I'm designing a PCB using the STM32F103CB, and have a question on the oscillator layout.

I've learned that I'm supposed to do a local GND plane for the oscillator circuit, and that the local GND plane should connect to the global GND plane in a single point. However, when I look into ST:s Oscillator design guide, it says the following (on page 42): "The oscillator ground plane should be connected to the nearest microcontroller ground."

I've implemented this as follows: enter image description here

enter image description here

It's a 4 layer design, and C201 is decoupling for the 3.3V supply that's next to the oscillator pins.

The connection between the local oscillator GND plane and the global GND plane is now through the MCU, instead of using a via down to the adjacent global GND plane on the next layer. My question is, is this a bad idea? Am I misinterpreting what the ST design guidelines says? Should I have a connection both to the MCU GND and a via down to the global GND plane?


1 Answer 1


The way I have had the best success with is to essentially Island the oscillator and make its only ground connection at the ground pin closest to the oscillator, having the via that connects it to the main ground plane be on the opposite side of this ground pin, I would aslo reduce the size of this island to only directly about the oscillator,

In this case I would also look into islanding the second layer plane directly beneat the capacitors and crystal and dropping a via ring around it to essentially cage it in on all sides to that one pin,

This makes a sheild for the crystal on all sides, so any fast current spikes cannot easily couple in,

  • \$\begingroup\$ Could you please illustrate with an imatges? \$\endgroup\$ Feb 10 at 18:53

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