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ATtiny25/45/85 [DATASHEET]
2586Q–AVR–08/2013
19. Self-Programming the Flash
The device provides a Self-Programming mechanism for downloading and uploading program code by the MCU
itself. The Self-Programming can use any available data interface and associated protocol to read code and write
(program) that code into the Program memory. The SPM instruction is disabled by default but it can be enabled by
programming the SELFPRGEN fuse (to “0”).
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 re-written. When using alternative 1, the Boot Loader provides an effec-
tive 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 essen-
tial that the page address used in both the Page Erase and Page Write operation is addressing the same page.
19.1
Performing Page Erase by SPM
To execute Page Erase, set up the address in the Z-pointer, write “00000011” 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 writ-
ten to PCPAGE in the Z-register. Other bits in the Z-pointer will be ignored during this operation.
Note:
The CPU is halted during the Page Erase operation.
19.2
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 CTPB 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.
If the EEPROM is written in the middle of an SPM Page Load operation, all data loaded will be lost.