- 1 - Universität Dortmund Alma Mater Studiorum Facoltà di Ingegneria, Bologna Marco Benocci, PhD...

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Universität DortmundUniversität DortmundUniversität Dortmund

Alma Mater StudiorumFacoltà di Ingegneria, Bologna

Marco Benocci , PhD – Microcontroller Interfaces – MPHS 2010/2011

Microcontrollers Interfaces& Orcad

Marco Benocci, [email protected]

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Signal processing: math library. Real-time nature: latency, scheduling. Portability: lifetimey constraint (specially when battery-driven).

Microcontrollers Interfaces

Embedded “specialized processor targeted for a specific application”

Photo: http://www.geekzone.co.nz

o Signal re-samplingo Signal generation:

• Periodic: Cosinusoidal, Sinusoidal, Square, Rectangular, Saw tooth, Dirac seq.

• Non-periodic: Noise, Ramp, Step, Dirac.

o Operators : Multiplication, Division, Sine, Cosine, Arc sine, Arc cosine, Absolute, Square root, Natural logarithm, Binary logarithm or base 2 logarithm, Common logarithm or base 10 logarithm, Exponential, Power, Randomo Windowing : Bartlett, Blackman, Hamming, Gauss, Hann, Kaiser, Welcho Vectors: Power, Minimum, Maximum, Negate, Zero padding, Copy, Partial Convolution, Convolutiono Filtering: FIR, least mean square, interpolation, IIRo Transforms (Complex Fast Fourier Transform, Complex inverse Fast Fourier Transform, Real to complex Fast Fourier Transform) o IMA/DVI ADPCM

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Microprocessor (Central Processing Unit, CPU)• First design in late-1960s. MP944 implemented the F-14A Central Air Data

Computer. Intel 4004 in 1971.• General purpose (i.e. Intel Core, PowerPC).• Features: ALU + sequencer + register (no memory or peripherals). Three

basic tasks: perform mathematical operations, move data between memory locations and follow sets of instructions.

• The job of starting up the computer specifically involves the bootstrap loader.

• The assembler translates semantic instructions developed by designers into a language the CPU can use.

Microcontroller (MCU)• The microcontroller is the integration of a number of useful functions into a

single IC package specialized form of microprocessor designed to be self-sufficient and cost-effective.

• Texas Instruments TMS1802 single-chip (4-bit) calculator device was designed in 1971.

• Features: processor + data/program memory + digital IO.• Application areas: automobiles, office machines, toys, and appliances.

Digital Processor: general VS special purpose

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Focus on MCU

Performance Metrics:

• Low Power consumptions: Harvester needs less than 1uWi.e. Atmel PicoPower (165μA/MHz).

• Cheap Price: consumer products.i.e. MC9S08QG4 costs less than 1$ featuring 4 KB FLASH, 256 B RAM.n.b. the package influences the final price due to the soldering.

• Easiness of integration: number and kind of Interfacesi.e. STM32 (ARM Cortex M-3) provides USB Host, I2C, SPI, Ethernet, SDIO, CAN.

• High computational power: MIPS (Million of Instructions Per Seconds)i.e. ARM Cortex A-8: 2,000 MIPS @ 1.0 GHz.

CORTEX • ARM Cortex family is a complete processor core that provides

a standard CPU and system architecture.

• Three main profiles: A profile for high end applications, R for real time and M for cost-sensitive and microcontroller applications.

• The Cortex-M3 provides a standardised microcontroller core which goes beyond the CPU to provide the entire heart of a microcontroller (including the interrupt system, SysTick timer, debug system and memory map).

