Noise Control

Introduction
A modern microcontroller consists of tens of thousands of transistors potentially radiating interference in a large frequency spectrum. Therefore noise sensitivity and generation must be considered early in the design process.

Microcontrollers (MCUs) allow the design of integrated and flexible controls at an everdecreasing cost. Because of this, they are spreading rapidly among most electronic applications, particularly in noise-sensitive equipment for power control or automotive use. Because a MCU operates with sequential logic, the control of an application can be lost during a disturbance - as with analogue control - but also after a power glitch in the system.

Also, a modern MCU includes several tens of thousands of transistors switching in the MHz range, potentially radiating interference of high magnitude in a large frequency spectrum. Consequently, noise sensitivity and generation must be considered as early as possible in MCU based designs. The major noise receptors and generators are the tracks and wiring on the printed circuit board (PCB), particularly those near the MCU. The first actions to prevent noise problems thus concern the PCB layout and the design of the power supply.

Optimised PCB Layout
Noise is basically received and transmitted through tracks and components which, once excited, act as antennas. Each loop and track includes parasitic inductances and capacitances which radiate and absorb energy once submitted to a variation of current, voltage or electromagnetic flux. An MCU chip itself presents high immunity to, and low generation of, electromagnetic interference (EMI) because its dimensions are small compared to the wavelengths of EMI signals (typically mms compared to tens of cms for EMI signals in the GHz range). So a single chip solution, with small loops and short wires, reduces noise problems.

At the PCB level, the number of possible antennas should be reduced. Special attention should be paid to the loops and wires such as supply, oscillator and I/0, connected to the MCU (see Figure 1). The oscillator loop must be particularly small because it operates at high frequency.
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Figure 1

A reduction of both the inductance and the capacitance of a track is generally difficult. Practical experience suggests that in most cases, the inductance is the first parameter to be minimised. Inductance can be reduced by making the lengths and surfaces of the track smaller. This is done by placing the track loops closer on the same PCB layer, or on top of one another (see Figure 2). The resulting loop area is small and the electromagnetic fields reduce one another. The ratio in order of magnitude relating to the inductance value and the area defined by the wire loop is about 10nH/cm2. Typical examples of low-inductivity wires are coaxial, twisted-pair cables or multiple-layer PCBs with one ground and one supply layer. The current density in the track can also be smaller due to track enlargement or the paralleling of several small capacitances mounted in the current flow.

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Figure 2

In critical cases, the distance between the MCU and the PCB, and therefore the surfaces of the loops between a MCU and its environment, must also be minimised. This can be achieved by removing any socket between the MCU package and the PCB, by replacing a ceramic MCU package with a plastic one, or by using surface mounting instead of dual in line packages.

Power supply filtering
The power supply is used by all parts of the circuit, so special attention must be paid to it. The supply loops must be decoupled to make sure that signal levels and power currents do not interfere. These loops can be separated using star wiring with one node designated as common for the circuit (see Figure 3). The close decoupling capacitance should be placed very close to the MCU supply pins to minimise the resultant loop. It should also be large enough to absorb, without significant voltage increase, parasitic currents coming from the MCU via the input protection diodes. Decoupling of the board can be achieved with electrolytic capacitors (typically 10 to 100uF) as the dielectric used in such capacitors provides a high volume capacitance. However, these capacitors behave like inductances at high frequency (above 10MHz), while ceramic or plastic capacitances keep a capacitive behaviour at higher frequency. A ceramic capacitance of, for example, 0.1 to 1uF, should be used as high frequency supply decoupling for critical chips operating at high frequency.

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Figure 3

The supply circuit must be sized so that its components can absorb energy peaks during supply overvoltages. For example, in a power supply with a capacitor in series between the mains supply and the MCU supply (typically +5V), this capacitance is a short circuit when a voltage spike occurs. The corresponding short circuit current has to be absorbed by a protection Zener diode. Depending on the maximum energy that must be withstood, a standard 0.5W Zener diode may have to be replaced by a 1.3W or 2W Zener diode. Additional filtering with serial resistance or inductance can be included to reduce the influence of voltage spikes and to absorb transients coming from the input supply line.

I/O configuration
In general, the fewer components that surround the MCU, the better the immunity versus noise. A ROMIess solution, for example, is typically more sensitive to, and a bigger generator of, noise than an embedded circuit. If the output buffers are embedded in the MCU, their switching speed has to be controlled to avoid parasitic oscillations when they are switching. A tradeoff between noise and speed has to be found by MCU designers.

I/0 pins which are not used in the application should be preferably grounded or connected via a large impedance (ie.100kW to a fixed potential, depending on the MCU reset configuration. Here, the trade-off is between immunity and consumption. If a current can be forced in an input pin, clamping protection with diodes has to be included in the circuit connected to the pin to divert the current from the MCU structure and to avoid the risk of latch-up.

Shielding
Shielding can help to reduce noise reception and emission, but its success depends directly on the material chosen as shield (high permeability, low resistivity), and on its connection to a stable voltage source including a decoupling capacitance via a low serial impedance (low inductance, low resistance). If the generator of major disturbances is near the MCU board and can be identified as a strong dV/dt generator (such as a transformer or Klystron), the noise is carried mainly by the electrostatic field. The critical coupling between the noise generator and the control board is capacitive. A highly conductive shield (such as copper) creating a Faraday cage around the control board may strongly increase the immunity.

If the strongest source of disturbance is a dI/dt generator (such as a relay), it is a high source of electromagnetic fields. The permeability of the shielding material, such as alloy, is therefore crucial to increase the immunity of the board. Also, the number and size of the holes on the shield should be reduced as much as possible to increase its efficiency. In critical cases, the implantation of a ground plane below the MCU and the removal of sockets between the device and the PCB can reduce the MCU noise sensitivity. Both actions lead to a reduction of the apparent surface and loop between the MCU, its supply, its I/0 and the PCB.

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Last revised: Saturday, 15 May 1999