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.

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 123 on page 291) used during programming. The page organization does not affect normal operation.

Application and

Boot Loader Flash

Sections

Application Section

The Flash memory is organized in two main sections, the Application section and the Boot Loader section (see Figure 133). The size of the different sections is configured by the BOOTSZ fuses as shown in Table on page 284 and Figure 133. These two sections can have different level of protection since they have different sets of Lock bits.

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 on page 275. The application section can never store any Boot Loader code since the SPM instruction is disabled when executed from the application section.

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 RWWand NRWW sections is given in Table Note: on page 284 and Figure 133 on page 275. 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.

Note that the user software can never read any code that is located inside the RWW section during a Boot Loader software operation. The syntax “Read-While-Write section” refers to which

Boot Loader Lock

Bits

Entering the Boot

Loader Program

Table 108. Boot Lock Bit0 Protection Modes (Application Section)(1)

Table 109. Boot Lock Bit1 Protection Modes (Boot Loader Section)(1)

Entering the Boot Loader takes place by a jump or call from the application program. This may be initiated by a trigger such as a command received via USART, or SPI interface. Alternatively, the Boot Reset Fuse can be programmed so that the Reset Vector Reset is pointing to the Boot Flash start address after a reset. In this case, the Boot Loader is started after a reset. After the application code is loaded, the program can start executing the application code. Note that the fuses cannot be changed by the MCU itself. This means that once the Boot Reset Fuse is programmed, the Reset Vector will always point to the Boot Loader Reset and the fuse can only be changed through the serial or parallel programming interface.

Table 110. Boot Reset Fuse(1)

Note: 1. “1” means unprogrammed, “0´means programmed

276

Addressing the

Flash During Self-

Programming

or if no SPM instruction is executed within four clock cycles. The CPU is halted during the entire page write operation if the NRWW section is addressed.

• Bit 0 – SPMEN: Store Program Memory Enable

This bit enables the SPM instruction for the next four clock cycles. If written to one together with either RWWSRE, BLBSET, PGWRT’ or PGERS, the following SPM instruction will have a special meaning, see description above. If only SPMEN is written, the following SPM instruction will store the value in R1:R0 in the temporary page buffer addressed by the Z-pointer. The LSB of the Z-pointer is ignored. The SPMEN bit will auto-clear upon completion of an SPM instruction, or if no SPM instruction is executed within four clock cycles. During page erase and page write, the SPMEN bit remains high until the operation is completed.

Writing any other combination than “10001”, “01001”, “00101”, “00011” or “00001” in the lower five bits will have no effect.

The Z-pointer together with RAMPZ are used to address the SPM commands. For details on how to use the RAMPZ, see “RAM Page Z Select Register – RAMPZ” on page 13.

Since the Flash is organized in pages (see Table 123 on page 291), the program counter can be treated as having two different sections. One section, consisting of the least significant bits, is addressing the words within a page, while the most significant bits are addressing the pages. This is shown in Figure 134. Note that the page erase and page write operations are addressed independently. Therefore it is of major importance that the Boot Loader software addresses the same page in both the page erase and page write operation. Once a programming operation is initiated, the address is latched and the Z-pointer/RAMPZ can be used for other operations.

The only SPM operation that does not use the Z-pointer/RAMPZ is setting the Boot Loader Lock bits. The content of the Z-pointer/RAMPZ is ignored and will have no effect on the operation. The (E)LPM instruction does also use the Z-pointer/RAMPZ to store the address. Since this instruction addresses the Flash byte by byte, also the LSB (bit Z0) of the Z-pointer is used.

278

Self-Programming

the Flash

Notes: 1. The different variables used in Figure 134 are listed in Table 114 on page 285. 2. PCPAGE and PCWORD are listed in Table 124 on page 291.

The program memory is updated in a page by page fashion. Before programming a page with the data stored in the temporary page buffer, the page must be erased. The temporary page buffer is filled one word at a time using SPM and the buffer can be filled either before the page erase command or between a page erase and a page write operation:

Alternative 1, fill the buffer before a page erase

•Fill temporary page buffer

•Perform a page erase

•Perform a page write

Alternative 2, fill the buffer after page erase

•Perform a page erase

•Fill temporary page buffer

•Perform a page write

If only a part of the page needs to be changed, the rest of the page must be stored (for example in the temporary page buffer) before the erase, and then be rewritten. When using alternative 1, the Boot Loader provides an effective Read-Modify-Write feature which allows the user software to first read the page, do the necessary changes, and then write back the modified data. If alternative 2 is used, it is not possible to read the old data while loading since the page is already erased. The temporary page buffer can be accessed in a random sequence. It is essential that the page address used in both the page erase and page write operation is addressing the same page. See “Simple Assembly Code Example for a Boot Loader” on page 282 for an assembly code example.