RL78/G14
Recommended PCB Layout for Reducing Noise
Figure 3.2 Recommended Layout (Left) and Non-recommended Layout (Right)
3.3
Differences between Recommended Layout and Non-recommended Layout
This section introduces the main differences between the recommended layout and the non-recommended layout.
3.3.1
Wiring of VDD and VSS
The VDD and VSS wiring of the recommended board are separated from peripheral power supply wiring at the
entrance of the main power supply. And the VDD wiring and VSS wiring of the recommended board are closer to each
other than the non-recommended board. Especially on the non-recommended board, the VDD wiring of the MCU is
connected to main power supply through jumper J1 before filter capacitor C9.
Reason: It is better to separate the MCU power supply wiring and peripheral power supply wiring. In this way, it can
avoid the noise into the MCU through peripherals. The MCU power supply wiring should be connected to main power
supply after connecting to filter capacitors. Otherwise, the filter capacitors will not work effectively.
3.3.2 Oscillator
Concerns
Oscillator circuits X1, C1, and C2 on the recommended board are closer to the MCU than the non-recommended one.
The wiring from oscillator circuit to the MCU on the recommended board is shorter than the non-recommended one.
On the non-recommended board, the oscillator circuit is not at the terminal of VSS wiring and not separated from other
VSS wiring.
Reason: Long wiring may have an antenna effect to catch noise. If noise enters the clock pins, clock waveform may be
deformed. This may cause program failure or runaway. Also if the potential difference is caused by the noise between
the oscillator VSS level and the MCU VSS level, the correct clock will not be input to the MCU.
3.3.3 Bypass
Capacitor
Bypass capacitor C4 on the recommend board is closer to the MCU than the non-recommended one. And the wiring
from the bypass capacitor to the MCU is shorter than the non-recommended one. Especially on the non-recommended
board, the leads of C4 are not directly connected to the VDD and VSS trunk wiring.
Reason: Longer wiring will generate larger impedance for noise signal. Longer wiring may resist the noise current
through the noise return circuit. Also long wiring between the bypass capacitor and the MCU may become a noise
antenna.
R01AN1876EC0100 Rev. 1.00
Page 6 of 14
Feb. 28, 2014
