INTRODUCTION TO AVR

Microcontroller: Microcontroller can be termed as a single on chip computer which includes number of peripherals like RAM, EEPROM, Timers etc., required to perform some predefined task.
Architecture of AVR MicrocontrollerArchitecture of AVR Microcontroller
Does this mean that the microcontroller is another name for a computer…? The answer is NO!
The computer on one hand is designed to perform all the general purpose tasks on a single machine like you can use a computer to run a software to perform calculations or you can use a computer to store some multimedia file or to access internet through the browser, whereas the microcontrollers are meant to perform only the specific tasks, for e.g., switching the AC off automatically when room temperature drops to a certain defined limit and again turning it ON when temperature rises above the defined limit.
There are number of popular families of microcontrollers which are used in different applications as per their capability and feasibility to perform the desired task, most common of these are 8051, AVR and PIC microcontrollers. In this article we will introduce you with AVR family of microcontrollers.
History of AVR
AVR was developed in the year 1996 by Atmel Corporation. The architecture of AVR was developed by Alf-Egil Bogen and Vegard Wollan. AVR derives its name from its developers and stands for Alf-Egil Bogen Vegard Wollan RISC microcontroller, also known as Advanced Virtual RISC. The AT90S8515 was the first microcontroller which was based on AVR architecture however the first microcontroller to hit the commercial market was AT90S1200 in the year 1997.
AVR microcontrollers are available in three categories:
1.      TinyAVR – Less memory, small size, suitable only for simpler applications
2.      MegaAVR – These are the most popular ones having good amount of memory (upto 256 KB), higher number of inbuilt peripherals and suitable for moderate to complex applications.
3.      XmegaAVR – Used commercially for complex applications, which require large program memory and high speed.
The following table compares the above mentioned AVR series of microcontrollers:
Series Name
Pins
Flash Memory
Special Feature
TinyAVR
6-32
0.5-8 KB
Small in size
MegaAVR
28-100
4-256KB
Extended peripherals
XmegaAVR
44-100
16-384KB
DMA , Event System included
AVR is an 8-bit microcontroller belonging to the family of Reduced Instruction Set Computer (RISC). In RISC architecture the instruction set of the computer are not only fewer in number but also simpler and faster in operation. The other type of categorization is CISC (Complex Instruction Set Computers). Click to find out differences between RISC and CISC. We will explore more on this when we will learn about the architecture of AVR microcontrollers in following section.
Let’s see what all this means. What is 8-bit? This means that the microcontroller is capable of transmitting and receiving 8-bit data. The input/output registers available are of 8-bits. The AVR family controllers have register based architecture which means that both the operands for an operation are stored in a register and the result of the operation is also stored in a register. Following figure shows a simple example performing OR operation between two input registers and storing the value in Output Register.
Working of AVR Microcontroller

The CPU takes values from two input registers INPUT-1 and INPUT-2, performs the logical operation and stores the value into the OUTPUT register. All this happens in 1 execution cycle.

In our journey with the AVR we will be working on Atmega16 microcontroller, which is a 40-pin IC and belongs to the megaAVR category of AVRfamily. Some of the features of Atmega16 are:

