1. Background info
I'm designing a board for an STM32F767ZI microcontroller. This microcontroller has a primary oscillator for the
SYSCLK (overall system clock) and a secondary oscillator for the
RTCCLK (real-time clock).
I selected the following crystal for the primary oscillator:
NX3225GD-8MHZ-EXS00A-CG04874 [DigiKey: 644-1391-1-ND]
And the following crystal for the secondary oscillator:
NX3215SA-32.768KHZ-EXS00A-MU00525 [DigiKey: 644-1386-1-ND]
I selected the same crystals as those on the NUCLEO-F767ZI board from STMicroelectronics.
2. Recommendations from STMicroelectronics
The chip manufacturer recommends to provide a separate local GND-plane underneath each crystal. This GND-plane must be tied to the nearest GND-pin on the chip. I got that from Application Note AN2867. The following figure is from that document:
You can see two crystals on the figure: X1 and X2. Each has its own local GND-plane, separated by a gap from the overall board GND-plane.
3. How to apply?
I wonder how I can apply these recommendations on the STM32F767ZI chip:
Please note the two oscillators on the left side:
RCC_OSC32 for the real-time clock and
RCC_OSC for the general system clock. Also note the 9 GND pins (named
VSS) on the chip.
Unfortunately, there is only one (!) GND pin on the left side of the chip. How can I properly apply the recommendations from STMicro? No matter what I try, I end up with one of the local GND-planes stretching out to reach a GND-pin far away ... probably not what the manufacturer had in mind.
On the advice of @isdi, I looked for the gerber files of this NUCLEO-F767ZI board. With a gerber viewer, I discovered that the general GND-plane runs under the oscillators as if they were no different from any other component. They definitely don't have their own local GND-plane.
Why is STMicro not following their own advice?