260
ATmega169V/L
2514A–AVR–08/02
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 109 and Table 110 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 lO
ck
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
WritingtoSPMCSR
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.
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 destina-
tion register. The BLBSET and SPMEN bits will auto-clear upon completion of reading
theLockbitsorifnoLPMinstructionisexecutedwithinthreeCPUcyclesornoSPM
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 120 on page 267 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 119 on page 267 for detailed description and
mapping of the Fuse High byte.
Bit 76543210
R0 1 1 BLB12 BLB11 BLB02 BLB01 1 1
Bit 76543210
Rd ––BLB12 BLB11 BLB02 BLB01 LB2 LB1
Bit 7654 3210
Rd FLB7 FLB6 FLB5 FLB4 FLB3 FLB2 FLB1 FLB0
Bit 7654 3210
Rd FHB7 FHB6 FHB5 FHB4 FHB3 FHB2 FHB1 FHB0
(27 pages)
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