249
ATmega165A/PA/325A/PA/3250A/PA/645A/P/6450A/P [DATASHEET]
8285E–AVR–02/2013
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.
affect the Flash access.
If bits 5...0 in R0 are cleared (zero), the corresponding Lock bit will be programmed if an SPM instruction is exe-
cuted 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 read-
ing the Lock 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 (EEWE) 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 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 des-
tination register. The BLBSET and SPMEN bits will auto-clear upon completion of reading the Lock bits or if no
LPM instruction is executed within three CPU cycles or no SPM instruction is executed within four CPU cycles.
When BLBSET and SPMEN are cleared, LPM will work as described in the Instruction set Manual.
The algorithm for reading the Fuse Low byte is similar to the one described above for reading the Lock bits. To
read the Fuse Low byte, load the Z-pointer with 0x0000 and set the BLBSET and SPMEN bits in SPMCSR. When
an LPM instruction is executed within three cycles after the BLBSET and SPMEN bits are set in the SPMCSR, the
value of the Fuse Low byte (FLB) will be loaded in the destination register as shown below. Refer to
Table 27-5 onpage 259 for a detailed description and mapping of the Fuse Low byte.
Similarly, when reading the Fuse High byte, load 0x0003 in the Z-pointer. When an LPM instruction is executed
within three cycles after the BLBSET and SPMEN bits are set in the SPMCSR, the value of the Fuse High byte
description and mapping of the Fuse High byte.
When reading the Extended Fuse byte, load 0x0002 in the Z-pointer. When an LPM instruction is executed within
three cycles after the BLBSET and SPMEN bits are set in the SPMCSR, the value of the Extended Fuse byte
Bit
765
43210
R0
1
BLB12
BLB11
BLB02
BLB01
LB2
LB1
Bit
765
43210
Rd
–
BLB12
BLB11
BLB02
BLB01
LB2
LB1
Bit
7
65
4
3210
Rd
FLB7
FLB6
FLB5
FLB4
FLB3
FLB2
FLB1
FLB0
Bit
7
65
4
3210
Rd
FHB7
FHB6
FHB5
FHB4
FHB3
FHB2
FHB1
FHB0