communication - university of washington · 2012-10-23 · cse 466 communication 31 01 01 0 011 0...

Communication

CSE 466 Communication 1

The space of digital communication methods


Unmodulated orbaseband

Modulated or passband

Wired RS-232, I2C, SPI10BASET-Ethernet, USB, Firewire, DVI, HDMI

Cable TV

Wireless Avalanche beepers [analog]?

WiFi, Bluetooth, GSM, LTE, IrDA

Could add analog / digital as another axis

What determines how much info we can transmit?

Shannon’s noisy channel coding theorem Says you can achieve error-free communicate at any

rate up to the channel capacity, and can’t do any better C: channel capacity, in bits / s W: bandwidth amount of frequency “real estate”,

in Hz (cycles / s) S: Signal power N: Noise power


2 2log log 1S N SC W WN N

Capacity of Additive White Gaussian Noise (AWGN) Channel

“Proof” of AWGN Channel Capacity formula


2 2log log

Number of distintguishable states per second is

2

W

WC

S N S NC WN N

S NN

The formula counts the number ofdistinguishable states

SN

f1 f2 f6…

Hand-waving

Units of bandwidth (cycles per sec) ultimately determines frequency bin width…This subtlety is where the simple “proof” is lacking

This formula is why the term “bandwidth” is used as shorthand to mean “communication rate”

Why may need modulation to use all the bandwidth (frequencies) that the channel has

Signal

Modulation Technique WaveformNo modulation (Baseband)

On-Off Keying (OOK)

Amplitude Modulation

Frequency Shift Keying (FSK)

Binary Phase Shift Keying (BPSK)

Direct Sequence Spread Spectrum (DSSS), etc


Communication methods Media and signaling conventions used to transmit data between digital

devices Different physical layers methods including:

wires, radio frequency (RF), optical (IR) Different encoding schemes including:

amplitude, frequency, and pulse-width modulation


Communication methods

Dimensions to consider bandwidth – number of wires – serial/parallel speed – bits/bytes/words per second timing methodology – synchronous or asynchronous number of destinations/sources arbitration scheme – daisy-chain, centralized, distributed protocols – provide some guarantees as to correct communication


Bandwidth Serial

Single wire or channel to transmit information one bit at a time Requires synchronization between sender and receiver Sometimes includes extra wires for clock and/or handshaking Good for inexpensive connections (e.g., terminals) Good for long-distance connections (e.g., LANs) Examples: RS-232, Ethernet, I2C, IrDA, USB, Firewire, Bluetooth, DVI,

HDMI, PCIe Parallel

Multiple wires to transmit information one byte or word at a time Good for high-bandwidth requirements (CPU to disk) More expensive wiring/connectors/current requirements Examples: SCSI-2, PCI bus (PC), PCMCIA (Compact Flash)

Issues Encoding, data transfer rates, cost of connectors and wires, modularity,

error detection and/or correction


Speed

Question: how does speed relate to bandwidth?If you’ve studied Fourier methods, dredge them up now…


Speed

Serial low-speed, cheap connections

RS-232 1K–20K bits/sec, copper wire medium-speed efficient connections

I2C 10K-400K bits/sec, board traces IrDA 9.6K-4M bits/sec, line-of-sight, 0.5-6.0m

high-speed, expensive connections USB 1.5M & 12 M bytes/sec; USB2 480M bytes/sec; USB3 5Gbits / s Ethernet 1.5M-1G bits/sec, twisted-pair or co-axial Firewire 12.5-50M bytes/sec DVI (single link) ~4Gb / s; HDMI 10Gb / s PCIe v3.0 1GB / s per lane; 16 lane slot = 16GB/s

Each PCIe lane is 2 differential pairs, one pair for each direction (4 wires total) Full duplex bidirectional link

Parallel low-speed, not too wide

SCSI-2 10M bytes/sec, 8 bits wide PCI bus, 250M bytes/sec, 32 bits wide PCMCIA (CF+), 9-10M bytes/sec, 16 bits wide


Speed

Issues length of the wires (attenuation, noise, capacitance)

other factors affecting bandwidth, i.e. highest freq that wire will pass

connectors (conductors and/or transducers)

environment (RF/IR interference, noise)

current switching (spikes on supply voltages)

number and types of wires (cost of connectors, cross-talk)

flow-control (if communicating device can’t keep up)


Timing methodology

Asynchronous fewer wires (no clock) no skew concerns synchronization overhead appropriate for loosely-coupled systems (CPU and peripherals) common in serial schemes

Synchronous clock wires and skew concerns no synchronization overhead can be high-speed if delays are small and can be controlled appropriate for tightly-couple systems (CPU and memory/disk) common in parallel schemes


