PIC question about crystal oscillator

ELCouz · Joined: 18 Jul 2007 Posts: 427 Location: Montreal,Quebec

Dear members,

I'm designing a small pcb (2x2) with 2x 18F4550 TQFP40 (each are very close).

I'm wondering if it's possible to use a single crystal osc. of 12mhz for the 2 pics ?

...or I must use 2 crystal osc. ?

Also, what do you recommend me to get the two pics working together , spi, i2c or rs232 ? (each pic will run at max speed, 48mhz sending over 6000 bytes per sec)

Best Regards,

Laurent

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

http://www.ccsinfo.com/forum/viewtopic.php?t=23649&highlight=two+pics+same+oscillator

Can you clarify:

Did you want to run two pics from the same crystal ?
or,
two PICs from the same external oscillator ?

A crystal looks like this:
http://rocky.digikey.com/WebLib/ECS/Web%20Photos/HC-49US.jpg

An oscillator looks like this:
http://rocky.digikey.com/WebLib/CTS/Web%20photos/MXO45.jpg

ELCouz · Joined: 18 Jul 2007 Posts: 427 Location: Montreal,Quebec

Thanks but...

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

These threads on the Microchip forum explain how to do it:
http://forum.microchip.com/tm.aspx?m=64265
http://forum.microchip.com/tm.aspx?m=232267

ELCouz · Joined: 18 Jul 2007 Posts: 427 Location: Montreal,Quebec

Thanks PCM Programmer,

I've learned that using a TTL Oscillator , you know the thing with 4 pins, you can drive as many pic you want easily... but for the crystal osc. (ex: HC49) it's another story and its more difficult to achieve.

I've never used a TTL oscillator before, you need a special circuitry to drive this or you simply apply some Vdd and Vss and you got some clock signal?

Thanks for all,

Laurent

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

Look on pages 18 and 19 in the PicDem2-Plus user's guide:
http://ww1.microchip.com/downloads/en/devicedoc/51275b.pdf
Look for "Y2". It's a "can" style oscillator. Disregard components
J7, R4, and C3 that are near Y2 on the schematic. They are used
with the optional RC oscillator. They have nothing to do with the Y2
oscillator.

If you order an oscillator, get one that is compatible with your PIC's Vdd
voltage (+3.3v or +5v, for example). The safe thing is to order one
that is correct for your voltage, and that says "TLL/CMOS compatible".

ELCouz · Joined: 18 Jul 2007 Posts: 427 Location: Montreal,Quebec

Wow thanks for the great help!

Those are pretty easy to use, the only downside , they are expensive and they are big (even with the half-size) compared to the cheap bare crystal osc. ! I will use them only when i need multiple clock signals.

Topic solved !

Best Regards,
Laurent

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

You can get them in much smaller sizes. A typical surface mount
package is 5x7mm.