Week-8_Serial-Communication_v1.3.pdf

8/20/2019 Week-8_Serial-Communication_v1.3.pdf

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CENG-336Introduction to Embedded

Systems Development

Spring 2015

Serial Communications


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CENG-336 Introduction to Embedded Systems Development 2

Early Serial Communications


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“Later” Serial Communications


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Serial Communications Serial communications means that information is transmitted from source

to destination over a single pathway.

Serial Data Links

A simplex serial data link transmits data in only one direction.

Usually, two serial data links are between a computer and terminal: one for

transmission in each direction. This is called full-duplex operation. Thecomputer echoes the character back to the terminal.

screen

keyboard

cpu

outputport

inputport

echo

Serial Link


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With half-duplex operation, there is only one line fortransmission and the characters are printed locally.

We can transmit in either direction, but it takes time toturn the line around.

screen

keyboard

cpu

outputport

inputport

Serial Link


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Since digital data are transmitted as a “stream of bits”,synchronization is required between the source anddestination of the data.

Asynchronous communication transmits one character (byte)at a time by adding start and stop bits to the character code.

Synchronous transmission transmits a block of data and aclock signal or known bit pattern that can be used forsynchronization.

A block number and checksum are also transmitted.


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ASCII Character Encoding

The asynchronously transmitted character is composed of

7-8 data bits,

an optional parity bit, and

either 1, 1.5, or 2 stop bits.

The most used form of character encoding is the American Standard Codefor Information Interchange (ASCII).

This is a seven bit code which allows for 128 characters.

Codes that are less than 20H are not alpha-numeric characters but control

codes such as carriage return or backspace.

numbers 0-9 are encoded as 30H-39H.

capital letters A-Z have codes 41H-5AH , and

lower case letters a-z are 61H-7FH.


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ASCII

When start and stop bits and a parity bit are added tothe ASCII code, 9 to 11 bits are transmitted.

The rate of transmission of a data bit stream in bits per

second (bps or b/s) is called the bit rate.

Standard bit rates are

110 ( the old teletype or TTY rate),

300 (early PC modems),

1200, 2400, 9600, 14.4K, 19.2K, 28.8K, 38.4K, 57.6K, and115.2 bps.


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Character Transmission

...mark = 1

space = 0

start

bit

data

bit 0

data

bit 1

parity

bit

data

bit n-1

stop

bit

stop

bit

start

bit

time

framebit time

General bit format for asynchronous serial data.

start 1 0 1 0 1 1 0 parity stop stop

b0 b1 b2 b3 b4 b5 b6 b7

[ b7 b6 b5 b4 b3 b2 b1 b0 ] = [ 1 0 1 1 0 1 0 1 ]

odd

parity bit (set to produce an odd number of 1’s.)

Typical transmission - 7 character bits plus parity. The charactercode is 35H which is the code for the number 5.

idle


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BAUD Rate

Data transmission rates are also specified in terms ofbaud rates.

A baud is a unit of signaling speed that is equal to thenumber of discrete events occurring per second.

The key idea here is that an event contains more than one bitof information.

Therefore, the baud rate can be less than the bit rate.

With only two levels of logic the baud rate equals the bit rate.

However, inside a modem, the logic levels are converted totones.


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BAUD Rate A typical implementation is the transmit sine waves with different phases,

e.g., sin(bt + 0o), sin(bt + 90o), sin(bt + 180o), and sin(bt + 270o).

The transmission can switch to any phase and since there are fourpossible phases, it takes two bits to code this information, i.e. 0o = 00, 90o= 01, etc.

Detecting the phase at a particular time produces 2 bits of information. Therefore, the bit rate is twice as fast as the baud rate in this case.

A 9600 bps modem typically has a baud rate of 2400 baud.

Transmission rates are sometimes referred to in units ofcharacters/second.

For example, if we have 10 bits/ character transmitted at 30 characters/sec.,

then the transmission rate is (10 b/ch)(30ch/s) = 300 b/s.


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RS-232C

The TTL logic signals are not what is transmittedbetween equipment such as modems computers andterminals.

One standard that describes the signals forcommunicating is the RS-232C standard.


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RS-232


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DCE/DTE

This standard causes as much trouble as any standard you can name

because it deals with two types of equipment,

Data Communications Equipment (DCE) and

Data Terminal Equipment (DTE).

Typically, a modem is DCE and a terminal or computer is DTE.

The problem is that the cable to connect DCE to DTE is different

from the cable to connect DTE to DTE.

Also, nonstandard use of the control signals is a problem.

Murphy’s law says that if you need to connect to a serial port, you

always have the wrong cable.


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Mechanical/Electrical/Procedural

RS-232C specifies 25 signal pins with a

male DTE connector and

a female DCE connector.

Most of these signals are not used in most applications so a 9 pin

subset is used.

Actually in some cases, only three wires are used.