Timing methodology

Issues clock period and wire delay

synchronization and skew

encoding of timing and data information

handshaking

flow-control

power consumption


Number of devices communicating

Single source – single destination point-to-point cheap connections, no tri-stating necessary

Single source – multiple destination fanout limitations addressing scheme to direct data to one destination

Multiple source – multiple destination arbitration between senders tri-stating capability is necessary collision detection addressing scheme priority scheme fairness considerations


Arbitration schemes Daisy-chain or token passing

devices either act or pass to next fixed priority order as many wires as devices fairness issues

Centralized request to central arbiter central arbiter implements priority scheme wires from/to each device can be costly can be dynamically changing priority/fairness

Distributed no central arbiter common set of wires (or ether) observed by all devices fixed priority/fairness scheme

Wired communication

Today we’re focusing on wired communication We talked about RS-232 earlier because it was needed

for the lab…now we’ll look at other wired communication schemes



Serial communication schemes

RS-232 (IEEE standard) serial protocol for point-to-point, low-cost, low-speed applications for PCs

I2C (Philips) TWI (Atmel) up to 400Kbits/sec, serial bus for connecting multiple components

Ethernet (popularized by Xerox) most popular local area network protocol with distributed arbitration

IrDA (Infrared Data Association) up to 115kbps wireless serial (Fast IrDA up to 4Mbs)

Firewire (Apple – now IEEE1394) 12.5-50Mbytes/sec, consumer electronics (video cameras, TVs, audio, etc.)

SPI (Motorola) 10Mbits/sec, commonly used for microcontroller to peripheral connections

USB (Intel – followed by USB2.0 and USB3.0) 12-480Mbits/sec, isochronous transfer, desktop devices

Bluetooth (Ericsson – cable replacement) 700Kbits/sec, multiple portable devices, special support for audio


Serial Peripheral Interface

Common serial interface on many microcontrollers

Simple 8-bit exchange between two devices Master initiates transfer and generates clock signal

Slave device selected by master

One-byte at a time transfer Data protocols are defined by application

Must be in agreement across devices


SPI Block Diagram 8-bits transferred in each direction every time Master generates clock MOSI: “Master Out Slave In”; MISO: “Master In Slave Out”

Connect MOSI to MOSI and MISO to MISO Very clean terminology, unlike “TX” and “RX” which are easy to confuse

Slave Select (SS) used to select one of many slaves Terminology varies:

Instead of SS, “Chip Select” (CS) Instead of MOSI and MISO, SIMOD and SOMI

SPI: point to point and daisy chain


End of lecture


Configuration details to watch out for CPHA (Clock PHase) aka ~CKPH (MSP430 terminology)

= 0 or =1, determines when data goes on bus relative to clock

CPOL (Clock POLarity) aka CKPL (MSP430) =0 clock idles low between transfers =1 clock idles high between transfers

This leads to 4 SPI clock modes


Mode CPOL/CKPL CPHA/ ~CKPH0 0 01 0 12 1 03 1 1

NB: on this slide,~ means negation,i.e. same as overbar

Takeaway message: make sure master and slave are configured thesame way!

SPI properties Pros

Simplest way to connect 1 peripheral to a micro Fast (10s of Mbits/s, not on MSP) because all lines

actively driven, unlike I2C Clock does not need to be precise Nice for connecting 1 slave

Cons No built-in acknowledgement of data Not very good for multiple slaves Requires 4 wires

3 wire variants exist…some get rid of full duplex and share a data line, some get rid of slave select



Inter-Integrated Circuit Bus (I2C)

Supports data transfers 10 kbit / s slow mode 100 kbit / s standard mode 400 kbit / s fast mode 1 Mbit /s fast mode plus 3.4Mbit / s high speed mode

Philips (and others) provide many devices microcontrollers with built-in interface A/D and D/A converters parallel I/O ports memory modules LCD drivers real-time clock/calendars DTMF decoders frequency synthesizers video/audio processors


+Vcc

device1

device2

devicen

SCL

SDA

Inter-Integrated Circuit Bus (I2C) Modular connections on a printed circuit board Multi-point connections (needs addressing) Synchronous transfer (but adapts to slowest device) Similar to TWI (Two-Wire Interface) on Atmegas Pull up resistors pull lines high Devices on bus go from “high impedance” to ground to generate a low on bus 1 Master (generates clock, initiates communication) Up to 112 slaves (7 bit IDs, 16 reserved)


SDA

SCL

START STOP

Serial data format

SDA going low while SCL high signals start of data SDA going high while SCL high signals end of data SDA can change when SCL low SCL high (after start and before end) signals that a data bit can be read


