Hot!dsPIC33EP128GS804 - guard ring around crystal

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musculus
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2017/12/07 02:59:41 (permalink)
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dsPIC33EP128GS804 - guard ring around crystal

Hi,
I found information in datasheet that there should be placed a guard ring around crystal (document DS70005258B, link: http://ww1.microchip.com/downloads/en/DeviceDoc/70005258b.pdf , page 17). To do this you need to place a track between pads of this chip. If case is TQFP 44 with pitch 0.8mm - it is possible but not every PCB manufacturer will make this board. If case is TQFP 48 with pitch 0.5mm there is almost impossible to find PCB manufacturer which can make this board. Is this guard ring really needed? Maybe there is other possible solution than this ring?
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    du00000001
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    Re: dsPIC33EP128GS804 - guard ring around crystal 2017/12/07 03:40:08 (permalink)
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    Usually there is some GND pin accompanying the clock pins. This is the point to connect the guard ring (that should also connect to the GND pins of the 2 capacitors that connect to the quartz.
    Regarding the connection to the "global ground": why not use some via ?

    PEBKAC / EBKAC / POBCAK / PICNIC (eventually see en.wikipedia.org)
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    musculus
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    Re: dsPIC33EP128GS804 - guard ring around crystal 2017/12/07 05:33:12 (permalink)
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    Unfortunately, dsPIC33EP128GS804 does not have GND pin directly adjacent to the oscillator pin. About via: I thought about this but datasheet suggests that the track should be between the pin of the oscillator and the neighboring pin.
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    du00000001
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    Re: dsPIC33EP128GS804 - guard ring around crystal 2017/12/07 05:59:26 (permalink)
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    OK - there's 1 pin inbetween.
    What might be feasible: the "vulnerable" pin is OSCI. Making the pin between this and Vss a "slow signal" pin would not be perfect, but some reasonably good compromise.
    Regarding routing traces between 0.5 mm spaced pins: it is possible although PCBs with such tiny structures do not come cheap. Sometimes you have to take compromises.

    PEBKAC / EBKAC / POBCAK / PICNIC (eventually see en.wikipedia.org)
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