Advanced Embedded Systems Design Lecture 8 Serial I/O BAE Fall 2004 Instructor: Marvin Stone Biosystems and Agricultural Engineering Oklahoma State University

Goals for Class Today Questions over reading (Chapter 18, PC-Link) Student presentation?? Serial I/O Demo Keil Compiler –Watch windows –Breakpoints –Code browser Review Target Hardware –Circuitry / cabling –Demo Set assignment

Serial I/O (RS 232) - Pont History –Serial communications needed as an efficient low speed data exchange between data terminal devices (teletype) and data communications equipment (modems) –EIA (telephone industries) standardized the interface in the early 1960s (RS-232-C in 1969) and the CCITT (Comite Consultatif Internatinale de Telegraphie et Telephonie) later standardized V.24 (functional description) and V.28 (electrical specifications). Application –Effective for low data rate communications (<10 kbaud) over short distances (<15 m) –Higher data rates may be used over short distances –Limited to Point-to-point applications –Few number of conductors required (minimum=3 for no handshaking duplex) –

Characteristics Point to point –Single receiver and single transmitter Direction –Simplex – single direction –Duplex – two directions Half duplex – one direction at a time Full duplex – both directions simultaneously Synchronous vs asynchronous –Synchronous – bit stream is transmitted with a clock synchronised at both ends of communication –Asychronous – Frames (characters) are sent as necessary without synchronization, bits are sent with an understood clock rate –DCE = Data Communications Equipment eg. Modem –DTE = Data Terminal Equipment eg. Computer

Packet structure amd baud rate Packet consists of: –Start bit (=1) –Data, 7 or 8, LSB first –May have a parity bit (Even, Odd, Mark or Space) –Stop bits, 1 or 2 (=0) Baud rate –Baud – State changes per second –Common baud rates: 300, 1200, 2400, 9600, 19,200, 38400, and 57,600 baud

Flow control Hardware –RTS/CTS Sender asserts Request to Send (RTS), Receiver asserts Clear to Send (CTS), Sender receives CTS and sends unless CTS is dropped. Software –Xon/Xoff Receiver signaled flow control through the data stream (needs full-duplex) The Xon character [0x11 (decimal 17)] is sent by the receiver to signal the sender to send The Xoff character [0x13 (decimal 19)] is send by the receiver to signal the sender to stop sending

RS 232 pin definitions DTE (computer) device has a male connector (socket with pins) Pin NumberSignal DescriptionDirection 1Carrier Detect (CD)In 2Received Data (RD)In 3Transmitted Data (TD)Out 4Data Terminal Ready (DTR) handshaking signalOut 5Signal Ground Common reference voltage 6Data Set Ready (DSR) handshaking signalIn 7Request To Send (RTS) flow control signalOut 8Clear To Send (CTS) flow control signalIn 9Ring Indicator (RI)In 9-Pin DB9-M male Connector on a DTE device (Looking into the connector)

RS 232 Voltage Levels Signal logic –> +3v = 0 = space –< -3v = 1 = mark

USB – Universal serial bus Next generation of serial I/O for PC to device Bus oriented (127 devices) Connector provides power Uses enumeration to ID devices High data rates –USB Mb/s or 1.5Mb/s –USB Mb/s Isocronous communications capability – “music” Excellent material on web – Development effort –Very significant effort to provide interface on embedded device. –Serial to USB devices attractive – use built in driver on host

Assignment Assemble target hardware / cabling Demo “Hello World” Read Pont, Chapter 19,20,21,22 Tutorial – 30 min –Review USB from a developers perspective