ATmega48PA/88PA/168PA/328P
26. Boot Loader Support – Read-While-Write Self-Programming,
ATmega88PA, ATmega168PA and ATmega328P
26.1 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 27-11 on page 299)
used during programming. The page organization does not affect normal operation.
26.2 Overview
In ATmega88PA, ATmega168PA and ATmega328P, 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 avail-
able 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 mem-
ory. 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.
26.3 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 26-2). The size of the different sections is configured by the
BOOTSZ Fuses as shown in Table 26-7 on page 289 and Figure 26-2. These two sections can
have different level of protection since they have different sets of Lock bits.
26.3.1
26.3.2
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 26-2 on page 281. The Application section can never store any
Boot Loader code since the SPM instruction is disabled when executed from the Application
section.
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 26-3 on page 281.
277
8161D–AVR–10/09
ATMEL [ ATMEL ]
