Datasheet

241
ATmega32A [DATASHEET]
Atmel-8155D-AVR-ATmega32A-Datasheet_02/2014
26.8 Self-Programming the Flash
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 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. See
“Simple Assembly Code Example for a Boot Loader” on page 261 for an assembly code example.
26.8.1 Performing Page Erase by SPM
To execute Page Erase, set up the address in the Z-pointer, write “X0000011” to SPMCR and execute SPM within
four clock cycles after writing SPMCR. 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 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 iterrupt occurs in the timed sequence, the four cycle access cannot be guaranteed. In order to ensure atomic
operation disable interrupts before writing to SPMCSR.
26.8.2 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 SPMCR
and execute SPM within four clock cycles after writing SPMCR. 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 SPMCR. 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.
26.8.3 Performing a Page Write
To execute Page Write, set up the address in the Z-pointer, write “X0000101” to SPMCR and execute SPM within
four clock cycles after writing SPMCR. 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.