Definitions of Embedded systems:
Using the term "Embedded Systems" in our name is confusing to some. In this case, people either have
no idea what the term means - or they have a very strict definition in mind, such as "assembly language
on a chip". But the work that we do is so much broader than that!
A general-purpose definition of embedded systems is that they are devices used to control, monitor or
assist the operation of equipment, machinery or plant. "Embedded" reflects the fact that they are an
integral part of the system. In many cases their embedded nature may be such that their presents is far
from obvious to the casual observer and even the more technically skilled might need to examine the
operations of a piece of equipment for some time before being able to conclude that an embedded
control system was involved in its function. "
Huh?.....Doesn’t this sound like” Blah Blah Blah Blah …………………Blah”
Let’s make it very simple
An embedded system is a special-purpose computer system designed to perform one or a few
dedicated functions, often with real-time computing constraints. It is usually embedded as part of a
complete device including hardware and mechanical parts. In contrast, a general-purpose computer,
such as a personal computer, can do many different tasks depending on programming. Embedded
systems control many of the common devices in use today.
Another one:
Any electronic system that uses a computer chip, but that is not a general-purpose workstation,
desktop or laptop computer. Such systems use microcontrollers (MCUs) or microprocessors (MPUs), or
they may use custom-designed chips. Deployed by the billions each year in myriad applications, the
embedded systems market uses the lion's share of all the electronic components in the world.
Most of the definitions of embedded systems revolve around the idea of it being a general purpose
computer designed to perform a specific task.
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Uses:
Embedded systems are employed in automobiles, planes, trains, space vehicles, machine tools, cameras,
consumer electronics, office appliances, network appliances, video games, cell phones, PDAs, GPS
navigation as well as robots and toys. Low-cost consumer products can use microcontroller chips that
cost less than a dollar.
Characteristics:
1) Embedded systems are designed to do some specific task, rather than be a general-purpose
computer for multiple tasks. Some also have real time performance constraints that must be
met, for reasons such as safety and usability; others may have low or no performance
requirements, allowing the system hardware to be simplified to reduce costs.
2) Embedded systems are not always standalone devices. Many embedded systems consist of
small, computerized parts within a larger device that serves a more general purpose. For
example, the Gibson Robot Guitar features an embedded system for tuning the strings, but the
overall purpose of the Robot Guitar is, of course, to play music. Similarly, an embedded system
in an automobile provides a specific function as a subsystem of the car itself.
3) The program instructions written for embedded systems are referred to as firmware, and are
stored in read-only memory or Flash Memory chips. They run with limited computer hardware
resources: little memory, small or non-existent keyboard and/or screen.
Listing ‘buzz-words’
(Micro-controllers, micro-processors, ARM, Thumb, AVR, PIC, 8051, OS, Real time, RTOS, firmware,
Linux, RT Linux, Assembly language, C, Java, BSP, Endian-ness, (cross)compiler, CISC, RISC, Debugger,
Emulator, Simulator, POSIX, MISRA, Device driver, Programmer, Flash, DSP, platform, architecture,
system Programmer, system software, application software, GUI, HMI, fuzzy logic, GNU, kernel,
scheduler, context switch, data structures, linker, locator, multi-processing, multi-tasking, multi-
threading, interrupts, hex code, object code, polling, pre-emption, protocol stack, TCP/IP, CAN, I2C, SPI,
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Zigbee, profiler, reentrant, recursive, simulator, interpreter, UML, Vxworks, Watchdog, Wince, FSF, open
source, closed source)
These are some of the ‘Buzz Words’ that one comes across whenever we talk about Embedded Systems.
The Layered Architecture categorizes them into different layers.
The layered Architecture:
Architecture Exclusions
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Since embedded system use microcontrollers (MCUs), lets get to know microcontrollers more into
details:
Get a feel of Microcontrollers
Microcontrollers are, for us robotists, a godsend. They showed us how we can make interesting projects
in the seemingly dull course structure. They had us addicted to the pure pleasure of creating things and
made us feel powerful. They gave us the first glimpses of what its like to be an ‘Engineer’.
How did such a thing as a microcontroller come into existence? Was it an outcome of an experiment
meant for something else or did anyone conceptualize it long before its birth?
Microcontrollers could be understood very easily by reading some very good books. But understanding
its history will make you feel more closer. This is an attempt to that effect.
