Datasheet

ManualsBrandsMicrochip ManualsAVR MicrocontrollersAVR RISC Microcontroller Flash 64KB TQFP

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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. Please refer to Simple Assembly Code

Example for a Boot Loader on page 376 for an assembly code example.

30.8.1. Performing Page Erase by SPM

To execute page erase, set up the address in the Z-pointer, write “X0000011” to SPMCSR and execute

SPM within four clock cycles after writing SPMCSR. The data in R1 and R0 is ignored. The page address

must be written to PCPAGE in the Z-register. Other bits in the Z-pointer must be written to zero during this

operation.

• Page Erase to the RWW section: The NRWW section can be read during the Page Erase.

• Page Erase to the NRWW section: The CPU is halted during the operation.

Note: If an interrupt occurs in the timed sequence the four cycle access cannot be guaranteed. In order

to ensure atomic operation disable interrupts before writing to SPMCSR.

30.8.2. Filling the Temporary Buffer (Page Loading)

To write an instruction word, set up the address in the Z-pointer and data in R1:R0, write “00000001” to

SPMCSR and execute SPM within four clock cycles after writing SPMCSR. The content of PCWORD in

the Z-register is used to address the data in the temporary buffer. The temporary buffer will auto-erase

after a page write operation or by writing the RWWSRE bit in SPMCSR. It is also erased after a System

Reset. Note that it is not possible to write more than one time to each address without erasing the

temporary buffer.

Note: If the EEPROM is written in the middle of an SPM Page Load operation, all data loaded will be

lost.

30.8.3. Performing a Page Write

To execute page write, set up the address in the Z-pointer, write “X0000101” to SPMCSR and execute

SPM within four clock cycles after writing SPMCSR. The data in R1 and R0 is ignored. The page address

must be written to PCPAGE. Other bits in the Z-pointer must be written to zero during this operation.

• Page Write to the RWW section: The NRWW section can be read during the Page Write

• Page Write to the NRWW section: The CPU is halted during the operation

30.8.4. Using the SPM Interrupt

If the SPM interrupt is enabled, the SPM interrupt will generate a constant interrupt when the SPMEN bit

in SPMCSR is cleared (SPMCSR.SPMEN). This means that the interrupt can be used instead of polling

the SPMCSR Register in software. When using the SPM interrupt, the Interrupt Vectors should be moved

to the Boot Loader Section (BLS) section to avoid that an interrupt is accessing the RWW section when it

is blocked for reading. How to move the interrupts is described in Interrupts chapter.