In this short series of blogs, we have looked at the noise and how it can disturb the operation of microcontroller-based systems. This final blog will examine some “rules” which we can apply to reduce the impact of sound.

We can apply some golden rules to reduce EMC in the design.

• Please keep track of the memory/clock trace to protect it from other signals.
• Consider filtering or buffering external connections.
• Use high-frequency bypass capacitors Vcc/Vss near every device.
• Always keep Vcc/Vss parallel and as close as possible to minimize current loops.
• Use parallel return/signal traces, particularly for signals that are fast or carry a large current.
• Consider using a multilayer board… with unbroken, dedicated Vss/Vcc plans.
• Avoid using a frequency higher than necessary. This will not only reduce noise, but also power consumption.

We’ll look at a few of them in more detail.

To minimize the antenna, we should keep the space between the power supply cables as small as possible. To keep noise levels to a minimum, we should minimize the current flowing to this antenna.

System power supplies are often the biggest source of noise within a system. It is important to design a power distribution system that uses bypass capacitors and EMI filtering. The Antenna Pattern area should be kept as small as possible between the power traces of the PCB so that the current loop (S0, S1, S2) can be reduced to the minimum. This can be achieved by using parallel tracks on Vcc and VSs lines.

The most effective values of capacitors are typically 0.01 uF and 0.1 uF. Combining different capacitor values in a noisy system may be worthwhile to improve noise performance.

Choose the lowest impedance bypass capacitor based on the noise frequency range. Ceramic and tantalum capacitors are suitable for most Microcontrollers. Use an electrolytic capacitance for filtering the PCB input power supply.

It is best to keep track of lengths as short as we can. The routes between the MCUs and other devices are like antennas that cause noise. Instead of using a parallel bus to communicate with external devices, consider using a serial bus such as I 2 C, SPI, or I 2 C. It reduces the amount of noise and also minimizes PCB space and power consumption. Keep in mind that these are high-frequency connections.

It is essential to take special care when designing traces that carry high currents. Avoid placing large current traces close to pins, such as reset or mode pins. These pins could be easily interfered by noise.

The previous blog has already discussed the oscillator, but it is important to pay attention to this area, especially if you are using the low-power crystal 32 kHz for low-power operation. It is important to adhere to the instructions in the hardware manual and the circuit recommendations from your oscillator provider. You can also take advantage of any oscillator specifications provided by the supplier (primarily if you use a 32-kHz oscillator). Another important recommendation is not to allow other signal lines to cross the oscillator tracks, which can cause crosstalk.

Do not feed ground between pins on the MCU.

Some other good layout techniques for microcontrollers include:

• When possible, use wide and short traces for Vcc/Vss.
• Inductive noise can be reduced by reducing the impedance in the power supply circuit.
• When possible, use Vss/Vcc plans. Return currents follow the signal path closely at higher frequencies (typically > 4 MHz). Plan the signal return paths carefully, especially for signals that have high winds.
• Breaking the ground plane will increase signal path impedance.
• Consider using current-limiting resistors at I/O pins.

Renesas uses various design techniques in our RA microcontrollers that minimize external noise. We use methods like the Separated CPU Bus (with Memory) from a Peripheral Bus, which minimizes disturbance of CPU operation. A distributed clock system and a module-stop function on each peripheral are also used. On-chip noise filters are used on sensitive inputs such as resets, oscillator outputs, etc. We also optimize the I/O and power supply designs.

Using advanced process technologies, we can integrate components that would otherwise be external. This helps improve reliability by removing the requirement for external circuitry like oscillators and energy management devices. On-chip oscillators (POR/LVD) and watchdog timers can reduce external noise entering the chip.

The microcontroller has software features that can be utilized to improve the overall system’s immunity to noise. Correct use of watchdog timers will recover applications from EMI-induced crashes. The Independent Watchdog Timer is a feature of many RA microcontrollers. One watchdog timer is clocked by the main clock, and the other from a dedicated oscillator on the chip. Correctly using both watchdog timers will allow the user to keep a watchdog running even when in low-power mode.

It is possible to use the application itself to track its progress and detect any unexpected events. Even with these features and techniques, the best way to ensure safe operation is to remove noise before it enters the chip.

This series of blogs should have given you some good ideas. There is much more to say about the noise that microcontrollers produce. We haven’t even touched on the different types of cables or protection circuits like Transils, Transorbs and Mosorbs. But I do hope you found this brief discussion about the noise that Microcontrollers make helpful. On our website, you can learn more about Renesas’ microcontrollers, including information on their noise performance. You can also access additional documentation regarding noise issues.