Let’s have a look at what it was like before the microcontrollers.
The computers started getting recognition during the 2nd World War. They were made of
vacuum tubes and switches. Needless to say, they were BIG!
John Bardeen, Walter Brattain and William Shockley discovered the transistor effect and
developed the first device in December 1947, while the three were members of the technical
staff at Bell Laboratories in Murray Hill, NJ.
In 1950's Shockley leaves Bell Labs to establish Shockley Labs in California. Some of the best
young electronic engineers and solid-state physicists come to work with him. These include
Robert Noyce and Gordon Moore.
Intel was started-up in 1969 in Santa Clara, headed by Noyce and Moore.
In 1970, A Japanese company, Busicom placed an order with Intel for custom calculator chips.
But Intel had problems working with the chip designs. Whiz Federico Faggin joins Intel and
solves the problems in weeks. The result is the 4004, a 4-bit processor.
This is how the Busicom calculator looked like. It is the 1st
comercial product which uses a microprocessor.
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Riding on it’s success, Intel begins its work on an 8- bit microprocessor 8008, and soon after
that, 8080. Both these devices were well recieved. Suddenly there was a surge of interest in
microprocessors.
Fagin left Intel to form his own company, Zilog. Z80, an 8-bit microprocessor was the 1st product
of Zilog.
Motorola, a car radio company, jumps into the field of microprocessors with its 6800 – an 8-bit
microprocessor.
Note that microcontrollers were not invented yet! All these devices were microprocessors’.
In 1976- Intel introduce an 8-bit microcontroller, the MCS-48.
4 years later, Intel comes up with another 8-bit microcontroller – the 8051. This was a legend.
Variants of the 8051 are still used dearly by many people.
Later, Intel kept the architecture of 8051 on market. Many companies adopted this architecture and
started making variants of 8051.
The momentum kept growing. And Companies like Atmel and Microchip are coming out with more
advanced microcontrollers like AVR and PIC respectively.
George Moore, co-founder of Intel, predicted that the number of transistors on a chip will double every
year. This was just an observation, which happened to be true during the last two decades. It is famously
known as Moore’s Law.
The quest for better and bigger processors continued. Today, we have attained the expertise to design a
64-bit microprocessor.
So let’s answer some questions that we call ‘silly’, but still need to be cleared..
What is a microprocessor?
Microprocessor is a collection of commonly used functions, (like arithmetic operations, logical
operations..) integrated in an IC. This device enables us to use these function on our choice of data, in
any order we want, any number of times. That is why we call it a general purpose device.
Then what is a microcontroller?
Microprocessors lack the ability to store data permanently. Hence it needs to be connected to external
memories for essential operations. Hence, microprocessor cannot be used as a stand-alone device. This
generated the need for microcontrollers. Microcontrollers come with a microprocessor and memory
integrated in a single IC. Microcontrollers are also packed with other features like I/O pins, timers etc..
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Different families of Microcontrollers
Some widely used families and what communities have to say about them
(courtesy: Spark fun electronics)
8051 ‐ The '8051 core' was the de facto standard in 8‐bit (and 4‐bit!) microcontrollers. Developed by
Intel in the 1980s, it still seems to be the instruction set they love to teach you in college. They are
based on archaic, but field proven instruction sets. Very old tech in my humble opinion, but these ICs
have been significantly improved over the years (now Flash based, ADC, SPI, etc.).
68HC08/11 ‐ Another very common instruction set developed by Motorola. Extremely popular, and a
micro commonly taught at university, it's the microcontroller I love to hate. These original micros often
lack on‐board RAM and flash based memory.
PIC - This is the classic micro from Microchip. Very simple, very proven, but it lacks many of the features
that other mfg's are building into their chips. This is a big deal for me. I was a die‐hard PIC person for
years and I've started to see the limits of PICs and the benefits of other micros!
AVR ‐ This (Atmel) is basically a direct competitor of PICs. They do everything a PIC does, but in my new
opinion, better, faster, cheaper, and simpler.
MSP ‐ These are very good micros by Texas Instruments (TI), not as beefy as AVR or PICs. However they
truly excel at low‐power applications. More on this later, but imagine running a complete system on one
AA battery for 5 years. This is in the realm of nano‐amp current consumption years. Crazy!