The mark and space signals are inverted by the standard to the

following levels:

A logic high or “mark” is between -3 V and -15 V under load

A logic low or “space” is between +3 V and +15 V under load.

Typically, +12 and -12 volts are used.

The voltage swing is > the TTL 5 volts for noise immunity.


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RS-232C

interfaceTTL levels RS-232C levels

MC1489A line receiver

MC1488 line driver

Signal Conversion to and from TTL and RS-232C levels.


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RS-232 Signals


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Serial Ports - RS-232 Standard

DTR Data terminal ready

TD Transmit data

RD Receive data

SG Signal ground

DSR Data set ready

RTS Ready to send

TS lear to send

D arrier detect

Signals for a DTE (Data Terminal Equipment – modem is aDCE (Data Communication Equipment)


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Signal meanings – RS232 Transmit data group

TD – transmit data – data from DTE to DCE

RTS – ready to send – DTE asserts before sending data and waits for CTSbefore sending

CTS – clear to send – DCE sends in response to RTS

Receive data group TD – transmit data – data from DCE to DTE

DSR – data set ready – DCE asserts before sending data and waits for DTR

before sending

DTR – data terminal ready – DTE sends in response to DSR

CD – carrier detect – modem is receiving a carrier from a modem at otherend

SG – signal ground – common return for all lines

Note: Not all manufacturers use the control lines


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DTE-to-DCE

• Connecting DTE to DCE equipment uses a straightthrough cable with wires connected to the proper pins.A DTE to DTE connection requires a null modem cable.

1 shield ground2 transmit3 receive4 request to send5 clear to send6 data set ready7 signal ground

20 data terminal ready

123456

720

Null modem connection.


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Variants


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BAUD / bps

Baud: bits per second Baud Application

110 ASR-33 Teletype

300 Early acoustic modems

1200 Direct-coupled modems c. 1980 2400 Modems c. 1990

9600 Serial terminals

19200

38400 Typical maximum 115200 ...


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Physical Variants


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PIC Port

USART serial port Universal Synchronous/Asynchronous Receiver/Transmitter

Supports RS-232 or RS-485


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PIC Universal Synchronous Asynchronous

Receiver Transmitter (USART)

USART is a serial I/O module that can be configured as a full duplex

asynchronous system or a half duplex synchronous system

The USART has the following modes:

Asynchronous (full duplex)

Synchronous - Master (half duplex)

Synchronous - Slave (half duplex)

The USART can communicate using a number of different

protocols:

RS232

SPI Microwire

I2C


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PIC Peripherals: USART: UART

Serial Communications Peripheral: UniversalSynchronous/Asynchronous Receiver/Transmitter

Only available in 14bit and 16bit cores

Interrupt on TX buffer empty and RX buffer full

Asynchronous communication: UART (RS-232C serial)

Can do 300bps - 115kbps

8 or 9 bits, parity, start and stop bits, etc.

Outputs 5V so you need a RS232 level converter (e.g.,MAX232)


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PIC Peripherals: USART: USRT

Synchronous communication: i.e., with clock signal

SPI = Serial Peripheral Interface

3 wire: Data in, Data out, Clock

Master/Slave (can have multiple masters)

Very high speed (1.6Mbps)

Full speed simultaneous send and receive (Full duplex)

I2C = Inter IC

2 wire: Data and Clock Master/Slave (Single master only; multiple masters clumsy)

Lots of cheap I2C chips available; typically < 100kbps(For example, 8pin EEPROM chips, ADC, DACs, etc.)


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PIC18F8722 EUSART

Enhanced Universal Synchronous Asynchronous ReceiverTransmitter

Includes two serial modules EUSART1 and EUSART2

Can operate in both asynchronous (RS232) and synchronousmodes (I2C or SPI)

Usage

First, select BAUD rate

Configure receive and transmit interrupts

Then, select synchronous/asynchronous mode Enable serial port (SPEN=1)

Read data from RCREGx when RCxIF is set

Write data to TXREGx to send data


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PIC18F8722 BAUD Rate Selection

BRG16: 8-bit or 16-bit BRG mode

BRGH: High (1) or low(0) baud rate (only for asynchronous)


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PIC18F8722 BAUD Rate Selection

PIC18F8722 datasheet gives necessary tables for baud rate selection

An important criteria is the "error" in the baud rate

Asynchronous communication relies on matching baud rate fortransmitter and receiver

Otherwise, errors might occur during transmission


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Important Details

Port pins corresponding to the RX and TX functionsmust be configured properly

Consult the datasheet for details

Its is often a good idea to properly initialize ALL

configuration bits and registers explicitly 8-bit vs 9-bit transmission

Synchronous or asynchronous mode

Enable bits: SPEN, TXEN, RXEN

If all else fails, you may need to use a logic analyzer oroscilloscope connected to the port pins to figure outwhat might be going on.


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Serial Communications Software

Asynchronous communications

Full-duplex structure requires independent components fortransmission and reception.

