1. Design Realization lecture 17 John Canny 10/21/03

2. Last Time  Electronics: A/D boundary

3. This time  Processors and networks  Printed-circuit board design  Sensors

4. 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.

5. 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.

6. 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.

7. Power supply bypass  Capacitors (and inductors or resistors) can be used to isolate component power supplies:

8. 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.

9. 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.

10. 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.

11. 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.

12. 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.

13. 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.

14. 1-wire system  DS2480B: a serial to 1-wire interface.  TXD and RXD are serial data (POL sets polarity)  GND, VDD, VPP are power lines.  1-wire goes to the bus.

15. 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

16. 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.

17. 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).

18. 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.

19. 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.

20. 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).

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

22. 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.

23. 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

24. 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.

25. 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

26. 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.

27. 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.

28. 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.

29. 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.

30. 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.

31. 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/ http://www.btdesigner.com/

32. 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.