Two PICs on one external oscillator?

ajt · Joined: 07 Sep 2003 Posts: 110

Are there any problems to be aware of if using a single packaged TTL oscillator to drive two PICs? The two PICs will be connected via I2C to pass some setup information between the two. Mostly they are both running as RS-232 transievers and setting I/O bits or forwarding the received messages. Both CCP peripherals on one device (or the other but not simultaneously) will be doing pulse generation using compare or PWM. Thanks.
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Al Testani

fpgeh · Joined: 07 Sep 2003 Posts: 19 Location: Vancouver, BC

With both PICs set to use an external oscillator it may work.

However, the TTL oscillators usually spec a maximum capacitive load they will drive, usually about 30pF to 50pF. The output may be reduced or have slow rise times if you exceed this. I briefly looked at same PIC datasheets but couldn't find the capacitive load presented by an OSC1 input. Maybe someone else knows or has tried this.

bwhiten · Joined: 26 Nov 2003 Posts: 151 Location: Grayson, GA

What frequency are you using? Two devices should be fine on a single oscillator output but take some simple layout precautions. Keep the two PIC loads and the oscillator close together as feasible and try to locate the oscillator equal distance to both. Use seperate oscillator trace runs for each PIC, in other words don't daisy-chain the clock from one PIC to the next. Make the clock traces as simple as possible with few if any vias.

ajt · Joined: 07 Sep 2003 Posts: 110

Josh Davis · Guest

Place a .01uf cap at the input to each os_in pin on the pic.

If your oscillator is speced for a external load of say 50 Ohms
Plase a series 25 Ohm resistor in series with each osc pin just before
the dc blocking cap.

The resistor can be ommited if the driven port i.e the pic osc pin
and the oscillator pin are high Z.

josh.