Design Realization lecture 17

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Design Realization lecture 17

• John Canny
• 10/21/03

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Last Time

• Electronics A/D boundary

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This time

• Processors and networks
• Printed-circuit board design
• Sensors

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The art of electronics

• Practical electronics departs in several ways
  from the ideal model
• There is no perfect wire. Every connection has
  finite resistance and finite inductance. If
  either high current, or high frequency current
  passes through a connection, it will cause a
  voltage drop.
• This is particularly acute for power supply wires.

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Stray capacitance

• There is no perfect connection point. Any two
  conductors near each other form a capacitor. Such
  stray capacitance can be strong between nearby
  conductors on either side of a PC board, or
  between pins on a chip.
• These effects are worst at high frequencies, and
  with high voltages.

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Feedback and isolation

• For both these reasons its a good idea to
  physically separate large signals from small
  ones, especially if the system does large
  amplification (say 100-1000 times) because the
  large signals are controlled by the small ones,
  which can lead to feedback and uncontrolled
  oscillation.
• Dont try for too much gain from a single stage
  amplifier.

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Power supply bypass

• Capacitors (and inductors or resistors) can be
  used to isolate component power supplies

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Printed circuit boards

• The most widely-used connection system for
  electronics.
• Typically epoxy or other plastic board with
  copper conductors.
• Usually two or more layers of conductor.
• Holes are drilled and copper-plated to allow
  component insertion connects between layers.
• There are other prototyping systems for circuits,
  but its often best to go straight to board
  design
• Start dealing with layout issues immediately.
• Avoids difficulties due to the prototyping
  hardware.

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PCB tips

• Main idea is to join the component pins that need
  to be joined, but there are some tips
• Ground and power conductors should be large, as
  straight and direct as possible.
• All conductors should be as short and direct as
  possible (avoid sharp turns which increase
  inductance).
• For two-sided boards, it often helps to prefer
  horizontal runs on one side, vertical on the
  other.

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PCB tips

• Keep large signals away from small ones.
• Place bypass capacitors physically close to the
  pins being bypassed.
• Use sockets for expensive components, or
  components that may need to be replaced.

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PCB systems

• ExpressPCB is a software system for fabricating
  small boards, which can be sent directly to the
  vendor for fab.
• Also draws schematics.
• EX USB and serial sensor boards.

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Processors and networks

• For a device to be intelligent it needs some
  computation (a processor) and some communication
  (a network).
• In the simplest case, the processor only
  provides communication between some sensors and a
  remote computer.
• This is the idea behind the 1-wire system.

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1-wire system

• The wire carries both power and communication,
  hence 1-wire.
• You still need a ground wire, so this is really a
  2-wire system.
• You talk to 1-wire devices directly through an
  interface chip, either serial or USB.

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1-wire system

• DS2480B a serialto 1-wire interface.
• TXD and RXD areserial data (POL setspolarity)
• GND, VDD, VPP arepower lines.
• 1-wire goes to the bus.

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1-wire system

• On a 1-wire bus, you can add A/D and D/A
  converters, thermometers, timers,
• Each device has a unique address. The main
  processor can query the bus to find all the
  devices on it. Then it calls them individually.
• Example

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1-wire advantages

• Simplest hardware, cheap, small devices. Two or
  3-wire bus possible.
• High-level drivers already written (in C, Java
  and MS Com).
• Interfaces for serial or USB.

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1-wire disadvantages

• Slow hard to achieve more than 30kb/s (although
  100k is theoretically possible). Only useful for
  slow sensors temp., light, etc.
• Missing interfaces PWM, r/c servo control
• Low quality software speed and reliability
  issues
• Some software only available for PCs (no source
  code).

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Single-chip microcontrollers

• Microchips PIC chips (also Atmel)
• Virtually everything is on one chip (PIC specs)
• Processor (8/16-bit)
• Program memory (512 32k words)
• Data memory (80 3k bytes)
• A/D converters (up to 16 x 10-bit channels)
• PWM converters (up to 14 x 10-bit channels)
• Standard serial bus up to 6Mb/s (RS232-RS485)
• Two-wire industrial bus (CAN bus, up to 2Mb/s)
• Hardware timers (real-time program threads)
• PIC pin counts range from 8 to 80 pins.

