MCU ATmega328 -Arduno Uno

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This is in continuation of Arduino Uno an Engineering Approach . Please go through that first  to  understand the background. We will validate the ATmega328P concepts with practical approach.

ATmega328/P Block Diagram 

Here is the full Device or SoC(System on Chip) or Platform  block diagram under discussion  :

  • mem9

Looks complex, Lets try to divide this to several tasks and focus on one at a time.

The core microprocessor AVR

 AVR got the name from its developers Alf and Vegard’s RISC processor. It is 8-bit little endian RISC processor from Atmel(Now Microchip). The program is going to execute on this Harvard Architecture RISC core.The Program Counter (PC) is 14 bits wide, thus addressing the 16 K program memory locations.


X,Y,Z are 16-bit registers used for indirect addressing. Other are 8-bit.

Device Memory MAP

Program memory :

Why should I learn this device  Program memory map ?  Since the device has very limited program memory. It is important to understand where this is being used and how can we optimize that for real field application.

Program memory consists of bootloader and user application. Size of the program memory can be configured by FUSE setting bits BOOTSZ1 and BOOTSZ0. Please note the size in words(16-bit word). FUSE setting bytes provide a mechanism to change default reset configuration of device. We can discuss it in clock related block.


How to verify/backup existing flash program ?

Reading of flash program is not straight forward. But we can use avrdude utility after installing WinAVR . avrdude options are available here.

Note: Avoid write/program feature of avrdude directly, Otherwise you may corrupt boot loader and later require external means to re-program the same. If boot loader gets corrupted or erased , You can’t communicate with device without external debugger.


How can we verify Application code ?

I discussed the method to fetch disassembly of  sketch/application data in  earlier blog. The HEX data shown below is from default.hex renamed as Arduino_uno_flash_read on left side and the application program disassembly from sketch build folder.


How can I verify Boot Loader code

Lets compare the data in optiboot_atmega328.lst at C:\Program Files (x86)\Arduino\hardware\arduino\avr\bootloaders\optiboot\  with hex dump taken from device by avrdude.


How to verify this with Fuse bits configured




All fuse bytes are read as zero. That is not possible. Lets try to understand Fuse bits related to bootsz.


It seems some issue with FUSE bits reading by avrdude , so we can use a AVR library function boot_lock_fuse_bits_get()

I will discuss the code used here in next post. The output shows hfuse=0xDE .

i.e. BOOTSZ1,BOOTSZ0 = 11bmem7

As per the boot loader table, The flash section should be from 0x3F00 to 0x3FFF. However as per build configuration it is 0x7E00-0x7FFF . Why ?  As I mentioned earlier the flash bank size is given in words(16-bit) , So the address has to be shifted left by one bit i.e. 0x3F00  will become 0x7E00. This is also indicated in  device datasheet  section “Addressing the Flash During Self-Programming” .



All documents are available here. I am referring to most of these :

This article is discussed at Atmega328 – Arduino Uno video

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