Файл: ATmega8.pdf

ATmega8(L)

Assembly Code Example for a Boot Loader” on page 216 for an assembly code

example.

Performing Page Erase by

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

SPM

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 will be ignored 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.

Filling the Temporary Buffer

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

(Page Loading)

“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

page buffer. The temporary buffer will auto-erase after a page write operation or by writ-

ing 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.

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.

Using the SPM Interrupt

If the SPM interrupt is enabled, the SPM interrupt will generate a constant interrupt

when the SPMEN bit in SPMCR is cleared. This means that the interrupt can be used

instead of polling the SPMCR 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. How to move the interrupts

is described in “Interrupts” on page 44.

Consideration While Updating

Special care must be taken if the user allows the Boot Loader section to be updated by

BLS

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.

Prevent Reading the RWW

During Self-Programming (either page erase or page write), the RWW section is always

Section During Self-

blocked for reading. The user software itself must prevent that this section is addressed

Programming

during the self programming operation. The RWWSB in the SPMCR will be set as long

as the RWW section is busy. During Self-Programming the Interrupt Vector table should

be moved to the BLS as described in “Interrupts” on page 44, 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 “Simple Assembly

Code Example for a Boot Loader” on page 216 for an example.

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