Renesas RA Family
RA2 Quick Design Guide
R01AN6060EU0100 Rev.1.00
Page 41 of 44
Sep.14.21
external influences on digital signals can have an acute influence on a digital signal, which can result in an
incorrect logic state at the moment when the data is sampled.
Analog signals are usually quite different. Analog signals may be periodic, but the evaluation of an analog
signal is typically a measurement of voltage over a range instead of logic state. The voltage level of an
analog signal is sampled based on a specific trigger event, and the resulting measurement is processed
using the analog circuitry in the MCU. The accuracy of an analog measurement is directly related to the
accuracy of the sampled voltage level. Any unwanted external influence which may change the voltage level
of an analog input signal, even slightly, can influence the accuracy of the measurement.
Due to the highly multiplexed nature of the I/O pins on Renesas RA2 MCU devices, many I/O pins can be
used for either analog or digital functions. This can result in situations where digital and analog functions may
overlap and result in data errors.
To minimize potential problems between digital and analog signal domains, consider the following guidelines.
•
When assigning I/O pin functions, select pin functions such that analog pins and digital pins are physically
separated as much as possible.
•
Each analog signal should be separated from all other signals as much as possible.
•
PCB routing should isolate each analog signal as much as possible. Avoid routing any other signals,
either analog or digital, in the same area.
•
Ensure that analog supply voltages and analog reference voltages include appropriate AC filters. This
may be in the form of recommended capacitors located near the MCU voltage pin, or appropriate
inductive filters. The goal is to provide voltage supply and reference voltage with little or no voltage ripple.
•
When using dedicated power layers in a PCB design, avoid routing digital signals in the areas of analog
voltages, and avoid routing analog signals in the areas of digital voltages.
For highly sensitive applications, it is highly recommended to evaluate the specific design using simulation
tools to understand the effect that circuit design has on the performance. For example, this may include
applications such as precision sensor designs, or very high-speed digital bus interfaces. Refer to the
“Electrical Characteristics” chapter in the Hardware User’s Manual for the specific requirements for each
peripheral function.
16.2
High Speed Signal Design Considerations
As clock speeds for digital signals increase, the influence of external stimuli on those signals can become
more significant. Some peripheral functions can be classified as "High Speed" digital signals. Additional
design considerations should be made for high speed digital signals.
Crosstalk is a condition where transitions on one signal have an inductive influence on another nearby
signal. When this crosstalk effect is strong enough, the first signal may cause errors on the second signal. To
reduce the effects of crosstalk, use the following general PCB routing guidelines.
•
Provide sufficient space between routed signals on the same routing layer. Generally, keep a minimum of
one trace width space between signals of the same digital group, and a minimum of 3-5 trace widths
space between signals of different digital groups.
•
Provide extra space between clock signals and data signals on the same routing layer. Generally, keep a
minimum of 3-5 trace widths space between clocks and any other digital signals.
•
Avoid parallel routing of digital signals on any adjacent routing layers. If signals must be routed on
adjacent signals layers, try to use only orthogonal crossings wherever possible.
If possible, separate PCB signal layers using power or ground layers between signal layers. The solid copper
of the power or ground layer can act as a "shield" for the digital signals.
Each standardized interface will have specific requirements. To ensure that the PCB is designed to avoid
signal crosstalk problems, we strongly suggest referring to the relevant standards for each interface in the
design.
16.3 Signal Group Selections
Some pin names have an added _A, _B, _C, _D, _E, or _F suffix to indicate signal groups. For RA2 devices,
the suffix can be ignored when assigning functionality. It is safe to select the most convenient pin assignment
for each function signal.
