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Questions relating to designing PCBs
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By Chupa
#31637
im using a PIC18F2550 in a project with a 20Mhz crystal and a 32KHz watch crystal on timer1. the osc pins and timer1 are right next to each other and the dat sheet recommends a ground ring around the crystals (page 134)
The Timer1 oscillator circuit draws very little power
during operation. Due to the low-power nature of the
oscillator, it may also be sensitive to rapidly changing
signals in close proximity.
The oscillator circuit, shown in Figure 12-3, should be
located as close as possible to the microcontroller.There should be no circuits passing within the oscillator
circuit boundaries other than VSS or VDD.
If a high-speed circuit must be located near the oscillator
(such as the CCP1 pin in Output Compare or PWM
mode, or the primary oscillator using the OSC2 pin), a
grounded guard ring around the oscillator circuit, as
shown in Figure 12-4, may be helpful when used on a
single-sided PCB or in addition to a ground plane.

image from the page:
Image

I am fairly new with Eagle cad, just mainly going by the tutorials posted here a couple weeks ago. I was wondering if someone could explain how to do this with Eagle, or where to see more information on doing this.
By Philba
#31638
You could draw a polygon around all 5 of those pins and then name the poly gnd (or what ever you are calling ground). Click on ratsnest. It will pretty much do it automatically. Set isolate to what ever clearance you want to have.

Usually, I draw the poly for the entire top and bottom layer. Non-gnd traces will get isolated. It's a great way to take care of ground connections right off the bat.
User avatar
By Chupa
#31639
Philba wrote:You could draw a polygon around all 5 of those pins and then name the poly gnd (or what ever you are calling ground). Click on ratsnest. It will pretty much do it automatically. Set isolate to what ever clearance you want to have.

Usually, I draw the poly for the entire top and bottom layer. Non-gnd traces will get isolated. It's a great way to take care of ground connections right off the bat.

thx for the info. quick question... IF i do a complete board ground like you said do I want to turn orphans on or leave it off?
By busonerd
#31665
Orphans off - otherwise you'll end up with floating unconnected areas that you may not know about.

If you really need a guard ring, I tend to hand draw a ground wire for the guard ring - that way you'll get a DRC flag if you accidentally route something through it.

Cheers,

--David Carne
By Philba
#31695
good point. though I usually visually inspect to ensure there is a guard.
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