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The purpose of this document is to help users understand how to design a good PCB layout with reduced noise performance. After implementing the countermeasures described in this document, a comprehensive system assessment is necessary. This document provides instructions for RL78 / G14 sample boards.
Instructions for the test board. This section shows an example of the recommended layout. The boards that are not recommended use the same schematic diagram and components. Only PCB layout is different. Through the recommended method, the recommended PCB board can achieve higher noise reduction performance. Recommended and unrecommended layouts use the same schematic design.
Schematic diagram of circuit around MCU
PCB layout of two test boards.
This section shows examples of recommended and non-recommended layouts. PCB layout should be designed according to the recommended layout to reduce noise performance. The next section explains why the PCB layout on the left side of Figure 1 is recommended. Figure 2 shows the PCB layout around the MCU of the two test boards.
Recommended layout (left) and non-recommended Layout (right)
The difference between recommended and non-recommended layouts
This section describes the main differences between recommended and non-recommended layouts.
VDD and VSS cables. The VDD and VSS wiring of the recommendation board is separated from the peripheral power wiring at the main power inlet. And the VDD wiring and VSS wiring of the recommended board are closer than those of the non-recommended board. Especially on non-recommended boards, the VDD wiring of the MCU is connected to the main power supply via the jumper J1 and then through the filter capacitor C9.
Oscillator problem. The oscillator circuits X1, C1 and C2 on the recommendation board are closer to MCU than those on the non-recommendation board. Wiring from the oscillator circuit to the MCU on the recommended board is shorter than not recommended. On non-recommended boards, the oscillator circuit is not on the terminals of the VSS wiring and is not separated from other VSS wiring.
Bypass capacitor. The bypass capacitor C4 on the recommended board is closer to MCU than the capacitor on the non-recommended board. And the wiring from the bypass capacitor to the MCU is shorter than recommended. Especially on non-recommended boards, C4 leads are not directly connected to VDD and VSS mainlines.