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Interfaces: policies

Visions

Real-time behavior Efficient, economical

i.e. centralized power supply Bandwidth and communication delay

Inverse relation between volume and urgency Electrical robustness

Single-ended vs. differential signals Fault tolerance

Error detecting & error correcting bus protocols Maintainability Diagnosability Security & Safety

Error detecting and error correcting bus protocols Privacy

Encryption, virtually private

Transfert Modalities

• FIFO Buffer: temporarily store acquired data

• Interrupts: the slowest but most common method

• DMA (Direct Memory Access):

is a system whereby samples are automatically stored in system memory while the processor does something else

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Interfaces: signal acquisition

Analog• Analog To Digital Converter (ADC)• Digital To Analog Converter (DAC)• …

DigitalSubsistem Control

Low speed serial interfaces Programmable Timers GPIO Real time clock Watchdog Timer Host Interface

Connectivity

USB PCI IEEE 802.3 (Ethernet) IEEE 802.11 a/b7g (WiFi) IEEE 802.15 (WPAN) IEEE 1394 (Firewire) Asynchronous Memory Flash Storage Card

Storage

Asynch Memory ATAPI/Serial ATA Flash Storage Card SDRAM/DDR

Data Streaming

Low Speed Serial Interfaces USB 2.0 Full Speed (12Mbps) 10BaseT Ethernet IEEE 802.11b Synch Serial Audio/Data Ports Host Interface IEEE 802.11a/g 100BaseT Ehternet USB 2.0 High Speed

(480Mbps) IEEE 1394 (Firewire) Parallel Video/Data Ports Gigabit Ethernet PCI/ PCI Express

Signal Conditioning

Sensors and ActuatorIEEE1451

Data acquisition system components:

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• The IEEE 1451 standard defines the architecture that achieve to the sensors, instruments and systems to work together with relative ease.

• The IEEE 1451 vision underlines the change of the computer role: the intelligence is distributed over the network. The innovative concept of Smart sensor aims to: move intelligence closer to the point of measurement/control; create confluence of transducers, computation and communication towards common goal; make sensor cost effective to integrate/maintain distributed systems.

IEEE 1451

A TIM contains:• from 1 to 255 transducers (can be a mix of

sensors and actuators);• signal conditioning and processing

electronics;• address logic (or microprocessor) to

implement a standardized Transducer Interface (wired or wireless) defined by IEEE 1451.X (.2, .3, .5, .6, ) between the TIM and NCAP;

• a TEDS.

A NCAP integrates:• a neutral smart transducer object and data

models that allow NCAP to communicate sensor data and information to any network;

• NCAP to NCAP communications (defined by IEEE 1451.1);

• application programming interfaces (API) and a common set to access transducers from a network (defined by IEEE 1451.0).

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Smart sensors is a transducer or an actuator easy to install, maintain, modify and upgrade.

• Integration of extensible Transducer Electronic Data Sheet (TEDS): a memory area inside the sensor where sensor identification information, calibration data, measurement range are stored.

• Simplify the data exchange over the network (standard engineering units).

• Self-identification, self-diagnostic .

• Time aware’ for time stamping and correlation: Triggering and control model defines how channels are accessed.

IEEE 1451.2: Smart Sensor

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• The physics of their operation. One physical principle can be used to measure many different phenomena. e.g. piezoelectric effect can measure force, flexure, acceleration, heat, and acoustic vibrations.

• The particular phenomenon they measure. One phenomenon can be measured by many physical principles. e.g. sound waves can be measured by the piezoelectric effect, capacitance, electromagnetic field effects, and changes in resistance.

• By a particular application. e.g. one could group all sensors together that can be used to measure distance.

• Active VS passive.Passive: the physical phenomena observed modifies some electrical characteristics of the sensor that can be observed supply external power (e.g. RFID).Self-generating sensor: the power is absorbed by the observed physical phenomena and transform in electric power in output (e.g. RFID & Sensing).

• Energy.Middelhoek’s classification energy domain such as electrical, thermanl, radiation, nechanical, magnetic, (bio)-chemical.

