Datasheet

336
ATmega16U4/32U4 [DATASHEET]
Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016
27. 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
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.
General information on SPM and ELPM is provided in “AVR CPU Core” on page 9.
27.1 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 28-11 on page 359) used during
programming. The page organization does not affect normal operation.
27.2 Application and Boot Loader Flash Sections
The Flash memory is organized in two main sections, the Application section and the Boot Loader section (see
Figure 27-2 on page 338). The size of the different sections is configured by the BOOTSZ Fuses as shown in
Table 27-8 on page 349 and Figure 27-2 on page 338. These two sections can have different level of protection
since they have different sets of Lock bits.
27.2.1 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). The
Application section can never store any Boot Loader code since the SPM instruction is disabled when executed
from the Application section.
Refer to
Table 27-2 on page 339.
27.2.2 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
section can be selected by the Boot Loader Lock bits (Boot Lock bits 1). Refer to
Table 27-3 on page 339.
27.3 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.