SDA

SCL

1 3 4 5 6 7 8 ack2

Byte transfer

Byte followed by a 1 bit acknowledge from receiver Open-collector (open drain) wires

sender allows SDA to rise receiver pulls low to acknowledge after 8 bits

Multi-byte transfers first byte contains address of receiver all devices check address to determine if following data is for them second byte usually contains address of sender


Ethernet

Inspired by early wireless network: “Aloha” network from U. Hawaii Local area network

“Classic”: 10Mbps serially on shielded co-axial cable “Switched”: 100Mbps, 1000Mbps, 10,000Mbps

Developed by Xerox in late 70s still most common LAN

High-level protocols to ensure reliable data transmission CSMA-CD: carrier sense multiple access with collision detection


Ethernet

RJ-45

BNCBayonetNavyConnector

10: 10 Mbps100: 100 MbpsBASE: Baseband signaling

(not modulated RF, like the 100s MHz signals on your TV cable)5: 500m max range2: 200m max rangeT: Twisted pair


0 1 0 1 0 01 1 0

Serial data format

Manchester encoding signal and clock on one wire (XORed together) "0" = low-going transition "1" = high-going transition

preamble at beginning of data packet contains alternating 1s and 0s 10MHz square wave for 6.4us….allows rcv to synch clock to tx preamble is 64 bits long: 10101. . . 01011

Extra 1 signals Start Of Frame (SOF)


preamble 8 bytes: 7x10101010 + 10101011

destination address (6 bytes)

source address (6 bytes)

Type/length (2 bytes)

data (46-1500 bytes)

Checksum [CRC!] (4 bytes) computed from data

Ethernet packet

Packet size: 64 bytes (min!) to 1518 bytes + 8 bytes of preamble


Arbitration

CSMA/CD “Carrier Sense Multiple Access with Collision Detection”

Wait for line to be quiet for a while then transmit detect collision average value on wire should be exactly between 1 and 0 if not, then two transmitters are trying to transmit data

If collision, stop transmitting wait a random amount of time and try again if collide again, pick a random number

from a larger range (2x) and try again Exponential backoff on collision detection

“Random exponential backoff” or “binary exponential backoff” Key innovation in Ethernet…Bob Metcalf’s thesis at Harvard

Try up to 16 times before reporting failure

CANBus


CANBus

Controller Area Network Developed by Bosch in 1983 Common in cars, robots, etc Multi-master broadcast serial bus Not full duplex Differential physical layer Non-Return to Zero (NRZ) line coding Bit rates to 1Mb / s for cables < 40m Longer cables can be used at lower data rates


http://en.wikipedia.org/w/index.php?title=File:CAN-Bus_Elektrische_Zweidrahtleitung.svg&page=1

From http://marco.guardigli.it/2010/10/hacking-your-car.htmlAnd http://www.picoauto.com/automotivetopics/canbus.html

What is NRZ?

Because of NRZ, CAN uses “bit stuffing”: inserts a bit of opposite polarity after 5 consecutive identical bits

000000 and 111111 are errors


RZSelf-clocking

http://en.wikipedia.org/wiki/File:RZcode.png

NRZNot self-clockingNo “rest” state

http://en.wikipedia.org/wiki/File:RZcode.png

“Open collector” connections to bus

Output goes between Hi-Z and ground Need an external pull up resistor Unlike a tristate-able output, this can sink current but not source current Good for connecting devices with different voltages Good for making busses (OK to connect multiple open collector outputs to

the same wire)


http://en.wikipedia.org/wiki/File:OpencollectorV3.png

CANBus Each message includes

a message ID that encodes priority Msg ID must be unique!! Type of data + sending node Including message deadline leads to higher bus utilization

up to 8 data bytes If two devices transmit simultaneously, message with more dominant

ID (more zeros) overwrites less dominant IDs: “priority-based bus arbitration”

“Wired AND” bus (0 is “dominant,” 1 is “recessive”) If bit a node writes is different than bit it reads collision

CSMA/BA Carrier Sense Multiple Access / Bitwise Arbitration Prioritized arbitration Delay-free for highest priority node Good for real-time prioritized systems


CANBusEach CAN node includes Host processor (interprets messages, decides what to send, …) CAN Controller (HW w/ synchronous clock)

Collects received data in a buffer, generates interrupt for his processor Transmits bits from host processor onto bus

Transceiver Adapts between differential bus signals and single-ended signals used by CAN

controller


http://ww1.microchip.com/downloads/en/devicedoc/21801e.pdf

EtherCAT

“Ethernet for Control Automation Technology” Targeted at same apps as CANBus Ethernet with

Short update times (cycle times) Low communication jitter Low hardware costs

Not currently very well-known, but Used in Willow Garage PR2 robot!


EtherCAT based!