ARM ‐ Why are all these three letters? I don't know actually... ARMs are the new kids on the block and
they are huge. Very powerful, very low‐cost, extremely low power consumption they are taking over the
world (85% of 32‐bit market is owned by ARM) but can be really intimidating if you've never played
with a micro before.
Overkill in majority of applications except mobiles, consumer multimedia items and SBCs
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I/O PORTs
The 1st Feature that needs to be learnt in any microcontroller is its Input-Output function. Let’s know
about the specific case of AVR series microcontroller, ATmega128. Though we are discussing specific to
a microcontroller here, try to understand the general concept behind the I/O Ports, its usage, It’s
functionality through the following discussion.
ATmega128 has a total of 64 pins. Of these, 53 pins can be used as Input or Outputs. These pins are
divided into 7 ports :
PORT A - 8 pins
PORT B - 8 pins
PORT C - 8 pins
PORT D - 8 pins
PORT E - 8 pins
PORT F - 8 pins
PORT G - 5 pins
Each of these pins can can be individually programmed as Inputs or Outputs. So you can use all 53 pins
as input or all 53 pins as outputs or any how...
But there are some steps that we need to perform to configure the pins as either.
Each port is associated with 3 registers: DDRx, PORTx, PINx. Each bit of these registers correspond to
each pin of the port.
For example, the pin 3 of port D is associated with BIT 3 of DDRD, BIT 3 of PORTD and BIT 3 of PIND
Here, we review how to configure pins as required.
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Output:
DDRx: Data Direction Register – Whichever pin is supposed to be an output, write 1 in the
corresponding DDRx bit.
PORTx: Write whatever data that needs to be output in the corresponding pins.
Example: Make pins 1,2,3 and 4 of PORTB as an output port and write 1 0 1 0 on those pins respectively:
DDRB: 7 6 5 4 3 2 1 0
Data: 0 0 0 1 1 1 1 0 = 1E (Hexadecimal)
------------------------------------------------------------------------------------------------------------------------------------
PORTB: 7 6 5 4 3 2 1 0
Data: 0 0 0 1 0 1 0 0 = 14 (Hexadecimal)
Code: DDRD = 0x1E;
PORTD = 0x14;
Input:
DDRx: Data Direction Register – Whichever pin is supposed to be an input, write 0 in the corresponding
DDRx bit.
PORTx: To enable pull up registers, (which should always be done if not for any special cases), write 1 in
the corresponding PORTx bit.
PINx: Read the input of the pins at the corresponding bits.
Example: Configure the pins 0,1,2 & 3 of port D as input and read the value at the pins.
DDRD: 7 6 5 4 3 2 1 0
Data: 0 0 0 0 0 0 0 0 = 00 (Hexadecimal)
------------------------------------------------------------------------------------------------------------------------------------
PORTB: 7 6 5 4 3 2 1 0
Data: 0 0 0 0 1 1 1 1 = 0F (Hexadecimal)
Code: DDRD = 0x00;
PORTD = 0x0F;
Value = PIND; //read status of Input Pins
Value = Value & 0x0F; //to mask the higher 4 bits.
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The Concept of Pull Up Resistors:
Pull up resistors are used on the input side so that the input pins are at an expected logic level even if
they are disconnected from the input switches. If Pull Up resistors are not used, the voltage level at the
input pin will be floating in such a case and hence the outcome of the circuit is unpredictable.
Consider the following circuit:
/ -------
_____/ -------| |---
| ---| |---
| ---| |---
\ / -------
GND
Here, a switch is connected at the input pin of a microcontroller. Note the absence of a pull up resistor.
When the switch is closed, the pin is directly grounded and the input pin reads a logic level 0. Thus logic
level 0 should indicate that the switch is closed. Now consider when the switch is open. The input pin is
not connected to anything else, hence the pin in open. Thus the input pin is said to be kept floating. i.e.
the voltage at the pin may vary from 0 V to 5V randomly. Thus we cannot be sure every time that when
the logic at pin is 0, it is because of the closed switch or the floating voltage.
Hence this circuit is not appropriate.
