Communications How to get something to almost talk to something else

1. CommunicationsHow to get something to almost talk to something else Raffi Krikorian MAS.863 1 December 2003

2. Getting stuff from A to B • Over wires • Traces on a PCB, wire from peripheral to computer… • Over the air • Optical pules, radio waves… • Inter-media • Putting it all together

3. Over wires

4. Wiring two PICs together (v1.0) • Parallel connection • As many wires as “simultaneous” bits • To transmit a 0xF, bring the lower four wires high, & the rest low

5. Wiring two PICs together (v2.0) • Serial connection • Using minimal number of wires to transmit “in time” • To transmit 0xF (low bit first), send high for 4 time units, then 4 low

6. Really fast Usage of large numbers of pins Requires bigger ICs Large number of wires/traces Space requirements on board (imagine modella) Large bundle of wires if off board Minimum number of pins (1 to 3) and wires Requires inter-device agreements and synchronization Speed (baud rate) Bit framing Requires extra software or hardware (USART) Parallel vs. Serial

7. Synchronous Serial • Clock on one wire, and synchronized data on other • Usually used to connect microcontrollers/processors to peripherals • SPI (Serial Peripheral Interface) -- 1 wire for each dir + 1 clock wire • I2C (Inter IC) -- 1 wire for bidirectional data (direction is handled by protocol) + 1 clock wire

8. Asynchronous Serial • Remove the clock wire • Need way to synchronize clocks due to clock drift • Agree on a speed • Use start bit to start running clocks • Transmit/Receive data • Stop clocks

9. Asynchronous Serial Framing • Agree on how the bytes/bits are to be sent • 8N1 : 8 data bits, no parity bit, 1 stop bit • Parity bits are the check against error • Stop bit indicate how long the line has to be quiet between bytes • Transmit least significant bit (LSB) first • Above transmitting b’11010110’

10. Serial standards and ICs • RS-232 • MAX202 (pref. 203) • 12V levels • 15m max cable length • P2P between devices • RS-485 • LTC1481 • 5V differential signals • Kms max cable length • Can be used as multi-drop between devices

11. Serial from the IC • Hardware USART good • PIC16F876 has it built in • Can use external peripheral • and use some other wired communication from microcontroller/processor to peripheral) • “Bit bang” the serial line • Rely on a compiler to do this work

12. USB • Universal Serial Bus • 12 Mbit/sec • 5m cable length • Use http://www.dlpdesign.com/usb/ • Have USB ICs • Premade boards that have a parallel (DLP-USB245M) and serial (DLP-USB232M) interface that convert to USB

13. Over the air

14. Radio • Modulate a carrier frequency with data • AM modulation takes a carrier frequency and plays with its amplitude • FM plays with the instantaneous frequency of the carrier • AM is relatively simple • but most “natural noise” is AM

15. Don’t build a radio! • Get a chip and just send serial data • Nordic VLSI nRF401 • Single chip 433MHz • 200 kbps @ 100 meters • Texas Instruments TRF6900 • Mostly single chip 900MHz • 38.4 kbps @ 100 meters • rfPIC Microcontrollers • PIC with built in FM transmitters

16. Infra-red • Use IR transmitter (diode) and IR receiver (photoresistor) • Hook it up like a LED and just turn it on and off • Problems with surrounding noise and base illumination levels

17. Don’t rebuild IrDA! • Unless you are doing something really simple • Hook IR transceiver to a USART • MAX3120 • Single chip IRDA modulator : hook one end to USART other end to IR transceiver • 115 kbps @ 1 m

18. 802.11b • Wireless LAN • 2.4 GHz • Internet Protocol based radio • 11 Mbit/sec @ 30 m • Spoof a PCMCIA bus and just wire yourself into a Orinoco Gold card • Saves you from having to get the analogs right • IOSoft 802.11b development kit for the PIC • http://www.iosoft.co.uk/wlan2.php

19. Inter-media

20. Internet Protocol • Unreliable packet based protocol • IPv4 = RFC791 • Addressing • IPv4 addresses are 4 bytes wide • Obtain address either statically or via DHCP • Not necessarily globally unique due to NAT (waiting for IPv6) • Routing

21. IPv4 packet • The beginning of every IP packet starts with 20 bytes of this header (the options and the padding are optional) • “Data” immediately precedes this header

22. User Datagram Protocol (RFC768) • Unreliable packet layer on top of IP • UDP allows for fast access to send packets • Packets are small • Packets are “one-offs”, you send the bytes and are done • No flow control, no congestion control, no guarantees • Trivial as an implementation

23. Transmission Control Protocol (RFC793) • Creates a virtual “stream” on top of the Internet • Reliable, in-order packets • Uses bandwidth responsibly • Non-trivial for implementation • Still areas of research to make it efficient, determine whether it is a “stable” system

24. Using IP • Using IP protocols • Serial line (over RS-232 or RS-485) or simple radio • Probably the best and easiest bet • SLIP (RFC1055) as packet framing or, • PPP (RFC 1661) as a control protocol • Ethernet (CS8900) and 802.11 • Need to implement ARP (RFC826)