Fujitsu F2MC-16LX Series Application Note page 7

The high-speed current (di/dt) will be supported from the decoupling capacitor only.
Therefore use traces with max. width and min. length between Vss/Vcc pin and DeCap.
After DeCap use thin traces to route the trace to the power supply system.
use EMC filter for short length
µC-supply max. width
high Z low Z
VCC
C
GND
Figure 5: The noise current flows return over the ground line
The exactly use of decoupling capacitors for the Vcc and Vss pins is the basis to reduce the
noise, but also the return way between load and MCU ground is not neglect.
high-Z min. length
choking coil max. width
VCC
C
VSS
I I
slow fast
dt/di
Figure 6: The noise current flows return over the ground line
To ensure an efficient decoupling of the power supply, two capacitors should be placed close
on each Vcc pin. The values of both capacitors should have a relationship of about 1:100.
Typical values are e.g. 100nF (XR7) and 1nF (COG). The accurate value is depended on the
application board, e.g. impedance of PCB or the length of supply lines. However, all of the
DeCaps on the PCB should have the same value.
VDD
C
board
GND
Figure 7: The use of several values of DeCaps lead to undefined resonance frequencies,
that's why all DeCaps should have the same value.
µC
µC
clock
unit
IO-driver
&
core
I
load
L
board
IC1 IC2
min. length
low-Z
max. width
I
supply
HVCC
C
HVSS
I I
crossbar return
R C
Load
L
board
ICn
A
f
7