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PIC programming

• Simple to program available C compilers
• Some not reprogrammable, but many have either
  flash or UV-erasable program memory
• Limited program/data memory but seems to match
  typical PIC applications well.
• Arithmetic performance weak, most dont have
  hardware multiply but new generation of
  PIC/DSPs have much more arithmetic power.

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PIC advantages

• Very wide range of applications, low cost (10)
• Often a one-chip solution (plus timing crystal).
• Increasingly sophisticated network support
  (integrated CAN bus).
• Programming is quite easy (C compilers or
  assembly code).

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PIC disadvantages

• Limited memory for program or data.
• Slow arithmetic.
• High-level network support missing (e.g. ethernet
  or USB).

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SOC Systems on a Chip

• More powerful processor ethernet controller,
  e.g. Gridconnect GC-LX-001 (25)
• Communicate with other chips using SPI (Serial
  Peripheral Interface) a short-range serial
  protocol with a speed of 10 Mb/s.
• Can be used with a PIC chip for other I/O.
• Is it really a system-on-a-chip? (board costs
  300, chip has 180 pins!!). Complex supporting
  hardware.

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SBC Single-Board Computers

• Applications that are too large for one or more
  chips fall in the single-board computer realm
  (note a multi-PIC solution will often be cheaper
  than an SBC).
• There are many options here. CPUs range from
  simple 8-bit (68HC11) to Pentium IV PCs.
• Cheapest option (?) the Dallas TINI board
  ethernet, Java, 1-wire, CAN-bus, 100

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SBCs

• If you need an SBC beyond TINI you probably
  already know why ?.
• Too many choices to summarize here, but be sure
  to consider using a PDA
• Price/performance very good compared to OEM SBCs
• You get a screen, pointer and some interface
  buttons
• Performance primitives for Intel chipset
  hardware, useful for cryptography, signal and
  image processing.

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Networking

• Simplest way to communicate with a small CPU
  serial port or RS232.
• One transmit wire, one receive wire, plus ground
  3 wires only are sufficient.
• Flow control wires in both directions (4 total),
  need these if software expects them.
• Traditional RS232 works up to 115kb/s

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Serial Networking

• Slight tweaks on RS232 RS422 and RS485.
• RS422 is a faster version of RS232 individual
  signal wires are replaced by twisted pairs, which
  can be driven faster (10Mb/s, up to 40 ft).
• RS485 is a multi-drop version of RS422 in half
  duplex mode, many nodes can send and receive on
  the same twisted pair (10 Mb/s). RS485 is a true
  network and a good choice for networks of
  simple devices.

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Control Networking

• For industrial control, several bus standards
  exist, including CAN (Controller Area Network)
  and LIN (Local Interconnect Network).
• CAN has a full protocol stack like TCP/IP, for
  packet communication, routing, and error
  recovery. Hardware built into some PICs.
• It is a true multi-drop 2-wire standard, like
  RS485, no hubs are needed.
• CAN is designed to be reliable in very noisy
  environments (automobiles and industry), but is
  rather slow (1 Mb/s) and code is complex.

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USB

• USB or Universal Serial Bus is a popular option
  for external device communication. USB 2.0 is
  very fast (480 Mb/s), USB 1.1 is 12 Mb/s.
• Its a 4-wire system, which is really
  point-to-point. A network is built using hubs.
• Tricky to use for small networks, because of
  topology constraints (limited depth - limited
  nodes in a chain), and the need for many hubs.

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Ethernet

• Like USB, really a point-to-point topology, so
  hubs needed to build networks.
• Ethernet driver chips relatively complex and
  expensive and the protocol (TCP/IP) is complex.
• Still the best way to put a device on the
  network without a supporting computer.

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Irda

• Infrared protocol based on serial communication.
  Many serial modules (e.g. the ones in PICs)
  support Irda.
• 2 Mb/s common.
• Very limited range, high power required.

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Bluetooth

• Very popular wireless standard.
• Class 2 Bluetooth (20m range) available via
  compact (1 x ½) modules for 60-70.
• Communication via fast serial or USB. 768 kb/s
  typical.
• Some support for analog data,including speech.
• See http//www.btdesigner.com/

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Summary

• PC board design and layout issues.
• Processor classes 1-wire, single-chip
  micro-controllers, systems-on-a-chip,
  single-board computers, and PDAs.
• Networks RS232 and its variants, CAN-bus, USB,
  ethernet, Irda, Bluetooth.