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).

Figure 9-2. Flash Sections

Section Sizes

The sizes of these sections are set by the Boot Section End fuse (FUSE.BOOTEND) and the Application Code Section End fuse (FUSE.APPEND).

The fuses select the section sizes in blocks of 256 bytes. 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 the BOOT section. APPEND may either be set to ‘0’ or a value greater than or equal to BOOTEND.

Table 9-1. Setting Up Flash Sections
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
Note:
  1. See also the BOOTEND and APPEND descriptions.
  2. Interrupt vectors are by default located after the BOOT section. This can be changed in the interrupt controller.

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

Between the three Flash sections, directional write protection is implemented:
  • The code in the BOOT section can write to APPCODE and APPDATA
  • The code in APPCODE can write to APPDATA
  • The 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.