ST STM32U3 Application Note page 30

9 Recommended PCB routing guidelines
9.1 PCB stack-up
In order to reduce the reflections on high speed signals, it is necessary to match the impedance between the
source, sink and transmission lines. The impedance of a signal trace depends on its geometry and its position
with respect to any reference planes.
The trace width and spacing between differential pairs for a specific impedance requirement is dependent on the
chosen PCB stack-up. As there are limitations in the minimum trace width and spacing which depend on the type
of PCB technology and cost requirements, a PCB stack-up needs to be chosen which allows all the required
impedances to be realized.
The minimum configuration that can be used is four or six layers stack-up. An eight layers boards may be
required for a very dense PCBs that have multiple SDRAM/SRAM/NOR components.
The following stack-ups are intended as examples which can be used as a starting point for helping in a stack-up
evaluation and selection. These stack-up configurations are using a GND plane adjacent to the power plane to
increase the capacitance and reduce the gap between GND and power plane. So high speed signals on top layer
have a solid GND reference plane which helps to reduce the EMC emissions, as going up in number of layers and
having a GND reference for each PCB signal layer improve further the radiated EMC performance.
9.2 Crystal oscillator
Use the application note
9.3 Power supply decoupling
An adequate power decoupling for STM32U3 devices is necessary to prevent an excessive power noise and
ground bounce noise. Refer to
The following recommendations shall be followed:
AN6011 - Rev 1
Figure 13. Four layer PCB stack-up example
Layer_1 (Top)
Layer_2 (Inner1)
Layer_3 (Inner2)
Layer_4 (Bottom)
Figure 14. Six layer PCB stack-up example
Layer_1 (Top)
Layer_2 (Inner1)
Layer_3 (Inner2)
Layer_4 (Inner3)
Layer_5 (Inner4)
Layer_6 (Bottom)
[4] for further guidance on how to layout and route crystal oscillator circuits.
Section 2.2: Power supply schemes
Recommended PCB routing guidelines
Solder Mask
High Speed Signals+GND
Prepreg
GND Plane
Core
Power Plane
Prepreg
High Speed Signals+GND
Solder Mask
Solder Mask
High Speed Signals+GND
Prepreg
GND Plane
Core
Power Plane
Prepreg
Low Speed Signals
Core
GND Plane
Prepreg
High Speed Signals+GND
Solder Mask
for more details.
AN6011
page 30/38