How it works Master/Slave, Master/Master, and Slave/Slave (via Master)

supported Master side: conventional Ethernet MAC HW

i.e., plug an EtherCAT network into the back of your laptop! Need alternate driver SW

Slave side: custom hardware Ethernet packets ingested & regenerated by slaves

(This would not be possible in classic bus-style Ethernet)

Slave can extract or insert “EtherCAT datagrams” into the data portion of the Ethernet packet

Slave has to replace Ethernet CRC if it adds an EtherCAT datagram to the Ethernet Frame


Master:

How it works To master, it looks like there is just one Ethernet device out there (even if

there are multiple EtherCAT slaves) In each slave, processing is done by dedicated hardware

Ensures fast, real-time behavior “Telegrams” processed directly “on the fly”

Many EtherCAT datagrams fit in a single Ethernet packet Many devices can be addresses in a single EtherCAT datagram Frame overhead is amortized over many messages, improving net efficiency


Slaves:

EtherCAT Datagrams


Research examples using these commsschemes---Pretouch sensing


Seashell Effect Pretouch


Moviesshown here

HW ArchitectureIntegrating w/ Willow Garage PR2 Motor Control Board


EtherCATASIC

FPGA

Our sensor HW: mic, op-amp, micro w/ ADC, SPI interfaceTo integrate our new sensor into PR2 robot, replace PPS sensor, use SPIPicoBlaze soft microcontroller implemented in FPGAProgram PicoBlaze in Pico asm to talk to our new sensor over SPI

even though byte-level SPI is standard, at a higher level our dataformat is different, so we need to reprogram the SPI masterto change from PPS sensor to our mic sensor

PicoBlazeSoft processor from XilinxWhat is it? Configuration of FPGA gates to implement a microcontroller within FPGA fabricSpecs 8 bit SPI master peripheral 2x 8bit timers to allow easy comms timing 512 byte double-buffer for sending sensor data to computerWhy soft processor? Easier to program than FPGA Faster to reprogram: seconds vs minutes Safe: cannot brick the MCB with a bad PicoBlaze program…can easily brick MCB

with bad FPGA codeWhy PicoBlaze? Free license from Xilinx to use on Xilinx FPGAs Very small Simple, predictable timing (2 clock cycles per instruction)


End



Universal Serial Bus Connecting peripherals to PCs

Ease-of-use Low-cost Up to 127 devices Bus provides power (500mA @ 5V = 2.5W) Max transfer rates: 12Mb/s (USB 1.0), 480 Mb/s (USB 2.0),

5Gb/s (USB 3.0) Variable speeds and packet sizes Full support for real-time data for voice, audio, and video Protocol flexibility for mixed-mode isochronous data transfers

and asynchronous messaging PC manages bus and allocates slots (host controller)

Can have multiple host controllers on one PC Support more devices than 127


USB Peripherals


USB

Tree of devices– one root controller


USB Data Transfer

Data transfer speeds Low is <0.8v, high is >2.0v differential 480Mb/sec, 12Mb/sec, 1.5Mb/sec Data is NRZI encoded (data and clock on one wire) SYNC at beginning of every packet


NRZI Encoding

NRZI – Non-return to zero inverted Toggles signal to transmit a “0” and leaves the signal unchanged

for a “1” Also called transition encoding Long string of 0s generates a regular waveform with a frequency

half the bit rate Long string of 1s generates a flat waveform – bit stuff a 0 every 6

consecutive 1s to guarantee activity on waveform


NRZI Encoding (cont’d)


USB Data Transfer Types

Control Transfers: Used to configure a device at attach time and can be used for

other device-specific purposes, including control of other pipes on the device.

Bulk Data Transfers: Generated or consumed in relatively large and bursty quantities

and have wide dynamic latitude in transmission constraints. Interrupt Data Transfers:

Used for timely but reliable delivery of data, for example, characters or coordinates with human-perceptible echo or feedback response characteristics.

Isochronous Data Transfers: Occupy a prenegotiated amount of USB bandwidth with a

prenegotiated delivery latency. (Also called streaming real time transfers)


USB Packet Format

Sync + PID + data + CRC PID; Packet IDentifier

Handshake packets Token packets Data packets

Sync: 8 bits (00000001) PID: 8 bits (packet id – type) Data: 8-8192 bits (1K bytes)

CRC: 16 bits (cyclic redundancy check sum)

Other data packets vary in size May be as short as only 8 bits of PID


USB Protocol Stack

FTDIUSB chipimplementsright side

Communicatesto physicaldevicethrough SPI

Extensions

USB to go Allows hand held device to be a USB master

USB 2.0 USB 3.0


End of lecture


communication - university of washington · 2012-10-23 · cse 466 communication 31 01 01 0 011 0...

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