11 Serial 0515

Section 17.1

Objectives
Upon completion of this chapter, you will be able to:
List the advantages of serial communication over parallel
communication
CMPE328 Microprocessors Explain the difference between synchronous and
(Spring 2007-08) asynchronous communication
Define the terms simplex, half duplex, and full duplex and
diagram their implementation in serial communication
Serial Interfacing Describe how start and stop bits frame data for serial
communication
Compare the measures baud rate and bps (bits per second)
By Dr. Mehmet Bodur Describe the RS232 standard
Compare DTE (data terminal) versus DCE (data
communication) equipment
Describe the purpose of handshaking signals such as DTR,
RTS, and CTS
Describe the operation of a USART and use and 8251 IC

CMPE328 Spring 2007-08 Dr.Mehmet Bodur, EMU-CMPE 2

Section 17.1 Section 17.1

Basics of Serial Communication Simplex vs Duplex
Microprocessors are based mostly on 8-bit In Simplex Mode data flow Transmitter Receiver
is in one direction only.
registers. Thus, their fastest I/O is 8-bit In Half Duplex Mode data
parallel ports. flows in one direction, at a Transmitter 0 1 Transmitter
given time, Receiver
Receiver 1 0
But, wiring cost of a long distance A protocol and switches
connect the devices both to
communication is very expensive if you receive and also to transmit.
carry 8-wires. In Full Duplex mode,
transmitted data and
Remedy is to transfer data serially in bits received data goes Transmitter Transmitter

instead of in bytes or words. simultaneously through two Receiver Receiver
channels.

CMPE328 Spring 2007-08 Dr.Mehmet Bodur, EMU-CMPE 3 CMPE328 Spring 2007-08 Dr.Mehmet Bodur, EMU-CMPE 4


Asynchronous Transmission
Asynchronous vs Synchronous
How can the receiver get each data bit when it is
Start and Stop Bits
delivered by the transmitter? Start bit is required to synchronize the internal
In synchronous transmission an explicit clock clock of receiver.
signal describes the instants of valid data. Stop bit is required to test the clock frequency.
A single data bit is sent at each clock.
Only DATA is transmitted, internal clock is
generated locally. Voltage levels are 0 and 5V (TTL signal)
Minimum three signal lines required for full duplex,
Receive-DATA, Transmit-DATA, and CLOCK. TTL DATA
In asynchronous transmission clock is derived internal
using a-priory parameters and a start bit. clock
Start D0 D1 D2 D3 D4 D5 D6 D7 Stop
Transfer rate known, internal clock starts to pulse with 0 1 0 1 0 0 1 1 0 1
the start bit, at the known transfer rate. Internal clock stops with the time
Internal clock starts Bits are sampled at
Ony two signal lines required for full duplex the rising edge of stop bit. If Stop bit is not
with the start bit. high, it gives framing error.
Receive-DATA and Transmit-DATA the internal clock
bit#: 7 6 5 4 3 2 1 0
Data is 01100101b =65h. It is ASCII “e”


Computers may be Asynchronous
connected to
eachother at COM
port or TTL signal
Communication System Data Transfer Rate
levels as well. 1488 and 1489 COM port
CPU BUS
MODEM
Transmit
DRIVER
Transmit
Both devices shall know the data transfer
System UART Phone Line
Bus rate of the communication to synchronize
Receive Receive the internal clocks correctly.
TTL
Level Signals
RS232
Level Signals
Data transfer rate is measured in BAUD
Baud = bit/second
Phone BUS
Telecommunication
Media MODEM
DRIVER
CPU
(including start, stop, data, parity etc.)
Transmit UART
Transmit System
Bus
kilo Baud = 1000 Baud. (not 1024 Baud)
Receive Receive Mega Baud = 1000 000 Baud
RS232 TTL
Level Level
Signals Signals

