Lecture 3 - Serial Communicationkahfag.pdf

Lecture 3’s sequence
3.1
3.2
3.3
Serial communica on ─ The basics
Serial communication in ATmega16
Review of C Programming
Example application & Debugging tool

Serial communica on─ The basics
 Computers transfer data in two ways: parallel and serial:
 Parallel: Several data bits are transferred simultaneously, e.g., to printers and hard disks
 Serial: A single data bit is transferred at one time
 Advantages of serial communication: longer distances, easier to synchronize, fewer IO
pins, and lower cost.
 Serial communication often requires:
 Shift registers: convert a byte to serial bits and vice versa
 Modems: modulate/demodulate serial bits to/from audio tones

Synchronous versus asynchronous
 Synchronous serial communication:
 The clocks of the sender and receiver are synchronized
 A block of characters, enclosed by synchronizing bytes, is sent at a time
 Faster transfer and less overhead
 Examples: serial peripheral interface (SPI) by Motorola, binary synchronous communication
(BISYNC) by IBM.
 Asynchronous serial communication:
 The clocks of the sender and receiver are not synchronized
 One character (usually 8 bits) is sent at a time, enclosed between a start bit and one or two
stop bits. A parity bit may be included
 Examples: RS-232 by Electronic Industries Alliance, USART of ATmega16

Data framing examples
PAD
BCC
ETX
DATA FIELD
STX
SYN
SYN
BISYNC Control Characters
SYN (16h): synchronization
STX (02h): start of text
ETX (03h): end of text
BCC: block checksum char
PAD (FFh): end of frame block
Data Framing in Synchronous BISYNC
Sending character “A” (41h = 0100 0001)
8-bit data, 1 start bit, 2 stop bits, even-parity
1 0 0 0 0 0 1 0 0
Receiver
start bit (0)
parity bit
stop bits (1)
MSB
LSB
Data Framing in Asynchronous Transmission

Serial communication terminology
 Baud rate: The number of bits sent per second (bps). Strictly speaking, it is the
number of signal changes per second.
 Parity bit: A single bit for error checking, sent with data bits to make the total
number of 1’s.
 even (for even parity), or
 odd (for odd parity)
 Start bit: to indicate the start of a character. Its typical value is 0
 Stop bit: to indicate the end of a character. Its typical value is 1

RS-232 standard
 The RS-232 (latest revision RS-232E) is a widely used standard for serial
interfacing.
 It covers four main aspects:
 Electrical: voltage level, rise and fall time, data rate, distance
 Functional: function of each signal
 Mechanical: number of pins, shape & dimension of connectors
 Procedural: sequence of events for transmitting data

RS-232 standard
 It defines 25-pin D connectors, but 9-pin connectors are often used.
 RS-232 specifies the baud rate up to 20Kbps, and the cable length up to 15m.
In practice, it supports up to 56Kbps & 30m of shielded cables.
Male DB9 connector Female DB9 connector

RS-232 standard
 Data Terminal Equipment (DTE)
essentially refers to the computer.
 Data Communication Equipment
(DCE) essentially refers to a remote
device or modem.
 These terms are needed to explain
the pin functions.
Description
Name
Pin
DCE has detected a carrier tone
Carrier Detect
CD
1
Incoming data from DCE
Received Data
RXD
2
Outgoing data to DCE
Transmit Data
TXD
3
DTE is connected and turned on
Data Terminal Ready
DTR
4
Ground
GND
5
DCE is connected and turned on
Data Set Ready
DSR
6
DTE has data to send
Request To Send
RTS
7
DCE can receive data
Clear To Send
CTS
8
Synchronized with the phone’s ringing tone
Ring Indicator
RI
9

Modem connection
 RS-2322 was originally used with modems to connect two PCs over the public phone lines
 When Computer A has data to send, it assert its RTS pin
 Modem A will assert its CTS when it is ready to receive
 Computer A transmits data through its TXD
Computer A
DTE
Computer A
DTE
Modem A
DCE
Modem B
DCE

Null-modem connection
 RS-232 is now mainly used to connect a microcontroller with PC or peripheral devices (e.g.,
GPS receiver, infrared range finder, camera).
 This configuration is known as null-modem:
 Connect pin TXD of a DTE with pin RXD of the other DTE
 Wire other pins to support flow control
Full handshaking cable Simple cable

RS-232 interface and MAX232 chip
 Compared to TTL in computer
electronics, RS-232 interface uses
different voltage levels
 A level converter is required
between RS-232 interface and
TXD/RXD pins of microcontroller
 MAX232 chip is often used for
this purpose
TTL levels
RS-232 levels
Logic
[+2V, +5V]
[-12V, -3V]
1
[0V, +0.8V]
[+3V, +12V]
0

Lecture 3’s sequence
3.2
3.3
Serial communica on ─ The basics
Review of C Programming
Example application & Debugging tool
3.1

 ATmega16 has 3 subsystems for serial communication:
 Universal Synchronous & Asynchronous Receiver & Transmitter (USART)
 Serial Peripheral Interface (SPI)
 Two-wire Serial Interface (TWI)
 USART:
 We focus on this subsystem in this lecture
 Supports full-duplex mode between two devices
 Typically used in asynchronous communication
 Start bit and stop bit are used for each byte of data

 Serial Peripheral Interface (SPI):
 The receiver and transmitter share a common clock line
 The transmitter is treated as the master, the receiver as the slave
 SPI supports higher data rates
 Examples: liquid crystal display, high-speed ADC
 Two-wire Serial Interface (TWI):
 For connecting several devices, e.g., microcontrollers and display boards, using a two-wire bus
 Each device has a unique address and can exchange data with other devices in a small network
 Up to 128 devices are supported

Serial USART ─ An overview
 USART of the ATmega16 supports:
 baud rates from 960bps to 57.6kbps
 character size: 5 to 9 bits
 1 start bit
 1 or 2 stop bits
 optional parity bit (even or odd parity)
 Common baud rates are 19200, 9600,
4800, 2400, and 1200 bps

Serial USART ─ An overview
c) Register UBRR
to set baud rate
d) Register UDR to store
the sent/received byte
a) TxD and RxD pins
to another device
b) Registers to configure/
monitor USART

Serial USART ─ Hardware elements
 USART Clock Generator:
 to provide clock source
 to set baud rate using UBRR register
 USART Transmitter:
 to send a character through TxD pin
 to handle start/stop bit framing, parity bit, shift register
 USART Receiver:
 to receive a character through RxD pin
 to perform the reverse operation of the transmitter
 USART Registers:
 to configure, control and monitor the serial USART

Serial USART ─ Three groups of registers
 USART Baud Rate Registers:
 UBRRH and UBRRL
 USART Control and Status Registers:
 UCSRA
 UCSRB
 UCSRC
 USART Data Registers:
 UDR
 Understanding these registers is essential in using the serial port. Therefore, we’ll
study these registers in depth.

USART Baud Rate Registers : UBRR
 Two 8-bit registers together define the baud rate:
=
( )
( + )
⇒ =
( )
−
 Example: Find UBRR registers for baud rate of 1200 bps assuming system clock is 1MHz:
 UBRR = 1000000/(16 × 1200) ─ 1 = 51d = 0033H
 Therefore, UBRRH = 00H and UBRRL = 33H
 C code: UBRRH = 0x00; UBRRL = 0x33;

USART Control and Status Register A: UCSRA
0
1
2
3
4
5
6
7
MPCM
U2X
PE
DOR
FE
UDRE
TXC
RXC
1 to enable multi-processor com mode
1 to double the transmission speed
1 when there is Parity Error
1 when there is Data Over Run
1 when there is Frame Error
1 when USART Data Register is Empty
1 when no new data in transmit buffer (TX Complete)
1 when receive buffer has unread data (RX Complete)

USART Control and Status Register B: UCSRB
0
1
2
3
4
5
6
7
TXB8
RXB8
UCSZ2
TXEN
RXEN
UDRIE
TXCIE
RXCIE
Tx extra data bit for 9-bit character size
Rx extra data bit for 9-bit character size
bit UCSZ2 to decide character size
1 to enable USART transmitter: Pin D.1 = TXD pin
1 to enable USART receiver: Pin D 0 = RXD pin
1 to enable USART Data Register Empty Interrupt
1 to enable TX Complete Interrupt, valid only if Global Interrupt Flag = 1 and TXC = 1
1 to enable RX Complete Interrupt, valid only if Global Interrupt Flag = 1 and RXC = 1

USART Control and Status Register C: UCSRC
0
1
2
3
4
5
6
7
UCPOL
UCSZ0
UCSZ1
USBS
UPM0
UPM1
UMSEL
URSEL
Clock polarity, used with synchronous
Used with UCSZ2 to select character size
To select stop bit modes: 0 -> 1 stop bit, 1 -> 2 stop bits
To select parity mode: 00 no parity, 10 - even party, 11 - odd parity
To select USART modes: 0 - asynchronous, 1 - synchronous
Must be set to 1 to write to UCSRC. (Note: UCSRC and UBRRH share same location)

Setting character size
 Character size (5 to 9) is determined by three bits: UCSZ2, UCSZ1, UCSZ0
TXB8
RXB8
UCSZ2
TXEN
RXEN
UDRIE
TXCIE
RXCIE
UCPOL
UCSZ0
UCSZ1
USBS
UPM0
UPM1
UMSEL
URSEL
UCSRB
UCSRC
Character Size
UCSZ0
UCSZ1
UCSZ2
5-bit
0
0
0
6-bit
1
0
0
7-bit
0
1
0
8-bit
1
1
0
Reserved
0
0
1
Reserved
1
0
1
Reserved
0
1
1
9-bit
1
1
1

USART Data Register : UDR
 Register UDR is the buffer for characters sent or received through the serial port.
 To start sending a character, we write it to UDR:
 To process a received character, we read it from UDR:
UDR = 'a'; // start sending character ‘a’
unsigned char data;
data = UDR; // this read will clear UDR

