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AM2634-Q1: Connecting the crystal oscillator to ground

teshima yohei
Prodigy 130 points

Part Number: AM2634-Q1

Should the crystal oscillator ground be connected to VSS?
Or connect to VSSA?

Datasheet: In Table 6-1 of SPRSP74, the POWER column for XTAL_XI and XTAL_XO is VDDS_OSC.
However, "VDDS_OSC" is not written anywhere else.
I think VDDS_OSC is different from VDDA18_OSC_PLL.


Hardware Design Guide: SPRABJ8 3.4 Crystal Placement and Routing states "A ground ring shorted to the local VSS plane should be placed adjacent".
However, looking at the picture, GND is also connected to the VSSA's BGA Pads.
Also, what is a "ground ring"?

Japanese memo

水晶発振器のグランドはVSSに接続するのか?
それともVSSAに接続するのか？

データシート：SPRSP74の表6-1ではXTAL_XIとXTAL_XOはPOWERの欄がVDDS_OSCとなっている。
しかし「VDDS_OSC」はその他の箇所には記載されていない。
VDDS_OSCはVDDA18_OSC_PLLと別物と考えている。


ハードウェアデザインガイド：SPRABJ8 3.4 Crystal Placement and Routingでは「A ground ring shorted to the local VSS plane should be placed adjacent」と記載されている。
しかし絵を見るとGNDはVSSAのBGA Padsにも接続されている。
それと「ground ring」とは何ですか？

  • 0
    TI__Mastermind 18760 points

    Hi Teshima, 

    Regarding the crystal oscillator power: Yes, the pinlist should reference VDDA18_OSC_PLL as the XTAL_XI/XO referenced power. There is no power net on AM263x BGA pinout called VDDS_OSC. This is an error and will be fixed in a future datasheet revision. 

    The overall idea is to make sure that the high-impedance crystal inputs are shielded from other noise sources on the board. Placing the lines away from other switching lines is one way to avoid this coupling. Including a VSS return path between these signals and other adjacent signals is another method. 

    https://www.ti.com/document-viewer/lit/html/SPRABJ8B#GUID-C0D69EC4-E41E-4C8A-A46C-C17883E5B330/GUID-3FED9FAD-8122-4620-B50E-33983F5714ED 

    By "ground ring" I mean the process of  trying to surround these XTAL_XI/XO signal paths with VSS return path. 

    VSS BGA V1, T2 are the only return path BGA near the crystal path. Do you mean how on these EVM designs I shorted VSS to VSSA? VSSA is specifically the ADC return path near the ADC input section of the BGA. In many cases, having the lowest impedance path between VSSA/VSS is superior (from a power integrity perspective) compared to trying to separate out the the VSSA return path, and isolate those signal return paths. But I think that is a separate topic from this crystal routing. 

    Let me know if that helps.

    Thank you,

    -Randy

  • 0 in reply to Randy Rosales
    Prodigy 130 points

    Is it correct to set the power supply of the crystal oscillator to the same voltage as VDDA18_OSC_PLL and connect the GND of the crystal oscillator to VSSA?

    Connect VSS and VSSA with star GND.

    Japanese memo

    水晶発振器の電源をVDDA18_OSC_PLLと同じ電圧にして、水晶発振器のGNDをVSSAに接続で正しいでしょうか？

    VSSとVSSAはスターGNDで接続します。

  • 0 in reply to teshima yohei
    TI__Mastermind 18760 points

    Hi Teshima, 

    Without knowing anything else about the layout in question, I would suggest shorting VSS and VSSA near the AM263x BGA and connecting the crystal return path to the combined VSS/VSSA plane. 

    The crystal input buffer will reference the BGA VSS nets surrounding the XTAL_XI/XO BGA. These are VSS, T2 and  VSS, V1

    Thank you,

    -Randy

  • +1 in reply to Randy Rosales
    TI__Mastermind 18760 points

    Hi Teshima, 

    Is there anything else I can help with here? Any further questions? If not I am going to close this thread. 

    Thank you,

    -Randy