User Manual

269
ATmega48A/PA/88A/PA/168A/PA/328/P [DATASHEET]
8271E–AVR–07/2012
27. Boot Loader Support – Read-While-Write Self-Programming
The Boot Loader Support applies to ATmega88A/88PA/168A/168PA/328/328P
27.1 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 28-11 on page 290) used during program-
ming. The page organization does not affect normal operation.
27.2 Overview
In ATmega88A/88PA/168A/168PA/328/328P the Boot Loader Support provides a real Read-While-Write Self-Pro-
gramming 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 sec-
tion 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.
27.3 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-
ure 27-2). The size of the different sections is configured by the BOOTSZ Fuses as shown in Table 27-7 on page
280 and Figure 27-2. These two sections can have different level of protection since they have different sets of
Lock bits.
27.3.1 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 27-2 on
page 273. The Application section can never store any Boot Loader code since the SPM instruction is disabled
when executed from the Application section.
27.3.2 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-
tion can be selected by the Boot Loader Lock bits (Boot Lock bits 1), see Table 27-3 on page 273.
27.4 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