How to design a good PCB layout with reduced noise performance. After taking the countermeasures mentioned in this document, it is necessary to conduct a comprehensive and systematic evaluation. This document provides a description of the rl78 / G14 sample plate.
Description of the test board. We recommend the example of layout. The circuit boards that are not recommended to be used are made of the same schematic diagram and components. Only PCB layout is different. Through the recommended method, the recommended PCB can achieve higher noise reduction performance. The recommended layout and the non recommended layout adopt the same schematic design.
PCB layout of two test boards.
This section shows examples of recommended and non recommended layouts. PCB layout shall be designed according to the recommended layout to reduce noise performance. The next section will explain 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.
Differences between recommended and non recommended layouts
This section describes the main differences between recommended and non recommended layouts.
Vdd and VSS wiring. It is recommended that the Vdd and VSS wiring of the board be 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 the non recommended board, the VDD wiring of MCU is connected to the main power supply through jumper J1, and then through filter capacitor C9.
Oscillator problem. The oscillator circuits x1, C1 and C2 on the recommended board are closer to the MCU than those on the non recommended board. The recommended wiring from the oscillator circuit to the MCU on the board is shorter than the recommended wiring. On the non recommended board, the oscillator circuit is not on the terminal of VSS wiring and is not separated from other VSS wiring.
Bypass capacitor. The bypass capacitor C4 on the recommended board is closer to the MCU than the capacitor on the non recommended board. And the wiring from the bypass capacitor to MCU is shorter than the recommended wiring. Especially on non recommended boards, C4 leads are not directly connected to VDD and VSS trunk lines.