26. Boot Loader Support – Read-While-Write Self-Programming, ATmega88 and
ATmega168
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 27-9 on page 290) used
during programming. The page organization does not affect normal operation.
26.2 Overview
In ATmega88 and ATmega168, 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-resi-
dent 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 any-
more. 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 26-6 on page 282 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 274. 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 274.
270
ATmega48/88/168
2545M–AVR–09/07
ATMEL [ ATMEL ]
