280
8271D–AVR–05/11
ATmega48A/PA/88A/PA/168A/PA/328/P
27. Boot Loader Support – Read-While-Write Self-Programming
The Boot Loader Support applies to ATmega88A/88PA/168A/168PA/328/328P
27.1 Features
• Read-While-Write Self-Programming
• Flexible Boot Memory Size
• High Security (Separate Boot Lock Bits for a Flexible Protection)
• Separate Fuse to Select Reset Vector
• Optimized Page
(1)
Size
• Code Efficient Algorithm
• Efficient Read-Modify-Write Support
Note: 1. A page is a section in the Flash consisting of several bytes (see Table 28-11 on page 302)
used during programming. The page organization does not affect normal operation.
27.2 Overview
In ATmega88A/88PA/168A/168PA/328/328P the Boot Loader Support provides a real Read-
While-Write Self-Programming mechanism for downloading and uploading program code by the
MCU itself. This feature allows flexible application software updates controlled by the MCU using
a Flash-resident Boot Loader program. The Boot Loader program can use any available data
interface and associated protocol to read code and write (program) that code into the Flash
memory, or read the code from the program memory. The program code within the Boot Loader
section has the capability to write into the entire Flash, including the Boot Loader memory. The
Boot Loader can thus even modify itself, and it can also erase itself from the code if the feature
is not needed anymore. The size of the Boot Loader memory is configurable with fuses and the
Boot Loader has two separate sets of Boot Lock bits which can be set independently. This gives
the user a unique flexibility to select different levels of protection.
27.3 Application and Boot Loader Flash Sections
The Flash memory is organized in two main sections, the Application section and the Boot
Loader section (see Figure 27-2). The size of the different sections is configured by the
BOOTSZ Fuses as shown in Table 27-7 on page 292 and Figure 27-2. These two sections can
have different level of protection since they have different sets of Lock bits.
27.3.1 Application Section
The Application section is the section of the Flash that is used for storing the application code.
The protection level for the Application section can be selected by the application Boot Lock bits
(Boot Lock bits 0), see Table 27-2 on page 284. The Application section can never store any
Boot Loader code since the SPM instruction is disabled when executed from the Application
section.
27.3.2 BLS – Boot Loader Section
While the Application section is used for storing the application code, the The Boot Loader soft-
ware must be located in the BLS since the SPM instruction can initiate a programming when
executing from the BLS only. The SPM instruction can access the entire Flash, including the
BLS itself. The protection level for the Boot Loader section can be selected by the Boot Loader
Lock bits (Boot Lock bits 1), see Table 27-3 on page 284.
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