248
Atmel ATmega16/32/64/M1/C1 [DATASHEET]
7647K–AVR–12/13
24.7.7 Setting the Boot Loader Lock Bits by SPM
To set the boot loader lock bits, write the desired data to R0, write “X0001001” to SPMCSR and execute SPM within four clock
cycles after writing SPMCSR. 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.
See
Table 24-2 and
Table 24-3 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 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 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.
24.7.8 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.
24.7.9 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 destination 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 25-4 on page 256 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 (FHB) will be loaded in the
destination register as shown below. Refer to
Table 25-6 on page 257 for detailed 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 (EFB) will be loaded in the
destination register as shown below. Refer to
Table 25-4 on page 256 for detailed description and mapping of the extended fuse
byte.
Fuse and lock bits that are programmed, will be read as zero. Fuse and lock bits that are unprogrammed, will be read as one.
Bit
7
65
43
2
1
0
R0
1
BLB12
BLB11
BLB02
BLB01
1
Bit
7
65
43
2
1
0
Rd
–
BLB12
BLB11
BLB02
BLB01
LB2
LB1
Bit
765
4
3210
Rd
FLB7
FLB6
FLB5
FLB4
FLB3
FLB2
FLB1
FLB0
Bit
765
4
3210
Rd
FHB7
FHB6
FHB5
FHB4
FHB3
FHB2
FHB1
FHB0
Bit
765
4
3210
Rd
–
EFB3
EFB2
EFB1
EFB0