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About AM3358 Clock Circuit design

Hoang Manh
Intellectual 380 points

Hi all,

I'm designing a schematic using AM3358ZCZD72 device.

I'm confusing about clock circuit design when compare the reference design and Advisory 1.0.30: OSC0 and OSC1: "Noise immunity improved when Crystal Circuit is connected directly to PCB Digital Ground."

My current design is similar to AM335x Starter Kit's schematic: VSS_OSC0 and VSS_RTC are still connected to isolated ground signals, please refer my attated file for more detail.

Should I connect VSS_OSC0 and VSS_RTC to DGND according to Advisory 1.0.30?

Thanks and best regards,

Hoang

  • 0
    TI__Guru**** 393215 points

    Hi Hoang,

    Yes, it's better to tie these to DGND.

  • 0 in reply to Biser Gatchev-XID
    Intellectual 380 points
    Hi Biser,

    Thank you for the quick response.

    But please help me understand more clearly,

    So, these GND_OSC0 and GND_OSC1 should be connected to DGND through 0 Ohm resistors (or through an inductor) and when doing layout, these GND_OSC planes are still isolated from DGND? Or I just have to change GND_OSCx signals to DGND?

    Best Regards,
    Hoang
  • 0 in reply to Hoang Manh
    TI__Guru**** 393215 points

    If you are redesigning the layout it's best to connect them directly to DGND. The 0 Ohm resistors is just a workaround for existing layouts.

  • 0 in reply to Biser Gatchev-XID
    TI__Guru 61035 points
    The VDDS_OSC power supply decoupling capacitor should never be connected to VSS_OSC0 or VSS_RTC. It should have always been connected to VSS rather than VSS_OSC0 or VSS_RTC.

    VSS_OSC0 and VSS_RTC are not power supply terminals. These terminals were provided as Kelvin ground connections to their respective crystal circuits. However, we have determined the Kelvin ground allows more noise to be coupled into the crystal circuit than connecting the crystal circuit directly to the same PCB digital ground plane where all VSS terminals are connected.

    Regards,
    Paul