ATMEGA8_14 Datasheet, PDF (202/331 Pages) ATMEL Corporation – High-performance, Low-power Atmel

ATmega8(L)
Boot Loader
Support – Read-
While-Write
Self-
Programming
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 applica-
tion 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 mem-
ory. 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 differ-
ent 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 89 on page 218) 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 102 on page 204). The size of the different sections is configured by
the BOOTSZ Fuses as shown in Table 82 on page 213 and Figure 102 on page 204. 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 78 on page 205. 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 79 on page 205.
Read-While-Write
and No Read-
While-Write Flash
Sections
Whether the CPU supports Read-While-Write or if the CPU is halted during a Boot Loader soft-
ware 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 83
on page 214 and Figure 102 on page 204. The main difference between the two sections is:
• 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
Note that the user software can never read any code that is located inside the RWW section dur-
ing a Boot Loader software operation. The syntax “Read-While-Write section” refers to which
section that is being programmed (erased or written), not which section that actually is being
read during a Boot Loader software update.
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