Uart

Serial
Communication
Mohamad Abd Elhay

Copyright © 2012 Embedded Systems
Committee

Serial Communication:
is the process of sending data one bit at a time,
sequentially, over a communication channel or
computer bus.

Parallel Communication:
where several bits are sent as a whole, on a link with
several parallel channels.
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serial connection requires fewer interconnecting
cables and hence occupies less space.
 the cost of cable and synchronization difficulties
make parallel communication impractical.

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• For serial communications to be accomplished
there must be a transmitter (TX), and a
receiver (RX).
• The TX is responsible for taking a byte (or
several bits of data) in parallel, and convert
them to a series of successive bits.
• The RX on the other side, collects the
successive bits to reform the data once again.

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A Serial Line.(HIGH=RED & LOW=WHITE)

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1-Simplex:
one way communication where all signals can flow in only
one direction.
Example Radio & TX.

2- half-duplex system
It provides for communication in both directions, but only one
direction at a time (not simultaneously).
Example walkie-talkie.

3-Full-duplex
allows communication in both directions.
Example Telephone.
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synchronous communication
Is transmission of data with the use of an external
clock signal.

Example:
Serial Peripheral Interface (SPI).

Inter-Integrated Circuit (I2C).
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Asynchronous communication
Is transmission of data without the use of an external
clock signal.

Example:
Universal asynchronous receiver/transmitter (UART).

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Serial Communication In Embedded Systems:
We use serial communication (UART-SPI-I2C) to interface
microcontroller with different devices such as :
1-Computer.
2-LCD.
3-EEROM.
4-Ultrasonic.
Also, serial communication used in embedded systems
networks such as:
1- Controller Area Networks (CAN)
2- Local Interconnect Networks (LIN)
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• Bit time:
– The time taken to transmit a bit/sec.
• Bit rate:
– The number of bits transmitted per second.
– Bit rate = 1 / Bit time.
• Baud Rate:
– The number of symbols signaled in a second.
– Baud rate = 1 symbol / sec.
• Symbol = bit or byte or number of bits or
bytes
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• When using a device has a set of 8 different
symbols.
• Each symbol represents 3 bits of useful data.
• If it transmits 300 bits per second, it gathers
each 3 bits in 1 symbol.
• So it transmits 100 symbol per second
– 300 bit per second (bit rate)
– 100 baud (baud rate)
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 It also called Serial Communication Interface(SCI)
 full-duplex communication.
 Asynchronous communication.
 Compatible with PC .

 The Standard bit rates are:
100, 200, 300, 1200, 2400, 4800, 9600, 19200,

38400, 57600, 115200 bps.
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• Frame : A frame is the unit of transmission in serial
communications.
• A frame contains:
– Start bit: To declare the start of transmission.
– Data bits: 4,5,6,7, or 8 bits of useful data bits.
– Parity bit : To check for transmission errors.
– Stop bit: To declare end of frame.

• Parity bit is used to check the integrity of a frame
and signal if an error occurred during
transmission.
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• Parity :
– Parity bit is an extra bit added to the end of a
frame.

• Even parity :
– The number of ‘1’ symbols inside a frame must
always be even.

• Odd parity :
– The number of ‘1’ symbols inside a frame must
always be odd.

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Frame structure:

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RS232 Data Interface
 PC serial interface.
 SPACE (0) will be between +3 to +15 volts.

 MARK (1) will be between -3 and -15 volts.

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Microcontroller O/P voltages:
o0 v for logic 0.
o5v for logic 1.
MAX232
Is an IC that converts signals
from an RS-232 serial port to
signals suitable for use in TTL
compatible digital logic circuits
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Hyper Terminal from Start > Programs > Accessories >
Communications

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UCSRA: USART Control and Status Register A
Bit No
7
Name RXC

6
5
TXC UDRE

4
FE

3
DOR

2
PE

1
0
U2X MPCM

RXC this bit is set when the USART has completed receiving a byte
from the host
UDRE this bit set when UDR emty.
UCSRB: USART Control and Status Register B
Bit No
7
6
5
4
3
2
1
0
Name RXCIE TXCIE UDRIE RXEN TXEN UCSZ2 RXB8 TXB8

RXCIE: Receive Complete Interrupt Enable
TXCIE: Transmit Complete Interrupt Enable
RXEN: Receiver Enable
TXEN: Transmitter Enable
UCSZ2: USART Character Copyright © 2012 Embedded Systems
Size
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UCSRC: USART Control And Status Register C
Bit No
7
6
5
Name URSEL UMSEL UPM1

4
UPM0

3
2
1
USBS UCSZ1 UCSZ0

0

URSEL: USART register select.
The UCSRC and the UBRRH register shares same address so to determine which register user
want to write is decided with the 7th(last) bit of data if its 1 then the data is written to UCSRC
else it goes to UBRRH.

USART Mode Select
USBS
0
1

Stop Bit(s)
1 BIT
2 BIT

USBS: USART Stop Bit Select
UMSEL
0
1

Mode
Asynchronous
Synchronous

UCSZ2
0
0
0

UCSZ1
0
0
1

UCSZ0
0
1
0

Character Size
5Bit
6Bit
7Bit

0

1

1

8Bit

1
1
1
1

0
0
1
1

0
1
0
1

Reserved
Reserved
Reserved
9Bit

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UDR: Data Register
UBRR: USART Baud Rate Register:

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void MCL_UART_Init(void)
void MCL_UART_SendByte(unsigned char byte)
void MCL_UART_StartSendArray(unsigned char length, unsigned char * array)
void MCL_UART_StartReceving(void)
void MCL_UART_GetReceivedData(unsigned char length, unsigned char * arr_rx)
int Compare (unsigned char * arr_rx,unsigned char length)
//equal=1 //not equal=0

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References

• ESC Training Team

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info@escommittee.net

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