Now consider the circuit below:
VCC
/ \
|
|
/ | -------
_____/ -------| |---
| ---| |---
| ---| |---
\ / -------
GND
This circuit will eliminate our problem of floating pin. It is obvious that when the pin is open, the voltage
at the input pin is Vcc. But now imagine what will happen if the switch is closed.. What do you think will
the voltage be at the input pin? You don’t have the time to calculate that! You have shorted Vcc and
ground of your Power Supply! This is a very wrong method to eliminate our original problem of floating
voltage.
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Now see what’s happening here:
VCC
/ \
|
|
\
/ Pull-up resistor
\
|
|
/ | -------
_____/ -------| |---
| ---| |---
| ---| |---
\ / -------
GND
When the switch is closed, the pin is directly connected to ground and reads logic level 0. When the
switch is opened, the pin is connected to Vcc through a high value resistor; hence it reads a logic value 1.
Thus, the use of pull up resistor has solved our problem.
Note that a high value pull up resistor must be used to limit the current flow to ground when the switch
is closed.
http://www.thinklabs.in © T hi
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LCD PROGRAMMING
LCD or Liquid Crystal Display is nowadays fast becoming a preferred choice for an interface device due to
its ease of use, cheap rates and value for money performance.
Every LCD needs a LCD driver to interface between the microcontroller and LCD module. This driver is
included on the LCD module.
The LCD we are using is 16 X 2 LCD, which means 16 columns and 2 rows.
1 . . . . . . . . . . . . . . . . . . . . . . . . .16
The pin configuration of the LCD is given below:
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The LCD uses three control lines. They are referred to as EN, RS, and RW.
The EN line is called "Enable." This control line is used to tell the LCD that you are sending it data. To
send data to the LCD, your program should make sure this line is low (0) and then set the other two
control lines and/or put data on the data bus. When the other lines are completely ready, bring EN high
(1) and wait for the minimum amount of time required by the LCD datasheet (this varies from LCD to
LCD), and end by bringing it low (0) again.
The RS line is the "Register Select" line. When RS is low (0), the data is to be treated as a command or
special instruction (such as clear screen, position cursor, etc.). When RS is high (1), the data being sent is
text data which should be displayed on the screen. For example, to display the letter "T" on the screen
you would set RS high.
The RW line is the "Read/Write" control line. When RW is low (0), the information on the data bus is
being written to the LCD. When RW is high (1), the program is effectively querying (or reading) the LCD.
Only one instruction ("Get LCD status") is a read command. All others are write commands--so RW will
almost always be low.
4 Bit Mode:
The LCD can work in 8 bit parallel mode or a 4 bit mode. We will use the 4 bit mode as it reduces the
number of pins and simplifies the circuit.
The only difference with the 8bit version is DB0, DB1, DB2 and DB3 on the display module side. These
lines are not connected to the processor. Leave those lines unconnected.
In 4-bit mode, we have to read and write data bytes and command bytes in two separate 'nibbles' (4bit
parts).
INITIALIZING THE LCD
Before you may really use the LCD, you must initialize and configure it. This is accomplished by sending a
number of initialization instructions to the LCD.
1. The first instruction we send must tell the LCD we'll be communicating with it with a 4-bit data
bus. We also select a 5x8 dot character font. These two options are selected by sending the
command 28h to the LCD as a command. After powering up the LCD, it is in 8-bit mode. Because
only four bits are connected, the LCD receives only 20 this sets it to the 4 bit mode.
2. We've now sent the first byte of the initialization sequence. The second byte of the initialization
sequence is the instruction 0Fh. This command is sent to instruct “ Display on, Cursor blinking”
3. Other commands may be sent in this way (higher nibble 1st. then lower nibble). Refer to table
below for the command codes.
4. Now we are ready to send the data to be displayed in packs to two nibbles.
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To initialize some more parameters, or otherwise, you can use the following command table:
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Refer the following table for ASCII codes of characters.
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UART – Universal Asynchronus Reciever Transmitter
Although it has a big name, this module of AVR is one of the easiest to use. UART is a protocol which has
been standardized and commonly used by many devices. It is so popular, that even the serial port
behind out computer CPUs uses this protocol to communicate with other devices.
UART is a serial protocol, which means that it transmits data, one bit at a time. So, we need only on
conductor to transmit data.
It is a very use full practice of developers to use the UART as a debugging tool.
