www.renesas.com MICROCONTROLLER DESIGN 21 www.power-mag.com Issue 1 2024 Power Electronics Europe The art of noise: how to design to minimise interference In a perfect world, signal margins and power supply voltages are maintained for a safe, stable environment, says Graeme Clark, Principal Engineer, Renesas Electronics In an ideal world, signal voltage margins and signal timing margins are always positive, power supply voltages are always within the operating voltage range, and our environment is completely benign. Unfortunately, we live in the real world. The real world is dirty and noisy with power distribution systems that are not perfect. The supply voltage can drop below the operating voltage, resulting in system malfunction or failure. Switching transients create noise and reduce signal margins, and impedance discontinuities can distort signals, reducing signal operating margins. There is also radiated or conducted noise from internal and external sources and electrostatic discharge and lightning surges which can disrupt or even destroy systems. In addition, thermal stress, mechanical stress and component ageing can cause systems to fail. This article will look at some of these issues and the measures that can be applied to designs to remove or at least minimise some of them. In particular, some features implemented in microcontroller design can mitigate some of these issues. In the past, around 25 years ago, microcontrollers were designed and implemented on 1.0 m or 0.8 m CMOS technology. Today’s devices use significantly more advanced process technology, with line widths orders of magnitude smaller. For example, the latest Renesas RA family microcontrollers are implemented on the latest 40nm and 22nm process geometries. As the transistor size in the latest devices becomes smaller and the transistor switching frequency becomes faster, noise becomes an increasing concern. Figure 1 shows a simplified comparison between a 1.0µm device operating at 8MHz and a modern device operating at 100MHz on a 40nm process. It is clear that the switching time of the 40nm device is much faster and the signals can be faster than the typical noise signal. Noise becomes a larger concern as design moves towards smaller process geometries. Design practices include integrating features to help operate in such an environment, with carefully designed power supply circuits, optimised I/O buffers and specialist protection circuitry. It is still important to minimise any effects in designs as much as possible; once noise enters the device, it is much harder to remove. Noise sources External noise sources are often one of the biggest threats to reliable operation. These can include switching noise from a power supply, noise caused by sparks from industrial machinery or motors, for example. Other sources include induction noise from relays, transformers, buzzers or fluorescent lamps, as well as static discharges, typically, but not exclusively, from the body of users, and of course lightning. Internal noise can come from a wide variety of sources. Current loops on a PCB can be a significant source of radiated noise. If current flows in a closed loop formed by the microcontroller and its I/O signals, as shown in Figure 2, this current Figure 1: Simplified comparison between a 1.0µm device and one implemented in a 40nm process. Figure 2: An example of PCB current loops. loop can act like an antenna and significant noise can be radiated, especially if the current is large. With a badly designed ground plane, where there is a voltage difference between the ground at different points on the PCB, current can flow between these points, which can act like an antenna. Badly designed oscillator circuits can also radiate noise. This is why it is advisable to follow the recommended circuit parameters and ground plan. This is especially important as a badly designed oscillator can inject noise into many parts of the circuit. A badly designed oscillator circuit can also fail. Another key area is the I/O system, especially with multiple high-speed devices on an external bus. A badly designed I/O system, where over- or under-shoot occurs, can cause devices to exceed the electrical specification. This can damage devices over time, causing failure, as well as increased power consumption and noise radiation. Noise on pins Noise can affect any pin on a microcontroller. However, microcontrollers’
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