Universal Asynchronous Receiver Transmitter (UART)

7/26/2019 Universal Asynchronous Receiver Transmitter (UART)

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T . R E V A T H I

T IF AC C O R E I N A U T O M O T I V E I N F O T R O N I C S

V I T U N I V E R S I T Y , V E L L O R E

Serial Communication Interfaces- UART (RS232) , RS485


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Contents

Basic Concepts of Serial communication

Types

Advantages

Applications

UART Features in ARM Implementation Steps Transmitter & Receiver


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Need of Serial Communication

Low Pin counts.

Supporting high data rate compared to IO lines.

Communication can be performed with just one IO

pin rather than to use multiple pins when compared

with Parallel Communication.

Data Synchronization.


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Serial Communication

Serial Communication Data Bits will be sent in a serial

way over a single line.

Asynchronous data transfer utilizes EIA-232 (Electronics

Industry Association) Standard.

Serial communication is often used either to control or to

receive data from an embedded microprocessor.


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Advantages of Serial Communication

Transmission distance is more.

Cable connection is simpler.

Low Cost in implementation.


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Types of Serial communication

Asynchronous serial communication

Universal Asynchronous Receiver Transmitter (UART)

Synchronous serial communication

Serial Peripheral Interface (SPI)

Inter Integrated Circuit (I2C)


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Terms to be remembered

Baud Rate No. of bits transmitted over the serial link per

second. (Also a measurement of transmission speed in

Asynchronous communication)

Half-duplex Communication channel where both

transmission and reception are possible, but only in one

direction at a time

Full-duplex- Connection permits the sending and receiving

of data at the same time


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Data Communication System

Devices that uses serial cables for their communication are

split into two categories

Data Terminal Equipment

Data Communication Equipment


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Asynchronous Data Transfer

Allows data to be transmitted without the sender has to send a clock

signal to the receiver.

Special bits will be added to each word in order to synchronize the

sending and receiving of the data.

When a word is given to the UART for Asynchronous transmissions, a

bit called the "Start Bit" is added to the beginning of each word that is

to be transmitted.


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Asynchronous Data Transfer

When the entire data word has been sent, the transmitter

may add a Parity Bit that the transmitter generates.

Then at least one Stop Bit is sent by the transmitter.

If the Stop Bit does not appear when it is supposed to, the

UART considers the entire word to be garbled and will

report a Framing Error.


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Data Format Signal levels

Data is transmitted character by character bit serially

A Character consists of

One start bit (0)

7 8 data bits

An optional parity bit

One or one and a half or two stop bits (1)

Least significant bit is transmitted first

Most Significant bit is transmitted last


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Need of level conversion

All the microcontroller signals are indicated only by the positive signals of

5V TTL /15VCMOS logic.

If the signal is transmitted to a distance, may be at the end of a long cable,

by the time the signal reaches the receiver, its voltage may have attenuated

by the cable impedance or have noise riding on it.

In order to avoid and decrease the noise margin, the signal from the

microcontroller has to be converted to a level called Recommended

Standard 232 (RS-232) signal for point to point communication.


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RS 232 Electrical Specification

Signal Rate < 20 kbps.

The signal can transfer correctly within 15 meters.

The receiver input voltage range is -25 V to +25 V.

The receiver output is high when input is open circuit.

A voltage more negative than -3 V at the receiver input is

interpreted as a logic 1. A voltage more positive than +3 V at the receiver input is

interpreted as a logic 0.


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Max232 Level conversion circuit


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RS 232 Level Conversion


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Typical Connection

Types of Connectors


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Applications

Dial-up modems

GPS receivers

Bar code scanners

LED and LCD text displays

Satellite Phones

Test and measuring equipment

such as digital multi meters and

weighing systems

Updating Firmware on various

consumer devices.

Industrial field buses

Printers

Computer terminal

Serial mouse


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RS 485 Specifications

RS485 is a balanced line, Half-duplextransmission system allowingtransmission distances of up to 1.2 km.


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RS-485 Balanced Differential Bus


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RS-485 Signaling

For Input

If a logic high is received on the

input of the transmitter (DI = 1),

Line A is more positive than Line B

(VOA > VOB) on the driver output.

If a logic low is received on the

input of the transmitter (DI = 0),

the transmitter causes Line B to bemore positive than Line A (VOB >

VOA).

For Output

If Line A is more positive than line

B (VIA VIB > 200 mV) on the

input of the receiver, the receiver

output is a logic high (RO = 1).

If Line B is more positive than Line

A(VIB VIA > 200 mV) on the

input of the receiver, the receiveroutput is a logic low (RO = 0).


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RS 485 Network Topologies


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Rs-485 Driver IC SN75176

Voltage produced by the driver appears across a

pair of signal wires that transmit only one

signal. Both wires are driven opposite.

RS-485 driver has always the Enable direction

control signal.

Differential system provides noise immunity,

because much of the common mode signal can

be rejected by the receiver. So ground shifts and

induced noise signals can be nullified.