• Technology.e.g. MEMS

Sensors: Classification

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FS

Sensors: CalibrationAlma Mater Studiorum

Facoltà di Ingegneria, Bologna

• FSO (Full Scale Output)Upper – Lower [endpoint of output]

• CalibrationRelationship between sensor output e applied physical input

• ErrorMeasured value – true value [% of FSO]

• OffsetSensor output for zero applied input

• HysteresisMax [value approached with decreasing input - value approached with increasing input]

• SensitivityMax deviation of calibration point from straight line [% of FSO]

• AccuracyMax deviation of calibration point from straight line [% of FSO]

• RepeatabilityMax deviation of calibration point from straight line [% of FSO]

• ResolutionSmallest change in the physical variable that results in a detectable change in the sensor output

• Frequency responseChange with frequency of out/in magnitude ratio and phase difference for sinusoidally varying input

• Cross-sensitivitySensitivity of sensor of a variable than the physical quantity under measurement

• StabilityAbility of sensor to reproduce output for identical input and condition over time Vout

Saturation

BiasNon Linearity

Mechanical Misalignements

(Factory)

Bia

s D

rift

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Sensors: Signals

• continuous-time, continuous-valued (real)• discrete-time, continuous-valued (sampled)• continuous-time, discrete-valued (quantified)• discrete-time, discrete-valued (numeric)

Sampled Real

Quantified

Numeric

Time interval definition (t0, t0+t) Nature

(Time interval definition [t0, t0+t]) Features

Peak value (plus):

Peak value (minus):

Peak-Peak:

Mean:

RMS:

Signal “Nature” to Voltage Convertion

• Current to Voltage• Resistance to Voltage• Signal Energy to Voltage• Capacitance to Voltage

"Conditioning" of a signal basically means to manipulate a signal in such a way that it meets the requirements of the next stage for further processing.

translate the sensors output to a selected voltage

modifying the sensors dynamic range to maximize

the accuracy of the data acquisition system

removing unwanted signals

limiting the sensor's spectrum

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Conditioning the FSR

Resistance to Voltage

Thresholding Hardware Interrupt generation

If the sensor's voltage is greater than the threshold, the output of the circuit is maximum (typically 5V).

If the sensor's output is less than the threshold, the output of the circuit is minimum (usually 0V).

FSR

FSR

Force sensing resistors use the electrical property of resistance to measure the force (or pressure) applied to a sensor.

FSR – Force Sensor Resistor

Inseguitore(Av=1)

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ADC

The analog and continuous time signals measured by the sensor and modified by the signal conditioning circuitry must converted into the form a computer can understand.

Aliasing

Impossible to reconstruct fast signals after slow sampling: multiple fast signals share same sampled sequence (mind Harry Nyquist)

-1.5

-1

-0.5

0

0.5

1

1.5

Example: Signal: 5.6 Hz; Sampling: 9 Hz

Equivalent circuit for the sample and hold

Sample and hold circuitry

• The ADC must have a stable signal in order to accurately perform a conversion: the sample and hold circuitry take a snapshot of the sensor signal and hold the value.

•The switch connects the capacitor to the signal conditioning circuit once every sample period.

•The capacitor then holds the voltage value measured until a new sample is acquired.

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Marco Benocci , PhD – Microcontroller Interfaces – MPHS 2010/2011 14

ADC Architectures

There are many different ADC architectures: Successive Approximation (SAR); Sigma Delta (SD or ); Slope or Dual Slope; Pipeline; Flash...as in quick, not memory.

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ADC in MSP430 (SAR 12bit)

Key idea: binary searchSet MSB='1‘ (if too large: reset MSB)Set MSB-1='1‘ (if too large: reset MSB-1)

e.g. successive approximation

1100

1000

1010

1011

t

V

Vin

V-

V-

e.g. quantization noise for sine wave

N = (approximated - real signal) called quantization noise.

