ATmega48/88/168
Boot Loader Support 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-resident Boot Loader program. The Boot Loader program can
use any available data interface and associated protocol to read code and write (pro-
gram) 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
– Read-While-Write
Self-Programming,
ATmega88 and
ATmega168
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 sep-
arate sets of Boot Lock bits which can be set independently. This gives the user a
unique flexibility to select different levels of protection.
Boot Loader 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 122 on page
274) used during programming. The page organization does not affect normal
operation.
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 117). The size of the different sections is configured by
the BOOTSZ Fuses as shown in Table 109 on page 268 and Figure 117. These two
sections can have different level of protection since they have different sets of Lock bits.
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 105 on page 259. The Application section
can never store any Boot Loader code since the SPM instruction is disabled when exe-
cuted from the Application section.
BLS – Boot Loader Section
While the Application section is used for storing the application code, the The Boot
Loader software must be located in the BLS since the SPM instruction can initiate a pro-
gramming 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 106 on page
259.
Read-While-Write and No Whether the CPU supports Read-While-Write or if the CPU is halted during a Boot
Loader software update is dependent on which address that is being programmed. In
addition to the two sections that are configurable by the BOOTSZ Fuses as described
above, the Flash is also divided into two fixed sections, the Read-While-Write (RWW)
section and the No Read-While-Write (NRWW) section. The limit between the RWW-
and NRWW sections is given in Table 110 on page 268 and Figure 117 on page 258.
The main difference between the two sections is:
Read-While-Write Flash
Sections
•
When erasing or writing a page located inside the RWW section, the NRWW section
can be read during the operation.
•
When erasing or writing a page located inside the NRWW section, the CPU is halted
during the entire operation.
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2545D–AVR–07/04
ATMEL [ ATMEL ]
