Advanced Embedded Systems Design

1
Advanced Embedded Systems Design

• Lecture 8 Serial I/O
• BAE 5030 - 003
• Fall 2004
• Instructor Marvin Stone
• Biosystems and Agricultural Engineering
• Oklahoma State University

2
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

3
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 (lt10
  kbaud) over short distances (lt15 m)
• Higher data rates may be used over short
  distances
• Limited to Point-to-point applications
• Few number of conductors required (minimum3 for
  no handshaking duplex)
• http//www.techtutorials.info/serial.html

4
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

5
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

6
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

7
RS 232 pin definitions

• DTE (computer) device has a male connector
  (socket with pins)

Pin Number Signal Description Direction
1 Carrier Detect (CD) In
2 Received Data (RD) In
3 Transmitted Data (TD) Out
4 Data Terminal Ready (DTR) handshaking signal Out
5 Signal Ground Common reference voltage  
6 Data Set Ready (DSR) handshaking signal In
7 Request To Send (RTS) flow control signal Out
8 Clear To Send (CTS) flow control signal In
9 Ring Indicator (RI) In
9-Pin DB9-M male Connector on a DTE
device (Looking into the connector)
8
RS 232 Voltage Levels

• Signal logic
• gt 3v 0 space
• lt -3v 1 mark

9
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 1.1 - 12Mb/s or 1.5Mb/s
• USB 2.0 - 480Mb/s
• Isocronous communications capability music
• Excellent material on web
• http//www.usb.org/developers/whitepapers/usb_20t.
  pdf
• Development effort
• Very significant effort to provide interface on
  embedded device.
• Serial to USB devices attractive use built in
  driver on host

10
Assignment

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