Same cystal for Two Microcontroller

Started by R Ramesh December 29, 2003
Hi all
 
Can I use a Same clock ( 20MHz ) crystal for two Microcontroller ( 16F877, 16F872).
When I tested it, it is working . But for a long will it give any problem ?
 
Have anyone using like this ?
 
Thanks
Ramesh



At 09:17 PM 12/28/2003 -0800, R Ramesh wrote:

>Hi all
>
>Can I use a Same clock ( 20MHz ) crystal for two Microcontroller ( 16F877, 16F872).
>When I tested it, it is working . But for a long will it give any problem ?

Can be done, but there are a few things you need to watch out for.
First, don't connect the xtal pins in parallel to one crystal.
Second, make sure that the xtal output pin can drive an external load. (see spec sheet)

Pic one as the master, and the other as a slave.
Connect the crystal normally to the master.
Then drive the slave's xtal input pin through a small resistor (roughly 100 ohms), from the master's xtal output pin. The resistor reduces EMI radiation.

Reduce the capacitor on the master's xtal output pin slightly, by 5 pf or so to compensate for the extra loading.

Make sure there's a solid and discrete ground return from the slave processor directly to the master's ground pin.