ATMEGA640 datasheet(310/436 Pages) MICROCHIP

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ATmega640/V-1280/V-1281/V-2560/V-2561/V [DATASHEET]

DS40002211A

29. 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 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.

29.1 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 30-7 on page 328) used during program-

ming. The page organization does not affect normal operation.

29.2 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-

ure 29-2 on page 312). The size of the different sections is configured by the BOOTSZ Fuses as shown in Table

29-7 on page 320 and Figure 29-2 on page 312. These two sections can have different level of protection since

they have different sets of Lock bits.

29.2.1

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 29-2 on

page 313. The Application section can never store any Boot Loader code since the SPM instruction is disabled

when executed from the Application section.

29.2.2

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-

tion can be selected by the Boot Loader Lock bits (Boot Lock bits 1), see Table 29-3 on page 313.

29.3 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

is given in Table 29-1 and Figure 29-1 on page 311. 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

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