Advanced Embedded Systems Design

Lecture 8 Serial I/O BAE 5030 - 003

Fall 2004Instructor: Marvin Stone

Biosystems and Agricultural EngineeringOklahoma 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)

– http://www.techtutorials.info/serial.html

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

DCE(Modem)

DTE(Teletype)

DCE(Computer)

DTE(GPS)

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

7 or 8 data bits

Sto

p bi

t1

or 2

Par

ity b

it

Sta

rt b

it

1-LS

B

8-M

SB

Data Word(8 bits shown)

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 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)

1 2 3 4 5

6 7 8 9

RS 232 Voltage Levels

• Signal logic– > +3v = 0 = space

– < -3v = 1 = mark

Space Mark Space

Vol

tage3

25-3

Logic “1"

Logic “0"

InputLevels

15-5

Logic “1"

Logic “0"

-25

-15

5OutputLevels

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

Assignment

• Assemble target hardware / cabling• Demo “Hello World”• Read Pont, Chapter 19,20,21,22• Tutorial – 30 min

– Review USB from a developers perspective