Features:• resolution (i.e. 12bit)• maximum conversion rate (i.e. 200 ksps)• sampling periods controller (i.e. software or timers)• on-chip reference voltage generation (i.e. 1.5 V or 2.5 V)• individually configurable external input channels• single-channel, repeat-single-channel, sequence, and repeat-sequence conversion modes• number of storage registers• cross-talking

Resolution12bit

Quantum

Vin

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DAC

PWM signals are often used to create analog signals in embedded applications.

We create a sine wave level with pulse-width modulated (PWM) signals from Timer_B

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UART (universal asynchronous receiver / transmitter) , Goldon Bell 1971 (?)

Asynchronous. UART controller is the key component of the serial communications subsystem of a computer. The UART

takes bytes of data and transmits the individual bits in a sequential fashion. 7- or 8-bit data with odd, even, or non-parity LSB-first data transmit and receive Simple compatibility with RS232 The start bit is always a 0 (logic low), which is also called a space. Cheap: serial transmission of digital information (bits) through a single wire or other medium is much

more cost.f Standard baudrate: 2400, 19200, 57600,115200, 921600…

Focus: reliability The UART usually does not directly generate or receive the external signals used between different items

of equipment. Typically, separate interface devices are used to convert the logic level signals of the UART to and from the external signaling levels.

than parallel transmission through multiple wires.

UART

CTS Clear to Send This line indicates that the Modem is ready to exchange data.

DCD Data Carrier Detect

When the modem detects a "Carrier" from the modem at the other end of the phone line, this Line becomes active.

DSR Data Set Ready This tells the UART that the modem is ready to establish a link.

DTR Data Terminal Ready

This is the opposite to DSR. This tells the Modem that the UART is ready to link.

RTS Request To Send

This line informs the Modem that the UART is ready to exchange data.

RI Ring Indicator Goes active when modem detects a ringing signal from the PSTN.

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Pros• Asynchronous serial devices, such as UARTs, do not share a common

clock.• Compatible with RS232CCons• Each device has its own, local clock. • The devices must operate at exactly the same frequency (baudrate

automatical detection).• Logic (within the UART) is required to detect the phase of the transmitted

data and phase lock the receiver’s clock to this.

UART

RS232C• An old standard (1960), originally intended for connecting computer

equipment (computers or terminals, referred to as DTE) to communication equipment (DCE).

• RS232C is are commonly used in conjunction with UART because they share the same protocol.

• RS232 Voltages are +5..+25V for a logic 0, and -5V..-25V for a logic 1 (Reverse polarity)

DB-25

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I2C

I2C (inter-integrated circuit), Philips1982Philips Semiconductor I2C specification v2.1 (http://www.nxp.com/acrobat_download2/literature/9398/39340011.pdf)

IC (Inter Communication Bus) fabrication process (NMOS, CMOS, bipolar)

Half-duplex, synchronous, multi-master bus Two wires: serial data (SDA), serial clock (SCL) Each device is recognized by a unique address and can operate as either a transmitter

or receiver. A master is the device which initiates a data transfer on the bus and generates the clock

signals to permit that transfer. At that time, any device addressed is considered a slave. Standard-mode, up to 400 kbit/s in the Fast-mode, or up to 3.4 Mbit/s in the High-

speed mode. 10-bit extended addressing for new designs (7-bit addresses all exhausted).

Focus: minimizes interconnections eliminates the need for address decoders and other ‘glue logic’ the multi-master capability allows rapid testing of end-user equipment via external

connections simple design/upgrade

Clones: present restriction of timing and voltage levels, introduction of interrupt signal • Intel SMBus (System Management Bus): speed 10 a 100 kHz• Intel PMBus: speed < 400 kHz

DECT cordless phone base-station.

http://www.nxp.com/acrobat_download2/literature/9398/39340011.pdf

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I2C

Pros• multimaster• no chip select or arbitration logic required• transmission control • “Clock stretching” – when the slave (receiver) needs more time to process a bit, it can pull SCL low. The

master waits until the slave has released SCL before sending the next bit.• “General call” broadcast – addresses every device on the busCons • The number of interfaces connected to the bus is solely dependent on the bus capacitance limit of 400 pF.• Limited to about 10 feet for moderate speeds.• Need external hardware: lines pulled high via resistors, pulled down via open-drain drivers (wired-AND)• Half-duplex

