Renesas RX Family Installation and operating instructions

Application Note
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RX Family
Hardware Design Guide
Introduction
This application note presents notes on board design and layout examples when using RX Family MCUs.
Target Device
RX Family
Contents
1. Board Configuration.................................................................................................................2
1.1 Block Diagram.........................................................................................................................................2
1.2 Component Layout..................................................................................................................................3
1.3 Layer Structure........................................................................................................................................5
2. Board Design...........................................................................................................................7
2.1 Power Supply Pins ..................................................................................................................................7
2.2 VCL Pin ...................................................................................................................................................9
2.2.1 Two-Layer Board.................................................................................................................................11
2.2.2 Board With Four or More Layers.........................................................................................................13
2.3 Reset Pin...............................................................................................................................................14
2.4 Clock I/O Pins........................................................................................................................................16
2.5 Analog Input Pins ..................................................................................................................................21
2.6 Signal Pins with Large Current Flows ...................................................................................................23
2.7 Signal Pins with Rapid Level Changes .................................................................................................23
3. Board Wiring Pattern Examples.............................................................................................24
3.1 Example Two-Layer Board Wiring Pattern............................................................................................24
3.2 Example Four-Layer Board Wiring Pattern ...........................................................................................27
4. Reference Documents...........................................................................................................31
Revision History............................................................................................................................32

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1. Board Configuration
1.1 Block Diagram
Figure 1.1 shows a block diagram of the board configuration used in the design example.
RX Family
Main crystal
oscillator
Sub crystal
oscillator
Power supply
circuit
Reset circuit
3.3 V
Reset signal
Analog input Analog signal
Main clock
Subclock
External power supply
5 V
Figure 1.1 Block Diagram

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1.2 Component Layout
Figure 1.2 and Figure 1.3 show component layout examples. Note that the board thickness is typically
1.6 mm.
The feedback resistor (R2) and damping resistors (R3 and R5) are usually unnecessary. (In Figure 1.2 no
feedback resistors are mounted.) If the oscillator manufacturer stipulates that external feedback resistors or
damping resistors should be added, mount them according to the manufacturer’s specification.
In some cases 0 Ω damping resistors (R3 and R5) or wiring pattern connection can be used.
Analog input
Subclock
Reset circuit
DC jack
Power
supply circuit
RX Family
Main clock
Figure 1.2 Component Layout (Front Side)

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Figure 1.3 Component Layout (Back Side)

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1.3 Layer Structure
Figure 1.4 shows layer structure examples.
Second-layer wiring pattern
GND, signal, and analog GND
First-layer wiring pattern
Power supply, signal, and
analog power supply
Fourth-layer wiring pattern
Signal and analog GND
Third-layer wiring pattern
Solid power layer
Second-layer wiring pattern
Solid GND layer
First-layer wiring pattern
Signal and analog power supply
Two-layer board
Four-layer board
Figure 1.4 Layer Structure Examples

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Figure 1.5 shows an example wiring pattern for a two-layer board.
GND
GND
+3.3 V
+5 V
+3.3 V
+3.3 V
Power IC
RX Family
Reset IC
Main clock
Subclock Analog
GND
Analog power
supply
Single-point
connection to
analog GND
Red: +3.3 V route
Green: GND route
Arrange the paths of the power and GND traces from the power supply
circuit to VCC/VSS as a pair. A typical route is shown as an example.
Also, in this example the paths on the first and second layers are
superimposed on one another to form a pair.
Figure 1.5 Example Wiring Pattern for Two-Layer Board