·      16KB of Flash memory
·         1KB of SRAM
·         512 Bytes of EEPROM
·         Available in 40-Pin DIP
·         8-Channel 10-bit ADC
·         Two 8-bit Timers/Counters
·         One 16-bit Timer/Counter
·         4 PWM Channels
·         In System Programmer (ISP)
·         Serial USART
·         SPI Interface
·         Digital to Analog Comparator.
Architecture of AVR
The AVR microcontrollers are based on the advanced RISC architecture and consist of 32 x 8-bit general purpose working registers. Within one single clock cycle, AVR can take inputs from two general purpose registers and put them to ALU for carrying out the requested operation, and transfer back the result to an arbitrary register. The ALU can perform arithmetic as well as logical operations
over the inputs from the register or between the register and a constant. Single register operations like taking a complement can also be executed in ALU. We can see that AVR does not have any register like accumulator as in 8051 family of microcontrollers; the operations can be performed between any of the registers and can be stored in either of them.
AVR follows Harvard Architecture format in which the processor is equipped with separate memories and buses for Program and the Data information. Here while an instruction is being executed, the next instruction is pre-fetched from the program memory.
AVR Family Architecture
Since AVR can perform single cycle execution, it means that AVR can execute 1 million instructions per second if cycle frequency is 1MHz. The higher is the operating frequency of the controller, the higher will be its processing speed. We need to optimize the power consumption with processing speed and hence need to select the operating frequency accordingly.
There are two flavors for Atmega16 microcontroller:
1.      Atmega16:- Operating frequency range is 0 – 16 MHz.
2.      Atmega16L:- Operating frequency range is 0 – 8 MHz.
If we are using a crystal of 8 MHz = 8 x 106 Hertz = 8 Million cycles, then AVR can execute 8 million instructions.
Naming Convention.!
The AT refers to Atmel the manufacturer, Mega means that the microcontroller belong to MegaAVR category, 16 signifies the memory of the controller, which is 16KB.
Naming Convention of ATmega16 - AVR FamilyNaming Convention of ATmega16 – AVR Family
Architecture Diagram: Atmega16
Following points explain the building blocks of Atmega16 architecture:
·           I/O Ports: Atmega16 has four (PORTA, PORTB, PORTC and PORTD) 8-bit input-output ports.
·           Internal Calibrated Oscillator: Atmega16 is equipped with an internal oscillator for driving its clock. By default Atmega16 is set to operate at internal calibrated oscillator of 1 MHz. The maximum frequency of internal oscillator is 8Mhz. Alternatively, ATmega16 can be operated using an external crystal oscillator with a maximum frequency of 16MHz. In this case you need to modify the fuse bits. (Fuse Bits will be explained in a separate tutorial).
Architecture of ATmega16 - AVR Microcontrollers | Building BlocksArchitecture of ATmega16 – AVR Microcontrollers
·           ADC Interface: Atmega16 is equipped with an 8 channel ADC (Analog to Digital Converter) with a resolution of 10-bits. ADC reads the analog input for e.g., a sensor input and converts it into digital information which is understandable by the microcontroller.
·           Timers/Counters: Atmega16 consists of two 8-bit and one 16-bit timer/counter. Timers are useful for generating precision actions for e.g., creating time delays between two operations.
·           Watchdog Timer: Watchdog timer is present with internal oscillator. Watchdog timer continuously monitors and resets the controller if the code gets stuck at any execution action for more than a defined time interval.
·           Interrupts: Atmega16 consists of 21 interrupt sources out of which four are external. The remaining are internal interrupts which support the peripherals like USART, ADC, Timers etc.
·           USART: Universal Synchronous and Asynchronous Receiver and Transmitter interface is available for interfacing with external device capable of communicating serially (data transmission bit by bit).
General Purpose Registers: Atmega16 is equipped with 32 general purpose registers which are coupled directly with the Arithmetic Logical Unit (ALU) of CPU.
·           Memory: Atmega16 consist of three different memory sections:
1.         Flash EEPROM: Flash EEPROM or simple flash memory is used to store the program dumped or burnt by the user on to the microcontroller. It can be easily erased electrically as a single unit. Flash memory is non-volatile i.e., it retains the program even if the power is cut-off. Atmega16 is available with 16KB of in system programmable Flash EEPROM.
2.        Byte Addressable EEPROM: This is also a nonvolatile memory used to store data like values of certain variables. Atmega16 has 512 bytes of EEPROM, this memory can be useful for storing the lock code if we are designing an application like electronic door lock.
3.        SRAM: Static Random Access Memory, this is the volatile memory of microcontroller i.e., data is lost as soon as power is turned off. Atmega16 is equipped with 1KB of internal SRAM. A small portion of SRAM is set aside for general purpose registers used by CPU and some for the peripheral subsystems of the microcontroller.
·           ISP: AVR family of controllers have In System Programmable Flash Memory which can be programmed without removing the IC from the circuit, ISP allows to reprogram the controller while it is in the application circuit.
·          SPI:Serial Peripheral Interface, SPI port is used for serial communication between two devices on a common clock source. The data transmission rate of SPI is more than that of USART.
·           TWI: Two Wire Interface (TWI) can be used to set up a network of devices, many devices can be connected over TWI interface forming a network, the devices can simultaneously transmit and receive and have their own unique address.
·           DAC: Atmega16 is also equipped with a Digital to Analog Converter (DAC) interface which can be used for reverse action performed by ADC. DAC can be used when there is a need of converting a digital signal to analog signal.
Various microcontroller of MegaAVR series:
ATmega8 and Atmega32 are other members of MegaAVR series controllers. They are quite similar to ATmega16 in architecture. Low power version MegaAVR controllers are also available in markets. The following table shows the comparison between different members of MegaAVR family:
Part Name
ROM
RAM
EEPROM
I/0 Pins
Timer
Interrupts
OperationVoltage
Operating frequency
Packaging
ATmega8
8KB
1KB
512B
23
3
19
4.5-5.5 V
0-16 MHz
28
ATmega8L
8KB
1KB
512B
23
3
19
2.7-5.5 V
0-8 MHz
28
ATmega16
16KB
1KB
512B
32
3
21
4.5-5.5 V
0-16 MHz
40
ATmega16L
16KB
1KB
512B
32
3
21
2.7-5.5 V
0-8 MHz
40
ATmega32
32KB
2KB
1KB
32
3
21
4.5-5.5 V
0-16 MHz
40
ATmega32L
32KB
2KB
1KB
32
3
21
2.7-5.5 V
0-8 MHz
40

 

Source :- engineersgarage