serial communication part-16

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Serial communication


Basics of serial communication

Serial versus Parallel Data Transfer


UART

• RS232 standard and application, e.g.

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RS232 port(UART)

RS232 port (UART)

RS232 standard3 wires+10 V=‘0’=SPACE-10V=‘1’=MARK

Pin2Pin3pin5

Pin3Pin2pin5


U Universal A Asynchronous R Receiver T Transmitter

UART for data communication

• Serial data transmission means sending data bits one by one using one wire.

• Asynchronous transmission means a data (including one start bit , 8-bit data, and stop bits) can be sent at any time.

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Universal asynchronous receiver transmitter : UART

• RS232 is a serial communication standard• Since it is asynchronous, no external clock is needed, only 3 wires are

required for the simplest RS232 connection {GND, tx(transmit), rx(receive)}

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+10V=Logic 0=space

-10V= Logic 1=mark

Exercise: Sketch Bit Patterns for character A and B

Start 0 1 2 3 4 5 6 7 stop

Bit 0 to 7 (least sig. bit first )

time

www.techvilla.org.inCEG2400 Ch6. Serial Interface -- UART V4b

ASCII table

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EXAMPLES

• Sending ASCII • ‘5’=0x35=0011 0101b

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Start bit=0

10101100(stop bit=1)Least Sign. Bit first

What is this code ?

The whole ASCII serial character has 10 bits (1 start+8 data + 1 stop bit)

Stop bit=1

OscilloscopeProbe

T=bit period =1/baud rate


If the baud rate is 57600 bits per second Define ‘bit period’ :the amount of time to transmit a logic 1 or 0. (bit period= 1/57600 seconds)

• Each bit lasts for (1/Baud rate) seconds

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bit period =1/baud rate =(1 / 57600) seconds=17.4 us

Exercise: What is the “bit period” in seconds if baud rate is 2400?


Asynchronous Serial Communication• With asynchronous communication, the transmitter and

receiver do not share a common clock

Transmitter Receiver+

1 byte-wide Data

Data–

1 byte-wide Data

The Receiver

Extracts the data using its own clock

Converts the serial data back to the parallel form after stripping off the start, stop and parity bits

The Transmitter

Shifts the parallel data onto the serial line using its own clock

Also adds the start, stop and parity check bits

Add: Start, Stop, Parity Bits Remove: Start, Stop, Parity Bits


Asynchronous Serial Communication• Start bit—indicates the beginning of the data word

• Stop bit—indicates the end of the data word

• Parity bit—added for error detection (optional)

• Data bits—the actual data to be transmitted

• Baud rate—the bit rate of the serial port

• Throughput—actual data transmitted per sec (total bits transmitted—overhead)• Example: 115200 baud = 115200 bits/sec• If using 8-bit data, 1 start, 1 stop, and no parity bits, the effective

throughput is: 115200 * 8 / 10 = 92160 bits/sec


Asynchronous Serial Communication

• Asynchronous transmission is easy to implement but less efficient as it requires an extra 2-3 control bits for every 8 data bits

• This method is usually used for low volume transmission


Synchronous Serial Communication• In the synchronous mode, the transmitter and receiver share a common clock

• The transmitter typically provides the clock as a separate signal in addition to the serial data

Transmitter Receiver

Data

Clock

The Receiver Extracts the data using

the clock provided by the transmitter

Converts the serial data back to the parallel form

The Transmitter Shifts the data onto the serial

line using its own clock

Provides the clock as a separate signal

No start, stop, or parity bits added to data

1 byte-wide Data 1 byte-wide Data


Avr uart


Serial connection using rs232 protocol


RS232

• For asynchronous communication.• between two data serial links on a network ─Between a data

communication equipment and data terminal equipment.• RS232C ─a standard protocol used in IBM PC COM ports, keyboard,

computer-mice and • For the data serial link network in UART bit format


DB 9 connector


Max 232 ic


Avr connection


How to set-up avr UART

• The first step is to set the baud rate in both, the master and the slave. The baud rate has to be the same for both – master and slave.

• Set the number of data bits, which needs to be sent.• Get the buffer ready! In case of transmission (from AVR to some other

device), load it up with the data to be sent, whereas in case of reception, save the previous data so that the new received data can be overwritten onto it.

• Then enable the transmitter/receiver according to the desired usage.


• in UART, there is no master or slave since master is defined by the MicroController, which is responsible for clock pulse generation.

• Master and Slave terms occur only in the case of USART.• Master µC is the one which is responsible for Clock pulse generation

on the Bus.


Baud rate generation

• The baud rate of UART/USART is set using the 16-bit wide UBRR register.


UBRR register

• he 16-bit UBRR register is comprised of two 8-bit registers – UBRRH (high) and UBRRL (low).

• The USART Baud Rate Register (UBRR) and the down-counter connected to it functions as a programmable prescaler or baud rate generator.

• The down-counter, running at system clock (FOSC), is loaded with the UBRR value each time the counter has counted down to zero or when the UBRRL Register is written.

• A clock is generated each time the counter reaches zero.


Calculating baud rate and UBRR value


•BAUD = Baud Rate in Bits/Second (bps) •Bps = Bytes/Second, whereas bps = Bits/Second)•FOSC = System Clock Frequency (1MHz) (or as per use in case of external oscillator) •UBRR = Contents of UBRRL and UBRRH registers


Uart data Frame


FRAME STRUCTURE

• USART communication uses special protocol to transmit data reliable. It consists of several parts:

• 1 start bit;• 5,6,7,8 or 9 data bits;• no, even, or odd parity bit;• 1 or 2 stop bits.

serial communication part-16

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