When a connectivity between the PC and microcontroller is established, data could be obtained from PC
or Displayed on the Monitor. Computers using Windows have a utility called has Hyper Terminal in their
Start Menu > Programs > Accessories > Communications > Hyper Terminal.
Linux users can use same type of utility called as GTKterm.
Note that AVR actually has a USART module (Universal Synchronous Asynchronous Receiver
Transmitter). But communication with the computer requires UART protocol. Hence we focus our
attention on UART.
This goes for all the features of AVR: All you need to do to start using the function is just configure the
associated registers.
UART:
The initialization process normally consists of setting the baud rate, setting frame format and enabling
the Transmitter or the Receiver depending on the usage.
USART Registers
UCSRA
• Bit 7 – RXC: USART Receive Complete
This flag bit is set when there are unread data in the receive buffer and cleared when the receive buffer
is empty (i.e. does not contain any unread data). If the Receiver is disabled, the receive buffer will be
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flushed and consequently the RXC bit will become zero. The RXC Flag can be used to generate a Receive
Complete interrupt.
• Bit 6 – TXC: USART Transmit Complete
This flag bit is set when the entire frame in the Transmit Shift Register has been shifted out and there
are no new data currently present in the transmit buffer (UDR). The TXC Flag bit is automatically cleared
when a transmit complete interrupt is executed, or it can be cleared by writing a one to its bit location.
The TXC Flag can generate a Transmit Complete interrupt.
• Bit 5 – UDRE: USART Data Register Empty
The UDRE Flag indicates if the transmit buffer (UDR) is ready to receive new data. If UDRE is one, the
buffer is empty, and therefore ready to be written. The UDRE Flag can generate a Data Register Empty
interrupt. UDRE is set after a reset to indicate that the Transmitter is ready.
• Bit 4 – FE: Frame Error
This bit is set if the next character in the receive buffer had a Frame Error when received (i.e., when the
first stop bit of the next character in the receive buffer is zero). This bit is valid until the receive buffer
(UDR) is read. The FE bit is zero when the stop bit of received data is one. Always set this bit to zero
when writing to UCSRA.
• Bit 3 – DOR: Data OverRun
This bit is set if a Data OverRun condition is detected. A Data OverRun occurs when the receive buffer is
full (two characters), it is a new character waiting in the Receive Shift Register, and a new start bit is
detected. This bit is valid until the receive buffer (UDR) is read. Always set this bit to zero when writing
to UCSRA.
• Bit 2 – PE: Parity Error
This bit is set if the next character in the receive buffer had a Parity Error when received and the parity
checking was enabled at that point (UPM1 = 1). This bit is valid until the receive buffer
(UDR) is read. Always set this bit to zero when writing to UCSRA.
• Bit 1 – U2X: Double the USART transmission speed
This bit only has effect for the asynchronous operation. Write this bit to zero when using synchronous
operation.
• Bit 0 – MPCM: Multi-processor Communication Mode
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This bit enables the Multi-processor Communication mode. When the MPCM bit is written to one, all
the incoming frames received by the USART Receiver that do not contain address information will be
ignored. The Transmitter is unaffected by the MPCM setting.
UCSRB
• Bit 7 – RXCIE: RX Complete Interrupt Enable
Writing this bit to one enables interrupt on the RXC Flag. A USART Receive Complete interrupt will be
generated only if the RXCIE bit is written to one, the Global Interrupt Flag in SREG is written to one and
the RXC bit in UCSRA is set.
• Bit 6 – TXCIE: TX Complete Interrupt Enable
Writing this bit to one enables interrupt on the TXC Flag. A USART Transmit Complete interrupt will be
generated only if the TXCIE bit is written to one, the Global Interrupt Flag in SREG is written to one and
the TXC bit in UCSRA is set.
• Bit 5 – UDRIE: USART Data Register Empty Interrupt Enable
Writing this bit to one enables interrupt on the UDRE Flag. A Data Register Empty interrupt will
be generated only if the UDRIE bit is written to one, the Global Interrupt Flag in SREG is written to one
and the UDRE bit in UCSRA is set.
• Bit 4 – RXEN: Receiver Enable
Writing this bit to one enables the USART Receiver. The Receiver will override normal port operation for
the RxD pin when enabled. Disabling the Receiver will flush the receive buffer invalidating the FE, DOR
and PE Flags.