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SPI (Serial Peripheral Interface Bus), found on Motorola's M68HC11 family of CPUs in 2001

• Data is simultaneously transmitted and received. • Master/slave relationship.• Interprocessor communications in a multiple-master system.• 3-pin and 4-pin SPI operation• 7- or 8-bit data length

• 2 data signals:• MOSI – master data output, slave data input• MISO – master data input, slave data output

• 2 control signals:• SCLK – clock• /SS – slave select (no addressing)

• The serial clock line [SCK] synchronizes shifting and sampling of the information on the two serial data lines.• The master places the information onto the MOSI line a half-cycle before the clock edge that the slave device uses to latch

the data.• Four possible timing relationships can be chosen by using the Clock Polarity [CPOL] and Clock Phase [CPHA] bits in the

Serial Peripheral Control Register [SPCR]. • Data rates as high as 1 Mbit per second are accommodated when the system is configured as a master; rates as high as 2

Mbits per second are accommodated when the system is operated as a slave.

SPI

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Pros Full duplex communication Higher throughput than I²C or SMBus Flexibility for the bits transferred (arbitrary choice of message size, content, and purpose) Simple hardware interfacing

o Typically lower power requirements than I²C or SMBus due to less circuitry (including pullups)o Slaves use the master's clock, and don't need precision oscillatorso Slaves don't need a unique address -- unlike I²C or GPIB or SCSIo Signals are unidirectional allowing for easy Galvanic isolation

Cons Requires more pins on IC packages than I²C No in-band addressing; out-of-band chip select signals are required on shared buses No hardware flow control No hardware slave acknowledgment Supports only one master device Only handles short distances compared to RS-232, RS-485, or CAN-bus

SPI

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SPI vs. I2C

For point-to-point, SPI is simple and efficient

Less overhead than I2C due to lack of addressing, plus SPI is full duplex.

For multiple slaves, each slave needs separate slave select signal

More effort and more hardware than I2C

SPI

I2C

SPI Simultaneus transfertAs the register transmits the byte to the slave on the MOSI signal line, the slave transfers the contents of its shift register back to the master on the MISO signal line, exchanging the contents of the two shift registers.

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PWM (Pulse-Width Modulated): communication throw a width-capture of a precise external pulse

i.e. drive servo-motors

PWMAlma Mater Studiorum

Facoltà di Ingegneria, Bologna

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Interface a digital smart sensor

LIS302DL o 3-axis digital accelerometero Range: - ±2g or ±8go Output Interface: I2C or SPI (CMOS)o Max Data rate (ODR): 400Hzo Bandwidth: ODR/2o Turn-on Time: 3/ODRo Sensitivity: 18 or 72 mg/LSB

No external conditioning circuit needfor C/V convertion and voltage translation

Internal 8 bit ADC

Serial Bridge

Advantages provided by Smart sensors(ctrl flags, interrupts, energy duty-cycling, ...)

Capacitive Sensor

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Interface a digital sensor: SPI

1. Init (RW=0; AD5:0=0x20; D7:0=0x47) Enable X, Y, Z axis – DR=400; FS = ±2g | n.b. CTRL_REG 2 does not need deafult value change

2. Read X Low Data (RW=1; AD5:0=0x29) Read D7:0 value

Register Memory Mapping

Serial Bridge

Control Registers

SPI Communication Management

Enable Filter for Free-Fall Wake Up

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Fieldbus (or field bus) is the name of a family of industrial computer network protocols used for real-time distributed control, now standardized as IEC 61158.A complex automated industrial system — such as a manufacturing assembly line — usually needs an organized hierarchy of controller systems to function. In this hierarchy there is usually a Human Machine Interface (HMI) at the top, where an operator can monitor or operate the system. This is typically linked to a middle layer of programmable logic controllers (PLC) via a non-time-critical communications system (e.g. Ethernet). At the bottom of the control chain is the fieldbus which links the PLCs to the components which actually do the work such as sensors, actuators, electric motors, console lights, switches, valves and contactors (Wiki).

