258
ATmega16M1/32M1/64M1 [DATASHEET]
8209E–AVR–11/2012
26. Boot loader support – read-while-write self-programming
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:
1. A page is a section in the flash consisting of several bytes (see
Table 27-9 on page 278) used during programming.
The page organization does not affect normal operation.
26.1
Overview
In Atmel ATmega16M1/32M1/64M1, 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 (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.
26.2
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-they have different sets of Lock bits.
26.2.1
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 26-2 onpage 262. The Application section can never store any Boot Loader code since the SPM instruction is disabled
when executed from the Application section.
26.2.2
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-
26.3
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
(RWW) section and the No Read-While-Write (NRWW) section. The limit between the RWW- and NRWW sections
are: