Serial Communication
Thareendhra Keerthi Wijayasiriwardhane
Department of Industrial Management
Faculty of Science
University of Kelaniya
[email protected] Learning Outcomes
On completion of this session, you should be able to
Discuss serial vs. parallel communications in the context
of embedded systems
Contrast asynchronous vs. synchronous communications
Explain asynchronous data transfer protocol
Explain need of Line Drivers/USART to USB interfaces
Explain how Tx and Rx transmit and receive data
Explain the functionality of USART registers
Construct a microcontroller-based system and program it
to communicate with another system
2
Serial Communication
Used by embedded systems to communicate with other
systems
What are other systems?
Sends 1 bit at a time
Why not parallel?
3
USART
A peripheral device that implements both
asynchronous and synchronous serial communications
Asynchronous
Synchronous
Why separate
peripheral
device for data
communication?
4
Asynchronous Data Transfer
Each character is framed between a start bit and a stop
bit and then transmitted
Why baud rate is important?
How can receiver recognize transmitted characters?
5
Connection over Serial Port
Using a Line Driver
Why not connect directly?
6
Arduino Severino
ATmega328P with MAX232
7
Connection over USB Port
Using a UART to USB interface controller
8
Arduino Duemilanove
ATmega328P
with FT232RL
9
Connection over USB Port
Using another Microcontroller as UART to USB
interface controller
10
Arduino Uno
ATmega328P
with ATmega16U2
11
Asynchronous Transmitter
Which flag (UDRE or TXC) gives maximum throughput?
What’s the purpose of TXC flag?
12
Asynchronous Receiver
What’s the role of Pin Control?
What’s the need of Shift Registers?
13
USART Registers
UBRRn (USART Baud Rate Register n)
Used to set baud rate
16-bit register but only 12 bits are used
14
USART Registers
UDRn (USART Data Register n)
Refers to Transmit Data Buffer that holds the character
to be transferred to Transmit Shift Register
Also refers to Receive Data Buffer that holds the
character transferred from Receive Shift Register
15
USART Registers
UCSRnA (USART Control and Status Register n A)
RXCn (Receive Complete) flag bit is set when UDRn
contains unread data
TXCn (Transmit Complete) flag bit is set when entire
frame in Transmit Shift Register has been shifted out and
UDRn is empty
UDREn (USART Data Register Empty) flag bit is set
when UDRn becomes empty
16
USART Registers
UCSRnB (USART Control and Status Register n B)
RXCIEn (Receive Complete Interrupt Enable) bit and
TXCIEn (Transmit Complete Interrupt Enable) bit
enable interrupts on RXCn and TXCn flags, respectively
UDRIEn (USART Data Register empty Interrupt
Enable) bit enables interrupt on UDREn flag
RXENn (Receiver ENable) and TXENn (Transmitter
ENable) bits enable RX and TX, respectively
What is the need of enabling and disabling RX and TX?
17
USART Registers
UCSRnC (USART Control and Status Register n C)
UCSZn0 (USART Character SiZe), UCSZn1 bits and
UCSZn2 bit of UCSRnB specify character size for
communication
UCSZn2 UCSZn1 UCSZn0 Character Size
0 0 0 5-bit
0 0 1 6-bit
0 1 0 7-bit
0 1 1 8-bit
1 1 1 9-bit
18
System 16
charEncloser - Receives characters
from PC, encloses them with
square brackets and transmits
enclosed characters back to PC
19
ATmega328P USART Pinout
20
Hardware
Schematic Diagram
21
Hardware
Wiring Diagram
22
Firmware
Algorithm
23
Setting Baud Rate
Select a standard baud rate from 2400, 4800, 9600,
14400, 19200, 28800, 38400, 57600, 76800, 115200,
230400, 460800, to 921600 bps
Why not other values?
Set the baud rate
Determine and
set the value of UBRR
Why do you need to add UL suffix?
24
Baud Rate Error
Determine actual baud rate
Calculate baud rate error
What would happen if a baud rate
with higher error % is used?
How to minimize baud rate error?
25
C Program
26
Extending IDE with a Terminal
What’s a
Terminal
Emulator?
27
Communicating over Terminal
What are CR
and LF?
28
Firmware
Algorithm for Arduino
29
Firmware
Source Code (using Arduino Wrapper Functions)
30
Serial Monitor
Terminal emulator of Arduino IDE
31
System 17
charEncloserUsingInterrupts -
Receives characters from PC,
encloses them with square brackets
and transmits enclosed characters
back to PC using interrupts
32
Firmware
Algorithm
33
C Program
34
Using Labeled Constants
35
Questions
36
