Wednesday 6 April 2016

pcb design - How's my crystal oscillator layout?



The HC-49 xtal is an 8 MHz crystal, and the radial crystal is an RTC 32.768 kHz crystal. C11, C12 are 22pF, and C13, C14 are 18pF.


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The traces go directly into a micro (a PIC24F.)


I tried to follow Microchip's guidelines on this, but they advised using surface mount xtals and talked about "guard rings" which are alien to me.



Answer



It's a good idea to return the ground for the feedback capacitors directly to the nearest ground pin on the MCU, to reduce emissions.


A guard ring is simply a track connected to ground. It shouldn't have any other connections made to it, as it shouldn't carry any current.


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