• Bit 3 – TXEN: Transmitter Enable
Writing this bit to one enables the USART Transmitter. The Transmitter will override normal port
operation for the TxD pin when enabled. The disabling of the Transmitter (writing TXEN to zero) will not
become effective until ongoing and pending transmissions are completed (i.e., when the Transmit Shift
Register and Transmit Buffer Register do not contain data to be transmitted). When disabled, the
Transmitter will no longer override the TxD port.
• Bit 2 – UCSZ2: Character Size
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The UCSZ2 bits combined with the UCSZ1:0 bit in UCSRC sets the number of data bits (Character Size) in
a frame the Receiver and Transmitter use.
• Bit 1 – RXB8: Receive Data Bit 8
RXB8 is the ninth data bit of the received character when operating with serial frames with nine data
bits. Must be read before reading the low bits from UDR.
• Bit 0 – TXB8: Transmit Data Bit 8
TXB8 is the ninth data bit in the character to be transmitted when operating with serial frames with nine
data bits. Must be written before writing the low bits to UDR.
UCSRC
• Bit 7 – URSEL: Register Select
This bit selects between accessing the UCSRC or the UBRRH Register. It is read as one when reading
UCSRC. The URSEL must be one when writing the UCSRC.
• Bit 6 – UMSEL: USART Mode Select
This bit selects between Asynchronous and Synchronous mode of operation.
• Bit 5:4 – UPM1:0: Parity Mode
These bits enable and set type of Parity Generation and Check. If enabled, the Transmitter will
automatically generate and send the parity of the transmitted data bits within each frame. The
Receiver will generate a parity value for the incoming data and compare it to the UPM0 setting.
If a mismatch is detected, the PE Flag in UCSRA will be set.
• Bit 3 – USBS: Stop Bit Select
This bit selects the number of stop bits to be inserted by the transmitter. The Receiver ignores this
setting.
• Bit 2:1 – UCSZ1:0: Character Size
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The UCSZ1:0 bits combined with the UCSZ2 bit in UCSRB sets the number of data bits (Character Size) in
a frame the Receiver and Transmitter use.
• Bit 0 – UCPOL: Clock Polarity
This bit is used for Synchronous mode only. Write this bit to zero when Asynchronous mode is used. The
UCPOL bit sets the relationship between data output change and data input sample, and the
synchronous clock (XCK).
UMSEL bit setting
UMSEL Mode
0 Asynchronous Operation
1 Synchronous Operation
UPM Bits setting
UMSEL UPM0 Mode
0 0 Disabled
0 1 Reserved
1 0 Enable, Even Parity
1 1 Enabled, Odd Parity
USBS Bit Setting
USBS Stop Bits(s)
0 1-bit
1 2-bit
UCSZ Bits Setting
UCSZ2 UCSZ1 UCSZ0 Character Size
0 0 0 5-bit
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0 0 1 6-bit
0 1 0 7-bit
0 1 1 8-bit
1 0 0 Reserved
1 0 1 Reserved
1 1 0 Reserved
1 1 1 9-bit
UCPOL Bit Setting
Transmitted Data Changed Received Data Sampled (Input on RxD Pin)
USBS
(Output of TxD Pin)
0 Rising XCK Edge Falling XCK Edge
1 Falling XCK Edge Rising XCK Edge
UBRR
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• Bit 15 – URSEL: Register Select
This bit selects between accessing the UBRRH or the UCSRC Register. It is read as zero when reading
UBRRH. The URSEL must be zero when writing the UBRRH.
• Bit 14:12 – Reserved Bits
These bits are reserved for future use. For compatibility with future devices, these bit must be written to
zero when UBRRH is written.
• Bit 11:0 – UBRR11:0: USART Baud Rate Register
This is a 12-bit register which contains the USART baud rate. The UBRRH contains the four most
significant bits, and the UBRRL contains the eight least significant bits of the USART baud rate. Ongoing
transmissions by the Transmitter and Receiver will be corrupted if the baud rate is changed. Writing
UBRRL will trigger an immediate update of the baud rate prescaler.
