Course Introduction. Content 16 pages. Learning Time 30 minutes

Transcription

1 Course Introduction Purpose This course discusses techniques for analyzing and eliminating noise in microcontroller (MCU) and microprocessor (MPU) based embedded systems. Objectives Learn what EMI is and why it should be minimized. Understand decoupling capacitors and how they should be used. Find out how to measure noise currents and near-field emissions. Discover a way to evaluate the effectiveness of EMI prevention measures. Content 16 pages Learning Time 30 minutes

2 Design Goal: Reduce EMI EMI reduction is a goal shared by both the semiconductor experts who design MPUs and other LSI devices, and by the engineers who apply those chips in embedded systems - It includes techniques for decreasing the noise generated by a specific system, circuit, or device that might cause problems in other electronic systems, circuits, and devices

3 Explanation of Terms Anechoic chamber Balun CISPR 25 Core ECU EMI Harness LISN Power supply TEM Cell WBFC A room designed to block radiation from the outside and to minimize reflections off the room s walls, ceiling, and floor A passive electronic device that converts between balanced and unbalanced electrical signals International Special Committee on Radio Interference (CISPR) publication 25: Limits and methods of measuring radio disturbance characteristics for the protection of receivers on board vehicles. CISPR is a sub-committee of the International Electrotechnical Commission (IEC). A microcontroller chip is composed of a core, I/O ports, and power supply circuitry. The core consists of the CPU, ROM, RAM, and blocks implementing timers, communication, and analog functions. Electronic Control Unit Electromagnetic Interference Cables (wires) connecting a board and power supply or connecting one unit in a system to another Line Impedance Stabilization Network Two power supplies are applied to the LSI: Vcc and Vss. The core power supply internal to the LSI is VCL (internal step-down). The Vssbased power supply routed through the LSI is VSL. Transverse Electromagnetic Cell Workbench Faraday Cage

4 Why Is EMI Reduction Important? Example: In an automobile, noise radiated and conducted from the ECU to the radio and its antenna can disturb FM reception Radiated emissions from ECU Antenna FM band radio signals FM station Conducted emissions from power wiring Battery Wiring harness (power line) ECU FM Radio MPU

5 Use a Decoupling Capacitor One way to reduce EMI is to minimize rapid variations in the current that the main power source has to deliver to the MPU or other LSI device By using a decoupling capacitor (Cdc) to provide the fluctuations of the current drawn by the MPU, the current drawn from the power supply will be more stable and, thus, generate less EMI A C C = A + B Package B CPG Module Chip Vss* Vcc* current (measured by 1Ω resistor) Power supply Decoupling capacitor (Cdc) Main clock Measuring point (VDE method)

6 Minimize Inductance of Cdc Wires Decoupling capacitor Power supply 1 2 The external decoupling capacitor (Cdc1) supplies the fast transients in the Vss current flowing into the chip; the main power supply charges this decoupling capacitor gradually Current variations flowing in wiring inductances can produce unwanted induced voltages (V = L x di/dt) Lb, Lbg = 10nH Lowering the inductances (Lb, Lbg) of the wiring to the external decoupling capacitor reduces its impedance, enabling it to achieve better EMI suppression On-chip decoupling capacitors (Cchip, etc.) are very effective at suppressing EMI, even if their capacitance is small.

7 Different Design Approaches EMI decreases when... - the traces to the external decoupling capacitor are redesigned, reducing the values of Lb and Lbg from 10nH to 1nH - a 3,000pF internal decoupling capacitor is built into the chip Lb and Lbg decreased to 1nH On-chip capacitor = 3,000pF

8 Measuring Noise Current Noise current can be evaluated quantitatively (using the VDE method [1Ω], etc.) Efforts to reduce EMI can be evaluated by comparing circuit performance with and without various decoupling capacitors R (1Ω) Vcc Vss LSI VDE method measuring point Chip VDE Measurement Method IC Vcc 49Ω Spectrum Analyzer Vcc Vss i n R=1Ω 50Ω

9 Measuring Near-field Emissions Probe with sensor coil EMV-200 Shielded room Details of EMV-200: Sensor coil (magneticfield sensor) is at the tip of a probe mounted on a robotic arm that moves with precision in three dimensions. Sensor coil is directional, so the probe rotates to accurately detect magnetic fields generated by noise currents, as the arm moves in a pattern close above the board without making contact. Tester accurately maps the magnitude of the circuit s magnetic-field emissions at a specific measurement frequency.

10 Noise-current Measurement 1 Pitch: 2mm Height above board: 5mm VDE method (IEC ) and NF-probe analysis (at 80MHz) Scanned Area MPU: SH7055RF 40MHz (10MHz x 4) program execute Vcc = 3.3V, PVcc = 5.0V Decoupling capacitors 58dB FM Without decoupling capacitors 46dB FM With decoupling capacitors

11 Noise-current Measurement 2 Pitch: 2mm Height above board: 5mm VDE method (IEC ) and NF-probe analysis (at 80MHz) Scanned Area MPU: SH7055SF 40MHz (10MHz x 4) program execute Vcc = 3.3V, PVcc = 5.0V VCL capacitors Decoupling capacitors 58dB 42dB FM Without decoupling capacitors 46dB 24dB FM With decoupling capacitors

12 Noise-current Measurement 3 Pitch: 2mm Height above board: 5mm VDE method (IEC ) and NF-probe Analysis (at 80MHz) Scanned Area MPU: SH7058FCC 80MHz (10MHz x 8) program execute Vcc = 3.3V, PVcc = 5.0V VCL capacitors Decoupling capacitors 58dB 20dB FM With VCL capacitors, but without decoupling capacitors 46dB 22 28dB FM With VCL capacitors and decoupling capacitors

13 Evaluating Supply Decoupling Area under device showing pads for decoupling capacitors Typical decoupling capacitor Current measurement points (Vcc, PVcc, Vss) Power supply connections Pads for inductors (ferrite beads) Top of evaluation board Bottom of evaluation board

14 Evaluation Example Near-field tests* using the evaluation board allow comparisons of levels of RF current in the power supply lines * MPU: SH7055R (40MHz) Measurement frequency: 80MHz No filter components 12 bypass capacitors added Ferrite bead + 12 caps

15 Supply Decoupling Test Results Decoupling capacitors Using a ferrite bead and multiple decoupling capacitors is an effective way to reduce EMI Slit (moat) Ferrite bead Vcc Capacitor Power plane Ground plane (no slit) Noise Current (dbµv) Typical Circuit Board Example Capacitors + ferrite bead Capacitors added -20dB f = 80MHz Target level GND Core current I/O current Number of Decoupling Capacitors

16 Course Summary Importance of EMI reduction Decoupling capacitors EMI measurements Evaluating EMI reduction techniques For more information on specific devices and related support products and material, please visit our Web site: