Datasheet

ATmega164A/PA/324A/PA/644A/PA/1284/P
 2018 Microchip Technology Inc. Data Sheet Complete DS40002070A-page 284
26.8.4 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. 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. How to move the interrupts is
described in ”Interrupts” on page 69.
26.8.5 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.
26.8.6 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 as described in ”Interrupts” on page 69, 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 287
for an example.
26.8.7 Setting the Boot Loader Lock Bits by SPM
To set the Boot Loader Lock bits and general lock bits, write the desired data to R0, write “X0001001” to
SPMCSR and execute SPM within four clock cycles after writing SPMCSR.
See Table 26-2 and Table 26-3 for how the different settings of the Boot Loader bits affect the Flash access.
If bits 5..0 in R0 are cleared (zero), the corresponding Boot Lock bit will be programmed if an SPM instruction is
executed within four cycles after BLBSET and SPMEN are set in SPMCSR. The Z-pointer is don’t care during
this operation, but for future compatibility it is recommended to load the Z-pointer with 0x0001 (same as used for
reading the lO
ck
bits). For future compatibility it is also recommended to set bits 7 and 6 in R0 to “1” when writing
the Lock bits. When programming the Lock bits the entire Flash can be read during the operation.
26.8.8 EEPROM Write Prevents Writing to SPMCSR
Note that an EEPROM write operation will block all software programming to Flash. Reading the Fuses and
Lock bits from software will also be prevented during the EEPROM write operation. It is recommended that the
user checks the status bit (EEPE) in the EECR Register and verifies that the bit is cleared before writing to the
SPMCSR Register.
26.8.9 Reading the Fuse and Lock Bits from Software
It is possible to read both the Fuse and Lock bits from software. To read the Lock bits, load the Z-pointer with
0x0001 and set the BLBSET and SPMEN bits in SPMCSR. When an (E)LPM instruction is executed within
three CPU cycles after the BLBSET and SPMEN bits are set in SPMCSR, the value of the Lock bits will be
loaded in the destination register. The BLBSET and SPMEN bits will auto-clear upon completion of reading the
Lock bits or if no (E)LPM instruction is executed within three CPU cycles or no SPM instruction is executed
Bit 76543210
R0 1 1 BLB12 BLB11 BLB02 BLB01 LB2 LB1