9.3.1.1 Flash
The Flash is divided into a set of pages. A page is the basic unit addressed when programming the Flash. It is only possible to write or erase a whole page at a time. One page consists of several words.
The Flash can be divided into three sections in blocks of 256 bytes for different security. The three different sections are BOOT, application code (APPCODE) and application data (APPDATA).
Section Sizes
The sizes of these sections are set by the Boot Section End fuse (FUSE.BOOTEND) and Application Code Section End fuse (FUSE.APPEND).
The fuses select the section sizes in blocks of 256 bytes. As shown in Figure 9-2, the BOOT section stretches from the start of the Flash until BOOTEND. The APPCODE section runs from BOOTEND until APPEND. The remaining area is the APPDATA section. If APPEND is written to 0, the APPCODE section runs from BOOTEND to the end of Flash (removing the APPDATA section). If BOOTEND and APPEND are written to 0, the entire Flash is regarded as BOOT section. APPEND should either be set to 0 or a value greater or equal than BOOTEND.
| BOOTEND | APPEND | BOOT section | APPCODE section | APPDATA section |
|---|---|---|---|---|
| 0 | 0 | 0 to FLASHEND | - | - |
| > 0 | 0 | 0 to 256*BOOTEND | 256*BOOTEND to FLASHEND | - |
| > 0 | == BOOTEND | 0 to 256*BOOTEND | - | 256*BOOTEND to FLASHEND |
| > 0 | > BOOTEND | 0 to 256*BOOTEND | 256*BOOTEND to 256*APPEND | 256*APPEND to FLASHEND |
- Also see BOOTEND6.8.4.8 Boot End and APPEND6.8.4.7 Application Code End descriptions.
- Interrupt vectors are by default located after the BOOT section. This can be changed in the interrupt controller. Refer to 13.3.2.2 Interrupt Vector Locations
If FUSE.BOOTEND is written to 0x04 and FUSE.APPEND is written to 0x08, the first 4*256 bytes will be BOOT, the next 4*256 bytes will be APPCODE, and the remaining Flash will be APPDATA.
Inter-Section Write Protection
- Code in the BOOT section can write to APPCODE and APPDATA.
- Code in APPCODE can write to APPDATA
- Code in APPDATA cannot write to Flash or EEPROM.
Boot Section Lock and Application Code Section Write Protection
The two lock bits (APCWP and BOOTLOCK in NVMCTRL.CTRLB) can be set to lock further updates of the respective APPCODE or BOOT section until the next Reset.
The CPU can never write to the BOOT section. NVMCTRL_CTRLB.BOOTLOCK prevents reads and execution of code from the BOOT section.