of the time, due to stray capacitance, but it's unlikely to be reliable.

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joshua17ss2

Is it possible its running on the internal oscillator and just ignoring the connection to the external crystal?

picbits

More than likely the stray capacitance of the PCB or prototype board he's using is enabling the crystal to oscillate as Nigel said above.

colin mac

Same thing happened me the other day .I was wondering what was wrong until I removed the caps. It's still working away now.
I don't have the part no. for the crystal to find out what the story was.

speakerguy79

Breadboards have a lot of parasitic capacitance. That said, I've always used xtals with caps on breadboards.

eblc1388

I never have problem with crystals not working on breadboard.

The technique is simple.

Place the crystal directly across the MCU pins. Do not use any extra connection wire or links to connect the crystal and its two capacitors.

Most projects I saw on the Net placed the crystal some distance away from the MCU pins, then connect them using links so that it looks neat and tidy.

Don't do that.

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skyrock

can you explain further? what is the reason behind this?

regards.

ericgibbs

hi,
I agree, doing this way I never have a problem with the osc..

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Kryten

I agree with ericgibbs and eblc1388

The shorterway from the Crystal to the cnotroller the bether. This way you get the most correct freq. as you can control the stray capacitance (witch is on all PCBs more or less).

There are some ways to control the stray capacitance as shown on the pic below (snapped from DS1302 Datasheet). For Crystals that are used with PICs the caps are used for stability and to keep the crystals BandWith very tight. (I mean to have read somewhere that its like 3-4 Hz)

Attached Images

crystal.jpg (151.9 KB, 17 views)

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skyrock

I've finally found a way to make the crystal to work. After trying to abandon the 16f628 by using 16f84a, i noticed a statement from this spec, saying that it is recommended to use HS mode for crystal higher than 3.5mhz rather than XT mode. so this took me a week to finally solve.

http://www.biltek.tubitak.gov.tr/gel.../pic16f84A.pdf

Page:22, (or 24 of acrobat)

gramo

Dont abandon the 16F628! The 16F84 is not nearly as good a tool

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