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RS 485 Half Duplex Network

Half-duplex RS-485 links have multiple drivers and receivers on the same signal path.

This is the reason why RS-485 transceivers must have driver/receiver enable pins enabling only

one driver to send data at a time.


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Advantages of RS-485

Long distance linksup to 4000 feet.

Bidirectional communications possible over a single pair of twisted

cables.

Differential transmission increases noise immunity and decreases noise

emissions.

Multiple drivers and receivers can be connected on the same bus.

Wide common-mode range allows for differences in ground potential

between the driver and receiver.

TIA/EIA-485-A allow for data rates of up to 10 Mbps.


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Applications of RS-485

Motor Control

Industrial Automation

Battery powered applications

Tele-communications equipment

Industrial Process Control


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Comparison between various serial standards


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Features in LM4F120H5QR


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UART in LM4F120H5QR

Programmable baud-rate generator allowing speeds up to 5 Mbps for regular speed (divide by

16) and 10 Mbps for high speed (divide by 8)

Separate 16x8 transmit (TX) and receive (RX) FIFOs to reduce CPU interrupt service loading

Programmable FIFO length, including 1-byte deep operation providing conventional

double-buffered interface

FIFO trigger levels of 1/8, 1/4, 1/2, 3/4, and 7/8

Standard asynchronous communication bits for start, stop, and parity

Line-break generation and detection

Fully programmable serial interface characteristics

5, 6, 7, or 8 data bits

Even, odd, stick, or no-parity bit generation/detection

1 or 2 stop bit generation


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Block Diagram


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UART Signal Mapping


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Programming Procedure Stellarisware Implementation

Step 1: Set the clock source

Step 2:Select / Enable the peripheral

Step 3:Select the Port corresponds to the peripheral function

Step 4:Select the UART function for the respective pin

Step 5: Enable the Port corresponds to the peripheral function

Step 6:Configure the Baud rate, No. of Data bits & Parity

Step 7 : Start Transmission / Reception of Data


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SysCtlClockSet

Sets the clocking of the device.

Prototype:

void SysCtlClockSet(unsigned long ulConfig)

Parameters:

ulConfig is the required configuration of the device clocking.

Description:

This function configures the clocking of the device. The input crystal

frequency, oscillator to be used, use of the PLL, and the system clock

divider are all configured with this function.

Step 1: Set the clocksource


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SysCtlPeripheralEnable

Enables a peripheral.

Prototype:

void SysCtlPeripheralEnable(unsigned long ulPeripheral)

Parameters: ulPeripheral is the peripheral to enable.

Description:

Peripherals are enabled with this function. At power-up, all

peripherals are disabled; they must be enabled in order to operate or

respond to register reads/writes.

Step 2: Select / Enablethe peripheral

Step 3: Enable the Port

corresponds to the peripheral

function


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GPIOPinTypeUART

Configures pin(s) for use by the UART peripheral.

Prototype:

void GPIOPinTypeUART(unsigned long ulPort, unsigned char ucPins)

Parameters:

ulPort is the base address of the GPIO port. ucPins is the bit-packed representation of the pin(s).

Description:

The UART pins must be properly configured for the UART peripheral to function correctly. This

function provides a typical configuration for those pin(s);

The pin(s) are specified using a bit-packed byte, where each bit that is set identifies the pin to be

accessed, andwhere bit 0 of the byte represents GPIO port pin 0, bit 1 represents GPIO port pin 1,

and so on.

This function cannot be used to turn any pin into a UART pin; it only configures a UART pin for proper

operation.

Step 4: Select the UART

function for the respective pin

Step 5: Enable the Port

corresponds to the

peripheral function


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UARTConfigSetExpClk

Sets the configuration of a UART.

Prototype:

void UARTConfigSetExpClk(unsigned long ulBase,

unsigned long ulUARTClk,

unsigned long ulBaud,

unsigned long ulConfig)

Parameters:

ulBase is the base address of the UART port.

ulUARTClk is the rate of the clock supplied to the UART module. ulBaud is the desired baud rate.

ulConfig is the data format for the port (number of data bits, number of stopbits, and parity).

Step 6:Configure the Baud

rate, No. of Data bits & Parity


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To Transmit Data - UARTCharPut

Waits to send a character from the specified port.

Prototype:

void UARTCharPut(unsigned long ulBase,unsigned char ucData)

Parameters:


ucData is the character to be transmitted.

Description:

This function sends the character ucData to the transmit FIFO for

the specified port. If there is no space available in the transmit FIFO,

this function waits until there is space available before returning.

Transmitter


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To Receive Data - UARTCharGet

Waits for a character from the specified port.

Prototype:

long UARTCharGet(unsigned long ulBase)

Parameters:


Description:

This function gets a character from the receive FIFO for the specified

port. If there are no characters available, this function waits until a

character is received before returning.

Receiver


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Universal Asynchronous Receiver Transmitter (UART)

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