Section 17.1

Baud Rate Calculations Packet transmission time
If each bit takes T seconds,
the baud rate is B= 1/T. If a system sends a packet of 50 bytes at
Standard Baudrates are
150, 300, 600, 1200, 2400, 4800, 9600, 19200, etc. 1200 Baud, using 8-data, no-parity, one
Baudrate tolerance for a10-bit frame is 5%. stop bits, what is the transmission time of
Example: T=209μs B= 1/T = 4785 Baud the whole packet:
it is 4800 Baud within 5% tolerance. 1-byte frame is 1-start + 8-data + 1-stop bit
one frame period = 10 bits/byte.
T
packet is transferred by 50 x 10 bits = 500 bits
on the serial communication line.
Tpacket = 500 bits / 1200 Baud = 0.417sec.
Start D0 D1 D2 D3 D4 D5 D6 D7 Stop = 417 milliseconds.
0 1 0 1 0 0 1 1 0 1 time
bits 7 6 5 4 3 2 1 0
the transmitted data value is 01100101 b = 65h =‘e’


RS232 Socket Pins Handshaking Signals
Recommended standard
describe two kind of sockets. DTE DCE Modem-Terminal handshaking signals:
TxD 2 TxD 2
DTE: Device Terminal Equipment. RxD 3 RxD 3
Device status signals
Computers, Terminals, etc.
GND 7 GND 7 DTR: Data terminal ready (DTE is ok.).
DCE: Device Communication Equipment DSR: Data Set ready (DCE is ok.)
Modem (modulator-demodulator).
Equipment connections: Flow control signals
DTE DTE RTS: Request to Send (DTE sends char.)
DTE is connected to DCE TxD 2 TxD 2
without crossing. RxD 3 RxD 3 CTS: Clear to Send (DCE accepts RTS)
DTE is connected to DTE GND 7 GND 7

cross-wired.


UART 8251 8251 UART Device
Register addressing
8251 USART
A processor may transmit/receive data in ~CS C/~D=00 (data) TXRDY
serial format without any extra hardware. writes to the transmit buffer D[0..7] TXE
~TXC
But it costs to the processing time of the reads from the receive buffer TXD
RXD
processor.
~CS C/~D=01 (control) RESET
RXRDY
A UART (Universal Asynchronous Receiver Transmitter) is writes to the CLK
~RXC
SY/BR
a hardware device that shifts out data bits mode register right after a reset. C/~D
DSR
to transmit a data byte, and also shifts-in command register after mode is ~RD
~DTR
~WR ~CTS
data bits to receive a data byte. written. ~CS ~RTS
reads from the status register.



8251 system bus connection 8251 Clock Signals
8251 USART CLK is system clock input
TXRDY 8251 USART
Data buffer D[0..7] D[0..7] TXE Transmit TXC and RXC are transmit-receive TXRDY
Clock
address is 308h ~TXC
TXD serial
clock inputs. D[0..7] TXE
~TXC
data There are three baudrate factors
Control/Status RXD TXD
RXD
RXRDY Receive divide by 1, 16 and 64.
address is 309h, RESET RESET ~RXC Clock
Example. Find RXC oscillator frequency RXRDY
CLK CLK RESET ~RXC
TXRDY and A0 C/~D
SY/BR Clock
Generator
for 1200 Baud operation with baudrate CLK
SY/BR
DSR factor 1/64. C/~D
RXRDY are for ~IOR ~RD
~DTR modem DSR
~IOW ~WR Solution: fRXC =1200*64 Hz = 76.8 kHz. ~RD
interrupted Address ~CS
~CTS
~RTS
handshaking
Example: What shall be the baudrate ~WR
~DTR
~CTS
Decoder ~308h – ~309h
operation. factor for 4800 Baud operation if RXC is ~CS ~RTS
connected to 19.2 kHz ?
Solution: 19.2kHz/4.8kHz = 4, 1/4


