Setting the Boot Loader Lock To set the Boot Loader Lock Bits, write the desired data to R0, write “X0001001” to
Bits by SPM
SPMCR and execute SPM within four clock cycles after writing SPMCR. The only
accessible Lock Bits are the Boot Lock Bits that may prevent the Application and Boot
Loader section from any software update by the MCU.
Bit
7
6
5
4
3
2
1
0
R0
1
1
BLB12
BLB11
BLB02
BLB01
1
1
See Table 78 and Table 79 for how the different settings of the Boot Loader Bits affect
the Flash access.
If bits 5..2 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
SPMCR. 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 Lock
Bits). For future compatibility It is also recommended to set bits 7, 6, 1, and 0 in R0 to “1”
when writing the Lock Bits. When programming the Lock Bits the entire Flash can be
read during the operation.
EEPROM Write Prevents
Writing to SPMCR
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 SPMCR
Register.
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 SPMCR. When
an LPM instruction is executed within three CPU cycles after the BLBSET and SPMEN
bits are set in SPMCR, the value of the Lock Bits will be loaded in the destination regis-
ter. 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.
Bit
7
6
5
4
3
2
1
0
Rd
–
–
BLB12
BLB11
BLB02
BLB01
LB2
LB1
The algorithm for reading the Fuse Low bits is similar to the one described above for
reading the Lock Bits. To read the Fuse Low bits, load the Z-pointer with 0x0000 and set
the BLBSET and SPMEN bits in SPMCR. When an LPM instruction is executed within
three cycles after the BLBSET and SPMEN bits are set in the SPMCR, the value of the
Fuse Low bits (FLB) will be loaded in the destination register as shown below. Refer to
Table 88 on page 221 for a detailed description and mapping of the fuse low bits.
Bit
7
6
5
4
3
2
1
0
Rd
FLB7
FLB6
FLB5
FLB4
FLB3
FLB2
FLB1
FLB0
Similarly, when reading the Fuse High bits, 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 SPMCR, the value of the Fuse High bits (FHB) will be loaded in the destination reg-
ister as shown below. Refer to Table 87 on page 220 for detailed description and
mapping of the fuse high bits.
Bit
7
6
5
4
3
2
1
0
Rd
FHB7
FHB6
FHB5
FHB4
FHB3
FHB2
FHB1
FHB0
Fuse and Lock Bits that are programmed, will be read as zero. Fuse and Lock Bits that
are unprogrammed, will be read as one.
214
ATmega8(L)
2486M–AVR–12/03
ATMEL [ ATMEL ]