Example of UBRR setting for commonly Used oscillator Frequencies (Continued)
Fosc = 16,0000 MHz Fosc = 18,4320 MHz Fosc = 20,0000 MHz
Baud
Rate U2X = 0 U2X = 1 U2X = 0 U2X = 1 U2X = 0 U2X = 1
(bps)
UBRR Error UBRR Error UBRR Error UBRR Error UBRR Error UBRR Error
2400 416 -0.1% 832 0.0% 479 0.0% 959 0.0% 520 0.0% 1041 0.0%
4800 207 0.2% 416 -0.1% 239 0.0% 479 0.0% 259 0.2% 520 0.0%
9600 103 0.2% 207 0.2% 119 0.0% 239 0.0% 129 0.2% 259 0.2%
14.4k 68 0.6% 138 0.1% 79 0.0% 159 0.0% 86 -0.2% 173 -0.2%
19.2k 51 0.2% 103 0.2% 59 0.0% 119 0.0% 64 0.2% 129 0.2%
28.8k 34 -0.8% 68 0.6% 39 0.0% 79 0.0% 42 0.9% 86 -0.2%
38.4k 25 0.2% 51 0.2% 29 0.0% 59 0.0% 32 -1.4% 64 0.2%
57.6k 16 2.1% 34 -0.8% 19 0.0% 39 0.0% 21 -1.4% 42 0.9%
76.8k 12 0.2% 25 0.2% 14 0.0% 29 0.0% 15 1.7% 32 -1.4%
115.2k 8 -3.5% 16 2.1% 9 0.0% 19 0.0% 10 -1.4% 21 -1.4%
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230.4k 3 8.5% 8 -3.5% 4 0.0% 9 0.0% 4 8.5% 10 -1.4%
250k 3 0.0% 7 0.0% 4 -7.8% 8 2.4% 4 0.0% 9 0.0%
0.5M 1 0.0% 3 0.0% - - 4 -7.8% - - 4 0.0%
1M 0 0.0% 1 0.0% - - - - - - - -
Max
(1)
1Mbps 2Mbps 1.158 Mbps 2.304Mbps 1.25 Mbps 25 Mbps
Frame Formats :
A serial frame is defined to be one character of data bits with synchronization bits (start and stop bits),
and optionally a parity bit for error checking. The USART accepts all 30 combinations of
the following as valid frame formats:
• 1 start bit
• 5, 6, 7, 8, or 9 data bits
• no, even or odd parity bit
• 1 or 2 stop bits
A frame starts with the start bit followed by the least significant data bit. Then the next data bits,
up to a total of nine, are succeeding, ending with the most significant bit. If enabled, the parity bit
is inserted after the data bits, before the stop bits. When a complete frame is transmitted, it can
be directly followed by a new frame, or the communication line can be set to an idle (high) state.
Figure Below illustrates the possible combinations of the frame formats. Bits inside brackets are
optional.
Frame Formats
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St ---Start bit, always low.
(n) ----Data bits (0 to 8).
P ----Parity bit. Can be odd or even.
Sp ----Stop bit, always high.
IDLE ----No transfers on the communication line (RxD or TxD). An IDLE line must be high.
The frame format used by the USART is set by the UCSZ2:0, UPM1:0 and USBS bits in UCSRB
and UCSRC. The receiver and transmitter use the same setting. Note that changing the setting
of any of these bits will corrupt all ongoing communication for both the receiver and transmitter.
The USART Character SiZe (UCSZ2:0) bits select the number of data bits in the frame. The
USART Parity mode (UPM1:0) bits enable and set the type of parity bit. The selection between
one or two stop bits is done by the USART Stop Bit Select (USBS) bit. The receiver ignores the
second stop bit. An FE (Frame Error) will therefore only be detected in the cases where the first
stop bit is zero.
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ADC – Analog to Digital Converter
As the name says, this module of the AVR converts an input analog signal to a digital signal.
Why? All the microprocessors and microcontrollers are digital devices. This means that they only
understand 1 and 0? Now how do you represent analog values in this format?
For example, if I want to sense the temperature using a thermostat, how do I read its value? There
should be a device which converts this analog output of the thermostat and send it to the controller in
interpretable terms. This device is the ADC. ATmega8 has In Built 8 ADCs. In effect, I could connect 6
different thermostats to 8 pins of the ATmega8 and sense temperatures of 8 different places.
