Can Two Different ICs Use The Same Crystal Oscillator?

Depends on the circuit. Possibly a simple capacitive isolation between the two would suffice, if the circuit requires more elaborate isolation then the minimal cost of two crystals would be negligible.

Hope this helps

HI Guest,

Thanks for the reply. If I used capacitive isolation from each other (the 2 chips), would I not then be isolating one of them from the crystal?

My biggest concern is not price of components, but board space.

Mike

Hi JohnDG,

Thank you for your reply. The frequencies needed to time the devices are the same.

The microcontroller is a PIC16F88. It has an internal oscillator, but I rarely see them used, so I'm thinking that they aren't all that good; I thought "better to go external". It also appears that the internal oscillator of the PIC can be configured to be a system clock. My circuit is not all that complex, just 5 analogue inputs, two digital outputs, 3 digital inputs (keypad) and a serial LCD driver IC. Maybe I'm wrong about the quality of the PIC internal oscillator. What do you think about using the PIC internal oscillator for either the system and/or for the PIC itself?

I've used the internal PIC oscillator many times (tho' not for some years). Timing was more-or-less irrelevant in most of the apps, - the only ones where it mattered were frequency synthesizers for driving stepper motors at defined speeds.

The accuracy wasn't particularly critical, and using the internal osc. gave results which were certainly acceptable - you couldn't notice any inaccuracy when timing moves lasting up to about 100 seconds, using a stopwatch.

If you really need the accuracy, go for an external trimmed oscillator. It just depends on your application.

Hi JohnDG,

Thanks for the advise (and to you as well, Jack of All Trades)! In light of your info, I am going to let the PIC use its own osc. For the LED interface, I will use a 4MHz resonator w/caps.

Be advised that this is a hobby project, not commercial.

_{I'm just a ChemE with too much time on his hands}

Mike

It'll definitely work - but try to screen it to keep down rf emissions - I'm still a bit concerned that the two unsynchronised clocks could interfere (not affecting performance, but in terms of the emissions) and muck up nearby radio/TV reception.

Maybe I'm just being over-cautious.

If your digital outputs are high speed and synchronous you may need to use a common clock, but it sounds as though you could let them free run. I have driven a pair of PIC18 devices from an external buffered clock, but I also had 4 free running devices in the same product.

GA

Eloquent and fully correct (and faster than me!).

Have you checked the microcontroller website for application notes that may cover this or more compact clock circuits (perhaps an internal clock option)?

eg, http://www.maxim-ic.com/appnotes.cfm/an_pk/2154

http://www.piclist.com/techref/microchip/osc.htm

What's the application and devices?

Hi jack of all trades,

Thank you for replying. Please see my reply to JohnDG re my concerns about the quality/stability/robustness of the PIC internal oscillator. Do you have any experience with these?

No personal experience but have a look at the following document for more information on the internal RC oscillator (page 2-7), and other external clock options and tricks.

http://ww1.microchip.com/downloads/en/DeviceDoc/01146B.pdf

http://www.microchip.com/wwwproducts/Devices.aspx?dDocName=en010243

There is a reason they (and other microcontroller manufacturers) provide an internal clock source as a feature, a +/-5% frequency variation is fine in many applications.

Yes !

Haajee

I made two drawings to show you the way.

These two 33Ω (or 22Ω) resistors must be placed very close to the output of your Xtal oscilator. In this way you have no reflections from each track. So the two clock signals that travels to the 2 chips (microcontroller and interface) will not affect each other. If the Xtal oscilator is not able to drive both inputs of these two chips then you can use a clock buffer as in the follow drawing.

(Again you place two resistors 33Ω closed to the two outputs of the clock buffer in order not to have reflections.) However, I don't think that there will be a need for such a clock buffer.

I hope that it helps.

I forgot to say that the two long tracks (from the xtal oscilator to the chips) must be a kind of transmission line (microstrip or stripline) having an impedance of ≈50R.