4446 Design of Microprocessor-Based Systems
Serial Interface
Dr. Esam Al_Qaralleh
CE Department
Princess Sumaya University for Technology
1
External Interface
• Two ways of interfacing I/O devices
– Serial
• Cheaper
• Slower
– Parallel
• Faster
• Data skew
• Limited to small distances
11-2
External Interface (cont’d)
Two basic modes of data transmission
11-3
External Interface (cont’d)
• Serial transmission
– Asynchronous
• Each byte is encoded for transmission
– Start and stop bits
• No need for sender and receiver synchronization
– Synchronous
• Sender and receiver must synchronize
– Done in hardware using phase locked loops (PLLs)
• Block of data can be sent
• More efficient
– Less overhead than asynchronous transmission
• Expensive
11-4
External Interface (cont’d)
11-5
External Interface (cont’d)
Asynchronous transmission
11-6
External Interface (cont’d)
• EIA-232 serial interface
– Low-speed serial transmission
– Adopted by Electronics Industry
Association (EIA)
• Popularly known by its predecessor
RS-232
– It uses a 9-pin connector DB-9
• Uses 8 signals
– Typically used to connect a modem
to a computer
11-7
External Interface (cont’d)
• Transmission protocol uses three phases
– Connection setup
• Computer A asserts DTE (Data Terminal Equipment) Ready
– Transmits phone# via Transmit Data line (pin 2)
• Modem B alerts its computer via Ring Indicator (pin 9)
– Computer B asserts DTE Ready (pin 4)
– Modem B generates carrier and turns its DCE (Data Communication
Equipment) Ready
• Modem A detects the carrier signal from modem B
– Modem A alters its computer via Carrier Detect (pin 1)
– Turns its DCE Ready
– Data transmission
• Done by handshaking using
– request-to-send (RTS) and clear-to-send (CTS) signals
– Connection termination
• Done by deactivating RTS
11-8
Serial Data Transfer
 Asynchronous v.s. Synchronous
— Asynchronous transfer does not require clock signal. However, it transfers extra bits
(start bits and stop bits) during data communication
— Synchronous transfer does not transfer extra bits. However, it requires clock signal
Frame
Asynchronous
Data transfer
Synchronous
Data transfer
data
Start
bit B0 B1 B2 B3 B4
B5 B6
Stop bits
Parity
clk
data
B0
B1
B2
B3
B4
B5
Baud (Baud is # of bits transmitted/sec, including start, stop, data and parity).
11-9
8251 USART Interface
8251
D[7:0]
RS232
TxD
RD
RD
WR
A0
WR
C/D
CLK
CLK
RxD
TxC
RxC
A7
A6
A5
A4
A3
A2
A1
IO/M
11-10
11-11
Programming 8251
 8251 mode register
7
6
Number of
Stop bits
00:
01:
10:
11:
invalid
1 bit
1.5 bits
2 bits
5
4
3
2
0
Mode register
Baud Rate
Parity enable
0: disable
1: enable
Character length
Parity
0: odd
1: even
1
00:
01:
10:
11:
Syn. Mode
x1 clock
x16 clock
x64 clock
00: 5 bits
01: 6 bits
10: 7 bits
11: 8 bits
11-12
Programming 8251
 8251 command register
EH
IR
RTS
ER
SBRK
RxE
DTR
TxE
command register
TxE:
transmit enable
DTR: data terminal ready, DTR pin will be low
RxE:
receiver enable
SBPRK: send break character, TxD pin will be low
ER:
error reset
RTS:
request to send, CTS pin will be low
IR:
internal reset
EH:
enter hunt mode
11-13
Programming 8251
 8251 status register
DSR
SYNDET
FE
TxRDY:
RxRDY:
TxEMPTY:
PE:
OE:
FE:
SYNDET:
DSR:
OE
PE
TxEMPTY RxRDY TxRDY
status register
transmit ready
receiver ready
transmitter empty
parity error
overrun error
framing error
sync. character detected
data set ready
11-14
Simple Serial I/O Procedures
 Read
 Write
start
start
Check RxRDY
Check TxRDY
No
No
Is it logic 1?
Is it logic 1?
Yes
Yes
Read data register*
end
* This clears RxRDY
Write data register*
end
* This clears TxRDY
11-15
Errors
– Parity error: Received data has wrong error -transmission bit flip due to noise.
– Framing error: Start and stop bits not in their proper
places.
• This usually results if the receiver is receiving data at the
incorrect baud rate.
– Overrun error: Data has overrun the internal receiver
FIFO buffer.
• Software is failing to read the data from the FIFO.
11-16