Serial USART ─ Main tasks
 There are 4 main tasks in using the serial port:
 Initializing the serial port
 Sending a character
 Receiving a character
 Sending/receiving a formatted string

Initializing serial port
Set USART for asynchronous mode
Set USART communication parameters
(data bits, stop bit, parity bit)
Set baud rate
Enable transmitter and receiver
end
begin

Ini alizing serial port ─ Example
Initialize serial port of ATmega16 to baud rate 1200 bps, no parity, 1 stop bit 8
data bits. Assume a clock speed of 1MHz.
void USART_init(void) {
// Asynchronous mode, no parity, 1 stop bit, 8 data bits
UCSRC = 0b10000110;
// Normal speed, disable multi-proc
UCSRA = 0b00000000;
// Baud rate 1200bps, assuming 1MHz clock
UBRRL = 0x33;
UBRRH = 0x00;
// Enable Tx and Rx, disable interrupts
UCSRB = 0b00011000;
}

Sending a character
end
begin
Has UDRE flag been set to 1?
(register UCSRA)
Write the character to register
UDR for transmission
Yes
No

Sending a character ─ Example
MPCM
U2X
PE
DOR
PE
UDRE
TXC
RXC
UCSRA
0
0
0
0
0
1
0
0
1<<UDRE
0
0
0
0
0
UDRE
0
0
bit-wise AND
Write a C function to send a character through ATmega16 serial port.
void USART_send(unsigned char data) {
// Wait until UDRE flag = 1
while ((UCSRA & (1<<UDRE)) == 0x00){;}
// Write char to UDR for transmission
UDR = data;
}
Constant UDRE is defined in avr/io.h
#define UDRE 5
Bit-wise AND returns zero if bit
at position UDRE is 0

Receiving a character
end
begin
Has RXC flag been set to 1?
(register UCSRA)
Read the received character
from register UDR
Yes
No

Receiving a character ─ Example
MPCM
U2X
PE
DOR
PE
UDRE
TXC
RXC
UCSRA
0
0
0
0
0
0
0
1
1<<RXC
0
0
0
0
0
0
0
RXC
bit-wise AND
Write a C function to receive a character via ATmega16 serial port.
unsigned char USART_receive(void) {
// Wait until RXC flag = 1
while ((UCSRA & (1<<RXC)) == 0x00) {;}
// Read the received char from UDR
return (UDR);
}
Constant RXC is defined in avr/io.h
#define RXC 7
Bit-wise AND returns zero if bit at
position RXC is 0

Sending/receiving a formatted string
 In ANSI C, the header file <stdio.h> has two functions for formatted strings:
printf and scanf.
 Function printf sends a formatted string to the standard output device, which is
usually the video display.
 Function scanf reads a formatted string from the standard input device, which is
usually the keyboard.
unsigned char a, b;
a = 2; b = 3;
printf("first = %d, second = %d, sum = %d", a, b, a + b);
unsigned char a, b;
scanf("%d %d", &a, &b); // get integers a, b from input string
first = 2, second = 3, sum = 5

Sending/receiving a formatted strings
 Being able to send/receive formatted strings through a serial port is useful in
microcontroller applications.
 To this end, we configure the serial port as the standard input and output
devices.
 General steps:
1) Write two functions to send and receive a character via serial port.
2) In main(), call fdevopen() to set the two functions as the handlers for standard output
and input devices.
3) Use printf/scanf as usual. Formatted strings will be sent/received via serial port.

Sending/receiving a forma ed strings ─ Example
#include <avr/io.h>
#include <stdio.h>
int serial_send(char c, FILE *stream) {
// wait until UDRE flag is set to logic 1
while ((UCSRA & (1<<UDRE)) == 0x00){;}
UDR = c; // Write character to UDR for transmission
}
int serial_receive(FILE *stream) {
// wait until RXC flag is set to logic 1
while ((UCSRA & (1<<RXC)) == 0x00) {;}
return (UDR); // Read the received character from UDR
}
int main(void) {
unsigned char a;
// ... Code to initialize baud rate, TXD, RXD, and so on is not shown here
// Initialize the standard IO handlers
stdout = fdevopen(serial_send, NULL);
stdin = fdevopen(NULL, serial_receive);
// Start using printf, scanf as usual
while (1) {
printf("nrEnter a = ");
scanf("%d", &a); printf("%d", a);
}
}

Sending/receiving a forma ed strings ─ Example

Lecture 3’s references
 M. Mazidi, J. Mazidi, R. McKinlay, “The 8051 microcontroller and embedded systems
using assembly and C,” 2nd ed., Pearson Prentice Hall, 2006, [Chapters 10]
 M. Mazidi and J. Mazidi, “The 8086 IBM PC and compatible computers,” 4th ed.,
Pearson Prentice Hall, 2003, [Chapters 17]
 P. Spasov, “Microcontroller technology the 68HC11,” 3rd ed., Prentice Hall 1999,
[Chapters 10]
 H Huang . Huang, “MC68HC12 an introduction: software and hardware interfacing,”
Thomson Delmar Learning, 2003, [Chapter 9]

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