194
ATmega8A [DATASHEET]
8159E–AVR–02/2013
24. Boot Loader Support – Read-While-Write Self-Programming
24.1
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
Code Efficient Algorithm
Efficient Read-Modify-Write Support
Note:
) used during program-
ming. The page organization does not affect normal operation.
24.2
Overview
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 inter-
face 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.
24.3
Application and Boot Loader Flash Sections
The Flash memory is organized in two main sections, the Application section and the Boot loader section (see
Fig-204 and
Figure 24-2. These two sections can have different level of protection since they have different sets of
Lock Bits.
24.3.1
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 24-2 onpage 197. The application section can never store any Boot Loader code since the SPM instruction is disabled
when executed from the application section.
24.3.2
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 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 sec-
24.4
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 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