22 MICROCONTROLLER DESIGN www.renesas.com Issue 1 2024 Power Electronics Europe www.power-mag.com system pins are particularly sensitive to noise, as they typically control the fundamental operation of the device, and a failure induced by noise here could cause the device to malfunction. Special care should be taken to make sure the possibility of noise interfering with the normal operation of the system pins is minimised. System pins on a typical microcontroller can include the reset pin, the power supply pins, the oscillator pins, and the mode or special function pins. To minimise the chance of noise disturbing these pins, special care should be taken to make sure the power supply pins have solid voltage levels with the required filtering and that the ground plan does not have any current loops. Designers should also make sure the oscillator is placed as close to the chip as possible, that the PCB layout follows the recommendations of the supplier and that the reset pin is protected from fast transient signals. Rules to minimise noise effects: Guard memory/clock traces from other signals; Consider filtering and/or buffering external connections; Always use high-frequency Vcc/Vss bypass capacitors close to every device; Always run Vcc/Vss in parallel and as close together as possible to minimise current loops; Use parallel signal/return traces on PCB, especially for fast signals or traces carrying substantial current; Consider the use of a multi-layer layer board with dedicated, unbroken Vss/Vcc planes; Do not use a higher frequency than required – the correct frequency minimises noise and power consumption as well. Power traces on the PCB The area between the power supply lines should be kept as small as possible to minimise the potential antenna. Designers should also minimise the current flow to this antenna to keep radiated noise to a minimum. The system power supply is often one of the largest sources of internal noise. An effective power distribution system can be designed using bypass capacitors and EMI filters. Keeping the antenna pattern area between the power traces on the PCB as small as possible means that the surrounding current loop area (S0, S1, S2) is reduced. The most effective way to do this is to use parallel tracks for Vcc and Vss lines. A bypass capacitor connected across the power supply lines to every IC will significantly reduce noise. These should be as close as possible to each device. Typically, the most effective capacitor values are 0.01µF to around 0.1µF. In a particularly noisy system, it may be worth combining different values of a capacitor to try to improve the noise performance. Due to differences in the high-frequency characteristics of various types of bypass capacitors, designers should choose the most suitable, lowest impedance capacitor according to the noise frequency range. For most microcontrollers, ceramic and tantalum type capacitors are usually suitable. An electrolytic capacitor can be used for filtering at the PCB power supply input. The number of tracks between devices should also be minimised and the length of each should be as short as possible. Tracks between the microcontroller and other devices act like an antenna which causes noise. For example, the serial bus, such as I2C or SPI, could be used to talk to external devices rather than a full parallel bus. This minimises noise as well as power consumption and PCB space. These are typically high-frequency connections, so users should be sure to keep the traces short. Traces that carry high current in a design need special care. This is why large current traces should not be placed near the oscillator or other system pins, such as the mode or reset pin. These could easily be interfered with by noise. Special care needs to be taken with any external oscillator circuits, especially if the design uses the low power 32kHz crystal for low power operation. It is advised to follow the oscillator circuit layout in the hardware manual and follow the circuit recommendations of the oscillator supplier as well as to take advantage of an oscillator specification service if offered (especially for a 32kHz oscillator design). Other signal lines should not cross the oscillator traces to avoid crosstalk. It is important to keep signal and power supply tracks as far as possible from the oscillator and to not feed the ground between the pins of the microcontroller. A stable oscillator circuit with a large operating margin is much less likely to cause problems. Using an internal oscillator instead also solves a lot of these issues. Layout advice Other good layout practices for microcontroller systems include: The use of wide and short traces for Vcc/Vss wherever possible; Reducing the impedance of the power supply circuit to reduce inductive noise problems; Use Vss/Vcc planes where possible. (At higher frequencies, typically >4MHz, the return current follows as closely to the signal path as possible. Therefore, users should plan signal returns paths carefully, especially for signals with high currents.) The ground plane should not be broken, as this increases signal path impedance; Use current limiting resistors on I/O pins. A typical technique to minimise the issues caused by external noise include separating the CPU bus (with memory) Figure 3: Microcontroller system pins. Figure 4: The relationship between microcontroller and PCB.
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