This is best implemented with receive and transmit interrupts

A robust and reliable "protocol" must be designed

Synchronous communications

Half-duplex nature enables a simpler structure. Transmission

and reception are interleaved Receive and transmit interrupts are still useful

Protocol design is less critical but still important


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Asynchronous Communications

Independent receive and transmit ISRs should be implemented

ISRs use ring buffers for incoming and outgoing data

Main program handles generation and processing of data, using

buffers Starting transmission may be tricky. The first byte can be initiated

by main. ISR handles remaining transmissions.

receivetask

receivebuffer

transmitbuffer

receiveISR

transmitISR

RCREGx

TXREGx

main

transmittask


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Asynchronous Example

System to listen for commands on a full-duplex RS232connection and respond accordingly

First step: "Communication Protocol"

Imposes a common structure on the exchange of data

Agreed upon and known by both the sender and the receiver Often structured in the form of "packets". For example:

bits 7 6 5 4 3 2 1 0

byte 1 type size

byte 2-n command / response

byte n+1 checksum


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Program structure

void main() {init_interrupts();init_serial();while(1) {

receive_task(); // Handles reception of new packet from bufferprocess_task(); // Handles reception of new packet from buffersend_task(); // Builds and buffers response packet

}}void init_serial() {

TRISC = 0xDF; // Pin 7 input, Pin 8 outputBAUDCON1 = 0x48; // 16 bit baud rate generation, receive idleSPBRGH1 = 0x00;SPBRG1 = 0x56; // Baud rate for 115200 (See Table 20-3)TXSTA1 = 0x04; // Asynchronous, high speed, 8-bit mode, disable transmitter for nowRXSTA1 = 0x90; // Serial and receive enabled, 8-bit mode

}void init_interrupts() {

PIF1 = 0x00; // RC1IE = 1, TX1IE = 1PIE1 = 0x30; // RC1IE = 1, TX1IE = 1INTCON = 0xC0; // GIE = 1, PEIE = 1

}

warning: untested code, double check


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Interrupt service routines

void interrupt high_priority high_isr() {if (PIF1 & 0x20 != 0) receive_isr();if (PIF1 & 0x10 != 0) transmit_isr();

}

void receive_isr() {receive_buf_push( RXREG1 ); // Receive and buffer data

PIF1 &= ~0x20; // Clear receive interrupt flag}

void transmit_isr() {if (transmit_buf_isempty()) { // Disable further interrupts if transmit buffer empty

TXSTA1 &= ~0x20; // This should be set when new data is placed into buffer} else {

TXREG1 = transmit_buf_pop();}

PIF1 &= ~0x10; // Clear transmit interrupt flag}

warning: untested code, double check


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Receive task: Decodes incoming packet

void receive_task() {while (! receive_buf_isempty() ) { // Consume all available data

data = receive_buf_pop();switch (state ) {

case WAIT_HEADER:packet_type = data >> 6; packet_size = data & 0x3f;state = RECEIVE_DATA; i = 0;break;

case RECEIVE_DATA:command_data[i++] = data;if (i == packet_size) state = VERIFY_CHECKSUM;

break;case VERIFY_CHECKSUM:

if (verify_checksum() == 0) discard_command();else alert_process_task();state = WAIT_HEADER;break;

} }}

wait for size receive packet

verify checksum


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Send task: Builds and buffers outgoing data

Also initiates sending of the first byte by enabling thetransmitter

void send_task() {

if (new_outgoing_packet) {

header = (response_type


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A Serious Problem: Scenario

Serial connections are unreliable. Bytes may be receivedincorrectly, lost, or extra bytes may be received.

Example: What happens if the receiver misses a start bitand fails to receive one of the packet bytes?

The receiver will process the remaining bytes of the packet,but they will be one less than what they should be

The first byte of the next packet will be interpreted as thechecksum byte

The following byte, which should have been the first data byteof the next packet, will be interpreted as the size

The rest of the communications will all be garbage.


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A Serious Problem: Solution

Distinct identifier to indicate the start of a byte

Could be a unique sequence of values

You could use a 9-bit protocol and use the 9th bit to identifythe header

If only 8 bits are available, use the most significant bit toidentify the header, and pack data into 7 bit segments placedinto the packet body

If a header is received in any state, go bck to theWAIT_HEADER state

Implement timeouts

If the entire packet is not received after a certain timeout, goback to the WAIT_HEADER state


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CENG 336 I d E b dd d S D l 41

Conclusion

Asynchronous communications may be difficult

Properly initialize hardware, select correct baud rates and portconfigurations

Use receive and transmit interrupts, together with ring buffers

Design and implement a well-defined communication protocol

Use state machines whenever you can, especially for reception

Handle ALL errors, including buffer overruns, underruns,checksum errors, timeouts, incorrect headers, invalid contentetc.

Week-8_Serial-Communication_v1.3.pdf

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