Design Realization lecture 17

John Canny

10/21/03

Last Time

Electronics: A/D boundary

This time

Processors and networks

Printed-circuit board design

Sensors

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.

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.

Feedback and isolation For both these reasons it’s 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.

Don’t try for too much gain from a single stage amplifier.

Power supply bypass Capacitors (and inductors or resistors) can be

used to isolate component power supplies:

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.

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.

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.

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.

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.

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.

1-wire system DS2480B: a serial

to 1-wire interface.

TXD and RXD areserial data (POL setspolarity)

GND, VDD, VPP arepower lines.

1-wire goes to the bus.

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

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.

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).

Single-chip microcontrollers Microchip’s 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.

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 don’t have

hardware multiply – but new generation of PIC/DSPs have much more arithmetic power.

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).

PIC disadvantages Limited memory for program or data.

Slow arithmetic.

High-level network support missing (e.g. ethernet or USB).

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.

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

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.

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

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.

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.

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.

It’s 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.

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.

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.

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/

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.