Again, use of ADC boils down to the correct configuration of the following registers:
ADC Registers
ADMUX
• Bit 7:6 – REFS1:0: Reference Selection Bits
These bits select the voltage reference for the ADC. If these bits are changed during a conversion, the
change will not go in effect until this conversion is complete (ADIF in ADCSRA is set). The internal voltage
reference options may not be used if an external reference voltage is being applied to the AREF pin.
• Bit 5 – ADLAR: ADC Left Adjust Result
The ADLAR bit affects the presentation of the ADC conversion result in the ADC Data Register.
Write one to ADLAR to left adjust the result. Otherwise, the result is right adjusted. Changing the ADLAR
bit will affect the ADC Data Register immediately, regardless of any ongoing conversions.
• Bits 4:0 – MUX4:0: Analog Channel Selection Bits
The value of these bits selects which analog inputs are connected to the ADC. If these bits are changed
during a conversion, the change will not go in effect until this conversion is complete (ADIF in ADCSRA is
set).
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Voltage Reference selections for ADC
REFS1 REFS0 Voltage Reference Selection
0 0 AREF, Internal Vref turned off
Voltage Reference selections for ADC
REFS1 REFS0 Voltage Reference Selection
0 1 AVcc with external capacitor at AREF pin
1 0 Reserved
1 1 Internal 2.56V Voltage Reference with external capacitor at AREF pin
Input Channel Selections
MUX3..0 Single Ended Input
0000 ADC0
0001 ADC1
0010 ADC2
0011 ADC3
0100 ADC4
0101 ADC5
0110 ADC6
0111 ADC7
1000 -
1001 -
1010 -
1011 -
1100 -
1101 -
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1110 1.22V (VBG)
1111 0V (GND)
ADCSRA
• Bit 7 – ADEN: ADC Enable
Writing this bit to one enables the ADC. By writing it to zero, the ADC is turned off. Turning the
ADC off while a conversion is in progress, will terminate this conversion.
• Bit 6 – ADSC: ADC Start Conversion
In Single Conversion mode, write this bit to one to start each conversion. In Free Running mode, write
this bit to one to start the first conversion. The first conversion after ADSC has been written after the
ADC has been enabled, or if ADSC is written at the same time as the ADC is enabled, will take 25 ADC
clock cycles instead of the normal 13. This first conversion performs initialization of the ADC. ADSC will
read as one as long as a conversion is in progress. When the conversion is complete, it returns to zero.
Writing zero to this bit has no effect.
• Bit 5 – ADFR: ADC Free Running Select
When this bit is set (one) the ADC operates in Free Running mode. In this mode, the ADC samples and
updates the Data Registers continuously. Clearing this bit (zero) will terminate Free Running mode.
• Bit 4 – ADIF: ADC Interrupt Flag
This bit is set when an ADC conversion completes and the Data Registers are updated. The ADC
Conversion Complete Interrupt is executed if the ADIE bit and the I-bit in SREG are set. ADIF is cleared by
hardware when executing the corresponding interrupt Handling Vector. Alternatively, ADIF is cleared by
writing a logical one to the flag. Beware that if doing a Read-Modify- Write on ADCSRA; a pending
interrupt can be disabled.
• Bit 3 – ADIE: ADC Interrupt Enable
When this bit is written to one and the I-bit in SREG is set, the ADC Conversion Complete Interrupt is
activated.
• Bits 2:0 – ADPS2:0: ADC Prescaler Select Bits
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These bits determine the division factor between the XTAL frequency and the input clock to the ADC.
ADC Prescaler Selections
ADPS2 ADPS1 ADPS0 Division Factor
0 0 0 2
0 0 1 2
0 1 0 4
0 1 1 8
1 0 0 16
1 0 1 32
1 1 0 64
1 1 1 128
ADC Data Register ADCH & ADCL
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When an ADC conversion is complete, the result is found in these two registers. If differential
channels are used, the result is presented in two’s complement form.
When ADCL is read, the ADC Data Register is not updated until ADCH is read. Consequently, if
the result is left adjusted and no more than 8-bit precision is required, it is sufficient to read
ADCH. Otherwise, ADCL must be read first, then ADCH.
The ADLAR bit in ADMUX, and the MUXn bits in ADMUX affect the way the result is read from
the registers. If ADLAR is set, the result is left adjusted. If ADLAR is cleared (default), the result
is right adjusted.
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