Relevant examples:

• CAN: Controller Area Network

• Profibus: Process Field Bus

• TTP: Time-Triggered-Protocol

• FlexRay: designed to be faster and more reliable than CAN and TTP

• MAP: bus designed for car factories

• IEEE 488: designed for laboratory equipment

• EIB: European Installation Bus

Field Busses

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ORCAD/Cadence

• Capture contains extensive parts libraries that may be used to generate schematics that stand alone or that interact with PSpice, or Layout, or both simultaneously.

• The pins on a Capture part can be mapped into the pins of a PSpice model and/or the pins of a physical package in Layout.

• PSpice is a CAE tool that contains the mathematical models for performing simulations, and Layout is a CAD tool that converts a symbolic schematic diagram into a physical representation of the design.

• Netlists are used to interconnect parts within a design and connect each of the parts with its model and footprint.

• In addition to being a CAD tool, Layout also functions as a front-end CAM tool by generating the data on which other CAM.

• Computer-Aided Engineering (CAE) tools cover all aspects of engineering design from drawings to analysis to manufacturing.

• Computer-aided design (CAD) is a category of CAE that is related to the physical layout and drawing development of a system design.

• Electronic design automation (EDA) reduce development time and cost because they allow designs to be simulated and analyzed prior to purchasing and manufacturing hardware.

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Printed Circuit Board (PCB)

A PCB consists of two basic parts:

a substrate (the board) printed wires (the copper traces).

The substrate provides a structure that physically holds the circuit components and printed wires in place and provides electrical insulation between conductive parts.

A common type of substrate is FR4, which is a fi berglass–epoxy laminate. Substrates are also made from Tefl on, ceramics, and special polymers.

During manufacturing the PCB starts out as a copper clad substrate as shown in Fig. 1-2. Arigid substrate is a C-stage laminate (fully cured epoxy). The copper cladding may be copperthat is plated onto the substrate or copper foil that is glued to the substrate.

A substrate can have copper on one or both sides. Multilayer boards are made up of one or more single- or double-sided substrates called cores. A core is a copper-plated epoxy laminate. The cores are glued together with one or more sheets of a partially cured epoxy.

PCB

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Layout is used to design the PCB by generating a digital description of the board layers for photoplotters and CNC machines, which are used to manufacture the boards.

There are separate layers for :

• routing copper traces on the top, bottom, and all inner layers; • drill hole sizes and locations; • soldermasks; • silk screens; • solder paste; • part placement; • board dimensions.

PCB Design Process: OrCAD Layout

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Layout format files (.MAX)Layout uses .MAX extension while you are designing your board. Gerber FilesWhen you are ready to fabricate your board, Layout postprocesses the design and converts it into a format that the photoplotters and CNC machines can use (Gerber files).A separate Gerber file is created for each of the layers (distinguible by the extension).

Assembly layersThese files are used for automated assembly of a finished board. • Solder-paste layer. It is used to make a contact mask for selectively applying solder paste

onto the PCB’s pads so that components can be reflow soldered to the board. There may be a solder-paste layer for the top side of the board (.SPT) and one for the bottom side (.SPB).

• Assembly layer, which contains information for automatic component placement machines (pick-and-place machines) as to the part type, its position, and its orientation on the board. As with the soldermask, there may be an assembly layer for the top side of the board (.AST) and one for the bottom side (.ASB).

