ATmega165 datasheet(232/336 Pages) ATMEL

232 / 336 page

232

ATmega165/V

2573G–AVR–07/09

Boot Loader Support

– Read-While-Write

Self-Programming

The Boot Loader Support provides a real Read-While-Write Self-Programming mecha-

nism for downloading and uploading program code by the MCU itself. This feature

allows flexible application software updates controlled by the MCU using a Flash-resi-

dent 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.

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 105 on page

249) 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 109). The size of the different sections is configured by

the BOOTSZ Fuses as shown in Table 96 on page 244 and Figure 109. These two sec-

tions 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 92 on page 236. The Application section

can never store any Boot Loader code since the SPM instruction is disabled when exe-

cuted from the Application section.

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 pro-

gramming 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 93 on page

236.

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 97 on page 244 and Figure 109 on page 235. 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.

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