Reducing Noise Issues in Microcontroller Systems with RENESAS MCU

In my ideal digital world, of which I often dream, signal voltage margins are always positive, signal timing margins are always positive, power supply voltages are always within the opeRAting voltage range, and our environment is completely benign.

Unfortunately, none of us live in this ideal world, no matter how much I would like to. The real world is dirty and noisy, and the power distribution in our designs is never perfect. The supply voltage can drop below the operating voltage range resulting in system malfunction or failure, switching transients create noise and reduce signal margins, and impedance discontinuities distort signals reducing signal margins.

Then to make matters worse, we have radiated or conducted noise from internal and external sources to contend with, electrostatic discharge and lightning surges interrupt or destroy systems, and thermal stress, mechanical stress, and component aging all can cause systems to fail. In this short series of blogs, I'd like to look at some of these issues and the measures we can apply to our designs to remove or at least minimize some of these issues.

The table shown here shows the two types of Electromagnetic compatibility or EMC. EMC is defined as the ability of electronic devices to function properly in their intended electromagnetic environment.

EMC comprises two distinct types: EMI or electromagnetic interference, and EMS or electromagnetic susceptibility. EMI is generally known as radiated noise and refers to the disturbances caused by noise, whereas EMS is known as Noise Susceptibility and refers to the damage done to the system due to noise.

In the table below, you can see the different types of EMC and the typical sort of phenomena that they cause. The third column lists some typical products that can be affected by EMI and EMS, resulting in disturbance in operation or damage to the system. This list is by no means complete and is meant only to show some typical examples.

Twenty-five years ago, I was dealing with devices like those in our H8 family, which were implemented on what was then quite advanced process technology, using 1.0μm or even 0.8μm CMOS technology. Today's devices use significantly more advanced process technology, the latest devices in our RA microcontroller family being implemented on 40nm technology, with line widths 25 times smaller than the H8 I used to use.

As the transistor size in the latest devices becomes smaller and, probably, more importantly, the transistor switching frequency becomes faster, noise becomes an increasing factor in terms of causing a device to malfunction.

In the diagram above, you can see a simplified comparison between a device using older technology, showing transistor operation with a channel length of 1μm, with the device typically operating from what was then a fast 8MHz Clock, with a slow transistor switching speed and the latest devices operating at up to 200MHz or beyond, with a transistor channel length is 40nm.

In this case, you can see that the switching time is much faster, and for the latest devices, the signal we are trying to deal with can be faster than the noise signal. Therefore, noise has become a larger concern as we move toward smaller process geometry. RENESAS has taken many steps on our devices, designing them with features to help operate in such an environment, with carefully designed power supply circuits and optimized I/O buffers and specialist protection circuitry, but it's still very important to minimize any effects in our designs as much as possible, as at the end, if noise enters the device, it's much harder to remove.

External noise sources are often one of the biggest threats to a system operating reliably, and there can be many sources of noise that we have to manage in our environment. These can include, Switching noise from a power supply, noise caused by sparks from Industrial machinery, Motors, etc., Induction noise from Relays, Transformers, Buzzers, Fluorescence lamps, etc., static discharges, typically from the body of users but also from elsewhere, and of course lightning.

Internal noise can come from a wide variety of sources. Current loops on your PCB can be a significant source of radiated noise. If current flows in a closed loop formed by the MCU and its I/O signals, as shown in the diagram below, this current loop can act like an antenna, and significant noise can be radiated, especially if the current is large.

If you have a badly designed ground plane, where there is a voltage difference between the ground on different parts of the PCB, current can flow between these ground points; this act like an antenna, and noise can also be radiated. Other internal sources of radiated noise can include badly designed oscillator circuits. We always suggest you discuss using a specific oscillator with the oscillator manufacturer to ensure it will have stable performance in your system, and that you follow the recommended circuit parameters and ground plan positioning. This is especially important as not only will a badly designed oscillator inject noise into many parts of the circuit, especially the ground, but a badly designed oscillator circuit can also cause the device to fail.

Another common area where internal noise can be created is in the I/O system, especially if you are using multiple high-speed devices on an external bus. A badly designed I/O system, where care is not taken to avoid over or undershooting, can cause devices to exceed their electrical specification, and this can damage devices over time, causing failure, as well as increase the power consumption of the system and radiate noise into the rest of the system.

While today's microcontrollers, such as the devices in the RA and RX microcontroller families, have circuitry designed to avoid Latch-up, it's still good practice to avoid the potential for this condition. Noise can affect any pin of a typical microcontroller. However, microcontrollers systems pins are particularly sensitive to noise, as these pins typically control the fundamental operation of the device, and a failure induced by noise here could cause the device to malfunction. So special care should be taken to make sure the possibility of noise interfering with the normal operation of the system pins is minimized.

Systems pins on a typical microcontroller can include the rest pin, the power supply pins, the oscillator pins, and the mode or special function pins. To minimize the chance of noise disturbing these pins, we should take special care, making sure the power supply pins have solid voltage levels with the required filtering and that the ground plan doesn't have any current loops, we should make sure the oscillator is placed as close to the chip as possible, and the PCB layout follows the recommendations of the supplier, and that the rest pin is protected from fast transient signals.