OrCAD Layout: File format

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Basic procedure for generating a schematic in Capture and converting the schematic to a board design in Layout:

1. Start Capture and set up a PCB project using the PC Board wizard.

2. Make a circuit schematic using OrCAD Capture.

3. Use Capture to generate a Layout netlist and save it as a .MNL file for Layout.

4. Start Layout and select a PCB technology template (.TCH file).

5. Save the Layout project as a .MAX project file.

6. Use Layout to import the .MNL netlist into the .MAX file.

7. Make a board outline.

8. Position the parts within the board outline.

9. Autoroute the board.

10. Run the postprocessor to generate fi les used to manufacture the PCB.

Overview of the Design Flow

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Industry Standards

How big and what shape should the board outline be?

Where should the parts be placed and in what order?

What kind of layer stackup should be used?

How wide and far apart should the traces be routed?

What grounding and shielding techniques should be used?

Is there a “right” way to do it, and who says so?

There are several standards related to PCB design to solve these questions. The organizations below set standards that may be guides, rules for certification, or even laws:

• Institute for Printed Circuits (IPC-Association Connecting Electronics Industries)• Electronic Industries Alliance (EIA)• Joint Electron Device Engineering Council (JEDEC)• International Engineering Consortium (IEC)• Military Standards• American National Standards Institute (ANSI)• Institute of Electrical and Electronics Engineers (IEEE)

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Performance classesPCBs can fall into any of three end-use performance classes.

● Class 1, General Electronic Products, includes general consumer products (televisions, electronic games, and personal computers) that are not expected to have extended service lives and are not likely to be subjected to extensive test or repairability requirements.

● Class 2, Dedicated-Service Electronic Products, includes commercial and military products that have specifi c functions such as communications, instrumentation, and sensor systems, from which high performance is expected over a longer period of time. Since these items usually have a higher cost they are usually repairable and must meet stricter testing requirements.

● Class 3, High-Reliability Electronic Products, includes commercial and military equipment that has to be highly reliable under a wide range of environmental conditions. Examples include critical medical equipment and weapons systems. They typically have more stringent test specifi cations and possess greater environmental robustness and reworkability.

Producibility levelsThe three producibility levels are:

● Level A, general design—preferred complexity

● Level B, moderate design—standard complexity

● Level C, high design—reduced producibility complexity

PCB Issues

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Minimum recommended spacing

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Ground Issues

Typical signal and return connection schemes.Left) parallel connectedRight) Series connected

Signal and return connection parallel and series schemes in Layout

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• Solve routing problems• If the signal path is relatively close to the return path, the return signal will automatically flow through the GND plane directly below the signal trace (in DC circuits, current follows the path of least resistance).

• AC currents will follow the path of least impedance and, particularly on PCBs, the path of least inductance

In a typical PCB the power distribution system contains one or more power and ground planes.

The power and ground planes are like very wide traces (have little inductance) and are usually adjacent to each other (high capacitance).

Ok for the power distribution systemProblem occurs in high-speed digital systems when gates switch from one state to another: switching noise.

Common ground plane

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Switching Noise

Because the power and ground planes are not superconductors there is a drop in voltage between the supply pins of the gate and where power is connected to the PCB (same for the return plane).

Remember that there is always some amount of resistance and inductance even on the so-called ground plane.

Rail collapse: the drop in the positive railGround bounce: the rise in ground potential.

The primary purpose of bypass capacitors in digital circuits is to promote a stable PCB power distribution system and prevent rail collapse and ground bounce. The bypass capacitors act as lowpass fi ters and short out power supply transients (noise) before they get to the amplifliers.

the supply voltages across the PCB while the gate is switching

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Split power and ground planes

The solution to the problem of digital noise being injected into analog circuitry through thesupply planes is to segregate the analog components from the digital ones and eliminatecommon return paths. Segregating the components is straightforward; the components arephysically placed in different places on the board.

a) Continuous planeb) Split planec) Moated planed) Isolated, coutinuous plane

- 1 - Universität Dortmund Alma Mater Studiorum Facoltà di Ingegneria, Bologna Marco Benocci, PhD...

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