Alternative 2, fill the buffer after page erase
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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 Section 24.7.13 “Simple Assembly Code Example
for a Boot Loader” on page 250 for an assembly code example.
24.7.1 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 will be ignored during this operation.
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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.
24.7.2 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.
If the EEPROM is written in the middle of an SPM page load operation, all data loaded will be lost.
24.7.3 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.
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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.
24.7.4 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. 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 BLS section to avoid that an interrupt is accessing the RWW section
when it is blocked for reading.
24.7.5 Consideration While Updating BLS
Special care must be taken if the user allows the boot loader section to be updated by leaving boot lock bit11
unprogrammed. An accidental write to the boot loader itself can corrupt the entire boot loader, and further software updates
might be impossible. If it is not necessary to change the boot loader software itself, it is recommended to program the boot
lock bit11 to protect the boot loader software from any internal software changes.
24.7.6 Prevent Reading the RWW Section during Self-programming
During self-programming (either page erase or page write), the RWW section is always blocked for reading. The user
software itself must prevent that this section is addressed during the self programming operation. The RWWSB in the
SPMCSR will be set as long as the RWW section is busy. During self-programming the Interrupt vector table should be
moved to the BLS or the interrupts must be disabled. Before addressing the RWW section after the programming is
completed, the user software must clear the RWWSB by writing the RWWSRE. See Section 24.7.13 “Simple Assembly
Code Example for a Boot Loader” on page 250 for an example.
ATmega16/32/64/M1/C1 [DATASHEET]
247
7647O–AVR–01/15
MICROCHIP [ MICROCHIP ]