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2. Board Design
2.1 Power Supply Pins
Connect the power supply pins to GND via a bypass capacitor (decoupling capacitor). Use a capacitor with
good frequency characteristics, such as a ceramic capacitor. The wiring length between each power supply
pin and the bypass capacitor must be equal and as short as possible. In addition, pair the appropriate power
supply pins. For example, the VCC pin, AVCC pin, and VREFH pin should be paired with the VSS pin, AVSS
pin, and VREFL pin, respectively. Refer to the User’s Manual: Hardware of the product used for more details
on pairing pins. The traces for the power supply pins must be wider than traces for other signal lines and
must be connected to the power supply and GND via a bypass capacitor. Figure 2.1 shows example
connections of power supply pins and bypass capacitors.
CC
Bad power supply wiring pattern Good power supply wiring pattern
CCapacitor
Through hole
Wiring trace
VSS
VCC
VSS
VCC
RX Family RX Family
Connections between pins, capacitor, and
through hole are as short as possible.
Not appropriate because the power
supply pins are connected to GND
without an intervening capacitor.
Appropriate because the power
supply pins are connected to GND
via a capacitor.
Digital GND
Digital GND
Digital power
supply
Digital power
supply
Figure 2.1 Example Connections of Power Supply Pins and Bypass Capacitors
Refer to the application notes below for details of the analog power-supply pins. The latest versions and
guides to new products can be downloaded from the Renesas Electronics website.
•RX610 Group: Notes on Analog Power Supply Printed Circuit Board Patterns (R01AN0271EJ)
•RX62N Group, RX621 Group: Notes on Analog Power Supply Printed Circuit Board Patterns
(R01AN0269EJ)
•RX62T Group: Notes on Analog Power Supply Printed Circuit Board Patterns (R01AN0638EJ)
Board design hint
A bypass capacitor connected to the power supply pins can be placed so as to route noise to GND
during MCU operation.

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Figure 2.2 shows an example power supply pin circuit configuration, and Figure 2.3 shows an example
power supply pin wiring pattern
VCC
VSS
RX Family MCU
Connection to
power supply
Figure 2.2 Example Power Supply Pin Circuit Configuration
+3.3 V
GND
Bypass
capacitor Make connection to through
hole via a capacitor.
VSS pin
VCC pin
Figure 2.3 Example Power Supply Pin Wiring Pattern

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2.2 VCL Pin
Connect the VCL pin to GND via a capacitor with a rated capacitance matching that specified in the user’s
manual of the device used. The wiring length between the VCL pin and the capacitor should be 8 mm or less
(4 mm or less if possible).
Note that GND wiring patterns vary depending on the number of board layers. Section 2.2.1 describes two-
layer boards, and section 2.2.2 describes boards with four or more layers.
Board design hint
If the wiring length between the VCL pin and capacitor is longer than 8 mm, the parasitic resistance
increases, decreasing the effectiveness of the capacitor and possibly causing noise from the VCC
pin. When the wiring length between the VCL pin and capacitor is longer than 8 mm, the wiring
pattern should be designed so that the parasitic resistance is less than 1 Ω. Figure 2.4 shows a
formula for calculating the parasitic resistance.
w
lt
Wiring on printed circuit board
Parasitic resistance = R + 2pfL (Ω) < 1 (Ω)
R = lσwt
/(Ω)
f = 20 (MHz) (internal fluctuation in MCU when power is supplied)
L = 0.2l ln
[2l (w+t)
/
{}]
+0.5 (µH)
ωL = 2pfL(Ω)
Figure 2.4 Formula for Calculating Parasitic Resistance
In the following example, the formula shown in Figure 2.4 is used to calculate the parasitic resistance.
Example:
Wiring length (l): 0.008 m (8 mm)
Wiring width (w): 0.00025 m (0.25 mm)
Wiring thickness (t): 0.000035 m (0.035 mm)
Resistivity of copper (ρ= 1 / σ): 0.0000000169 Ω•m
Parasitic resistance = R + 2pfL
= (l / σwt) + 2 p20 (0.2 l (loge (2 l / (w + t)) +0.5))
= 0.01545 + 2 3.1416 20 0.00724
0.9253 (Ω)

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Figure 2.5 shows an example circuit configuration for the VCL pin.
VCL
VSS
RX Family MCU
Refer to the user’s manual of the device
used regarding the rated capacitance of
the capacitor connected to the VCL pin.
Figure 2.5 Example VCL Pin Circuit Configuration