ECE 371- Unit 11
Introduction to Serial I/O
TWO MAJOR CLASSES OF SERIAL
DATA INTERFACES
• ASYNCHRONOUS SERIAL I/O
- USES “FRAMING BITS”
(START BIT AND STOP BIT(S))
WITH EACH CHARACTER
Asynchronous Serial Data Format
idle
Start
bit
lsb
msb
Parity
bit
Data value transmitted is 0 0 1 1 1 0 1 1
Parity bit shown represents even parity option
Stop
bit(s)
• SYNCHRONOUS SERIAL I/O
- USES SINGLE “SYNCH”
CHARACTER AT BEGINNNG OF
DATA STREAM
OR
- USES SPECIAL CLOCK SIGNAL TO
DEFINE BEGINNING OF EACH NEW
CHARACTER
Synchronous Serial Data Transmission Format
Synch char Header
information
Data char
Data char
Data char
Data char
etc
OR
Data char
Data char
Data char
Data char
Data char
Data char
CLASSES OF SERIAL DATA APPLICATION
• COMPUTER TO PERIPHERAL DEVICE
• COMPUTER TO COMPUTER
Half-duplex Serial Data Interface Between Computer and
Peripheral Device
Serial data
Computer
System
Handshaking
Signals
Ground
Peripheral
Device
Full-duplex Serial Data Interface Between Computer and
Peripheral Device
Serial data
Computer
System
Handshaking
Signals
Ground
Peripheral
Device
Serial Data Communication
Between Computers, Using Modems
Computer
(DTE)
RS-232
Standard
Interface
Modem
Telephone
System
Modem
DCE
Data Communication Equipment
DTE = Data Terminal Equipment
RS-232 is a Standard Interface between DTE and DCE
Computer
(DTE)
RS-232
Standard
Interface
Laptop COM1 Port
CPU
Asynchronous
Serial Interface
Inside PC
Modem
Telephone Line
RS-232 Standard Interface
• Defines a 25-bit Connector
• Defines the function of each pin
• Defines EIA RS-232 Logic Levels
– Serial Data Physical Representation
• Logic “1”:
• Logic “0”:
-25 volts ≤ V1 ≤ -5 volts
+5 volts ≤ V0 ≤ +25 volts
EIA RS-232 Signals of Interest
DTE
TXD
RXD
GND
RTS
CTS
DCD
DSR
DTR
RI
DCE
Primary RS-232 Signals
•
•
•
•
•
•
•
•
•
RxD -- Received Data
TxD --Transmitted Data
RTS – Request to Send
CTS – Clear to Send
DTR – Data Terminal Ready - DTE Ready
DSR – Data Set Ready – DCE Ready
RI – Ring Indicator
DCD – Data Carrier Detect
GND -- Ground
EIA RS-232 Interface
COM Connector on PC
SERIAL COMMUNICATION INTERFACE (SCI) ON THE
MC9S12DP256B MICROCONTROLLER
• ASYNCHRONOUS SERIAL I/O
• COMPATIBLE WITH RS232 STANDARD
SCI Data Formats
See p. 211, text, for def.
Serial Data Format
•
•
•
•
•
•
•
Idle (no data being transmitted) = “1”
Start = “0”
Stop =“1”
Each data bit is T seconds in length
Baud = 1/T
Least Significant Bit Sent First
Most Significant Bit Sent Last
ASCII Code for Letter A = 0b01000001 = 0x41
ASCII Code for Letter B = 0b01000010 = 0x42
ASCII Code for Digit 1 = 0b00110001 = 0x31
Idle
A= 1 1 1
B= 1 1 1
1 = 1 1 1
Start LSB
0
0
0
1 0
0 1
1 0
MSB Stop
0
0
0
0
0
0
0
0
1
0
0
1
1
1
0
0
0
0
1
1
1
1
1
1
ASCII Character Set
7-bit Characters
d6 d5 d4 d3 d2 d1 d0
0 0 x x x x x
0 1 x x x x x
1 0 x x x x x
1 1 x x x x x
Control Characters
Numbers and Punctuation
Upper Case Characters
Lower Case Characters
Ctrl J = Line Feed
Ctrl M = Carriage Return
Ctrl Q = XON
Ctrl S = XOFF
Ctrl Z = End of File
2 Asynchronous Serial Ports
SCI0 and SCI1
SCI0 – Uses Port S[1:0]
SCI1 – Uses Port S[3:2]
RS-232 TTL Level Converters
TTL – RS-232 Conversion
Channel 0
RTS = CTS
DCD = DTR = DSR
RI – No Connection
Channel 1
Data Lines Only
TTL – RS-232 Conversion
DC to DC Power Supply