8251 mode/control settings
8251 Reset Sequence
8251 USART
Mode Register format for asynchronous mode:
and status bits
b7 b6 = { S2S1: nr.of stop bits 00: invalid / 01:1stop / 10: 1.5stop / 11: 2stop },
8251 reset sequence is b5
b4
= { EP: parity type 0: odd / 1: even },
= { PEN: parity enable 0: no-parity-bits / 1: parity-bits-present },
write three successive zeros to control address to b3 b2 = { L2L1: nr.of data bits 00: 5-bit / 01: 6-bit / 10: 7-bit / 11: 8-bit },
b1 b0 = { B2B1: baud rate factor 00: sync-mode / 01: /1 / 10: /16 / 11: /64 }
assure writing a reset to the command register. Control Register format for asynchronous mode:
b7 = { EH: Enter hunt mode to search sync char 1: enable / 0: disable }
write command 40h to reset (reset chip) b6 = { IR: Internal reset 1: resets the 8251A }
b5 = { RTS: Request to send, 1: RTS-output-forced-to-low }
After the reset, 8251 expects mode settings. b4 = { ER: Error Reset 1: reset error flags PE,OE,FE }
b3 = { SBRK: Send break char 1: forces TxD low }
write the mode settings to control address b2 = { RxE: Receiver enable 1: enable, 0: disable }
b1 = { DTR: Data terminal ready 1: DTR-output-forced-to-low }
There after 8251 needs command settings. b0 = { TxE: Transmitter enable 1: enable, 0: disable }
Status Register format for asynchronous mode:
write command for command settings. b7 = {DSR 1: DSR pin is active (low)}
b6 = {SY/BD 1: sync-or-break char detected}
Now the device is ready for transmit and receive b5 = {FE 1: Framing error occurs}
b4 = {OE 1: Overrun error occurs}
operations b3 = {PE 1: Parity error occurs}
b2 = {TxE 1: Tx finished transmitting all data}
b1 = {RxRDY 1: Data-in buffer is full}
b0 = {TxRDY 1: Data-out buffer is empty}

Section 17.4

For the circuit 8251 initialization For the circuit with ports 8251 coding
with ports 308h, 308h, 309h enabling
309h enabling Mode Register 8251, write a code mainloop
b7 b6 = { S2S1: nr.of stop bits 00: inval./ 01:1stop / 10: 1.5stop / 11: 2stop }, .model small
8251, write a b5 = { EP: parity type 0: odd / 1: even }, a) to initialize it for 1200 .code ….
code b4 = { PEN: parity enable 0: no-parity-bits / 1: parity-bits-present }, baud when TXC ; reset 8251 ; read the port
a) to initialize it for b3 b2 ={ L2L1: nr.of data bits 00: 5-bit / 01: 6-bit / 10: 7-bit / 11: 8-bit }, connected to 19.2kHz, to
mov dx,309h mov dx,300h
1200 baud when b1 b0 ={ B2B1: baud rate factor 00: sync-mode / 01: 1 / 10: 16 / 11: 64 } transmit characters to a
mov al,0 in al,dx
TXC connected to Control Register DCE with 8-bit data, no mov ah,al ; save it
19.2kHz, to b7 = { EH: Enter hunt mode to search sync char 1: enable / 0: disable } parity, one stop bit out dx,al
transmit b6 = { IR: Internal reset 1: resets the 8251A } configuration. out dx,al ; send the char,
characters to a b5 = { RTS: Request to send, 1: RTS-output-forced-to-low } b) to read port 300h, and out dx,al ; wait if buffer is full
DCE with 8-bit b4 = { ER: Error Reset 1: reset error flags PE,OE,FE } transmit the value in mov al,40h mov dh,309h
data, no parity, b3 = { SBRK: Send break char 1: forces TxD low } serial format out dx,al wdataout
one stop bit b2 = { RxE: Receiver enable 1: enable, 0: disable } in al,dx ; status byte
b1 = { DTR: Data terminal ready 1: DTR-output-forced-to-low } ; set mode
configuration. b0 = { TxE: Transmitter enable 1: enable, 0: disable }
Status Register format for asynchronous mode:
mov al, 4Eh and al,01h
b) to read port b7 = {DSR 1: DSR pin is active (low)} jz wdataout
300h, and Mode register shall be 01001110b = 4Eh b6 = {SY/BD 1: sync-or-break char detected} out dx,al
b5 = {FE 1: Framing error occurs} ; set command ; now send character
transmit the value Control register shall be 00110011b = 33h b4 = {OE 1: Overrun error occurs} mov al,ah
in serial format mov al, 33h
b3 = {PE 1: Parity error occurs} mov dx,308h
handshaking b2 = {TxE 1: Tx finished transmitting all data} out dx,al out dx,al
to modem is receiver is disabled. b1 = {RxRDY 1: Data-in buffer is full}
enabled. b0 = {TxRDY 1: Data-out buffer is empty} ; continue looping
only transmitter enabled.
jmp mainloop
end

Exercise
Solve the following two Final Exam
questions.
Final Fall 06
Final Fall 07

CMPE328 Spring 2007-08 Dr.Mehmet Bodur, EMU-CMPE 21

11 Serial 0515

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