Setting The Boot Loader Lock Bits; Eeprom Write Prevents Writing To Spmcsr; Reading The Fuse And Lock Bits From Software - Atmel ATmega128 Manual

Setting the Boot
Loader Lock Bits by
SPM
EEPROM Write
Prevents Writing to
SPMCSR
Reading the Fuse and
Lock Bits from
Software
2467S–AVR–07/09
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 soft-
ware update by the MCU.
Bit 7
R0 1
See Table 108 and Table 109
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 $0001 (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 pro-
gramming the Lock Bits the entire Flash can be read during the operation.
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.
It is possible to read both the Fuse and Lock bits from software. To read the Lock bits, load the
Z-pointer with $0001 and set the BLBSET and SPMEN bits in SPMCSR. When an LPM instruc-
tion 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 BLB-
SET and SPMEN are cleared, LPM will work as described in the Instruction set Manual.
Bit 7
Rd –
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 $0000 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 bits (FLB) will be
loaded in the destination register as shown below. Refer to
description and mapping of the Fuse Low bits.
Bit 7
Rd FLB7 FLB6
Similarly, when reading the Fuse High bits, load $0003 in the Z-pointer. When an LPM instruc-
tion is executed within three cycles after the BLBSET and SPMEN bits are set in the SPMCSR,
the value of the Fuse High bits (FHB) will be loaded in the destination register as shown below.
Refer to Table 118 on page 288
Bit 7
Rd FHB7 FHB6
When reading the Extended Fuse bits, load $0002 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 bits (EFB) will be loaded in the destination register as shown below.
Refer to Table 117 on page 287
Bit 7
Rd –
6 5 4
1 BLB12 BLB11 BLB02
for how the different settings of the Boot Loader Bits affect the
6 5 4
– BLB12 BLB11 BLB02
6 5 4
FLB5 FLB4 FLB3
for detailed description and mapping of the Fuse High bits.
6 5 4
FHB5 FHB4 FHB3
for detailed description and mapping of the Fuse High bits.
6 5 4
– – –
ATmega128
3 2 1
BLB01 1
3 2 1
BLB01 LB2 LB1
Table 119 on page 288
3 2 1
FLB2 FLB1 FLB0
3 2 1
FHB2 FHB1 FHB0
3 2 1
– – EFB1 EFB0
0
1
0
for a detailed
0
0
0
281