RL78/G14
Recommended PCB Layout for Reducing Noise
R01AN1876EC0100 Rev. 1.00
Page 7 of 14
Feb. 28, 2014
3.3.4
Wiring of RESET Pin
Reset circuits R1, D1, and C6 on the recommend board are closer to the MCU than the non-recommended one. And the
wiring from the reset circuit to the MCU is shorter than the non-recommended one. On the non-recommended board,
the leads of R1, D1, and C6 are not directly connected to the MCU VDD and VSS wiring, but connected to the main
power supply VDD and VSS wiring.
Reason: The reset signal initializes the internal MCU state. If fine noise-like pulses pass through the reset signal wiring,
the MCU may not completely initialize (partially initialize). It is better to shorten the wiring of the reset circuit to
reduce the noise effect.
3.3.5
Wiring of REGC Pin
The REGC circuit C3 on the recommend board is closer to the MCU than the non-recommended one. And the wiring
from the REGC circuit to the MCU is shorter than the non-recommended one. On the non-recommended board, the
VSS of C3 is connected to VSS lines before the MCU bypass capacitor C4.
Reason: The long wiring between C3 and the MCU REGC pin may become a noise antenna. The REGC should be
connected to VSS wiring after the bypass capacitor, otherwise the bypass capacitor effect will be weakened.
3.3.6
Wiring of TOOL0 Pin
The TOOL0 circuit R2 on the recommend board is closer to the MCU than the non-recommended one. And the wiring
from the TOOL0 circuit to the MCU is shorter than the non-recommended one. On the non-recommended board, the
VDD of R2 is not connected to the MCU VDD, but connected to the main power VDD wiring.
Reason: The long wiring between R2 and the MCU TOOL0 pin may become a noise antenna. If the potential difference
is caused by noise between TOOL0 VDD and MCU VDD level, it may affect the MCU working status.
3.4
Block Diagram of the Test Board
The test board will perform some sample functions under noise test. The MCU operation status can be shown by the
two groups of LEDs (Red and Yellow).
After the MCU resets, all LEDs will blink for 5 seconds. Then the two groups of LEDs will light from “0000
2
” to
“1111
2
” by “1
2
”. One group is controlled by the main loop; the other group is controlled by timer interrupt routine.
These two groups of LEDs will blink synchronously.
Figure 3.3 shows the block diagram of the test board.
