Communications How to get something to almost talk to something else Raffi Krikorian MAS December 2003

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

Over wires

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

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

Parallel vs. Serial 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)

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

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

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’ ’

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

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

USB Universal Serial Bus 12 Mbit/sec 5m cable length Use –Have USB ICs –Premade boards that have a parallel (DLP- USB245M) and serial (DLP-USB232M) interface that convert to USB

Over the air

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

Don’t build a radio! Get a chip and just send serial data Nordic VLSI nRF401 –Single chip 433MHz – meters Texas Instruments TRF6900 –Mostly single chip 900MHz – meters rfPIC Microcontrollers –PIC with built in FM transmitters

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

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 1 m

802.11b Wireless LAN –2.4 GHz –Internet Protocol based radio –11 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 b development kit for the PIC –

Inter-media

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

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

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

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

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 –Need to implement ARP (RFC826)