197
ATmega8A [DATASHEET]
8159E–AVR–02/2013
Note:
24.5
Boot Loader Lock Bits
If no Boot Loader capability is needed, the entire Flash is available for application code. The Boot Loader has two
separate sets of Boot Lock Bits which can be set independently. This gives the user a unique flexibility to select dif-
ferent levels of protection.
The user can select:
To protect the entire Flash from a software update by the MCU.
To protect only the Boot Loader Flash section from a software update by the MCU.
To protect only the Application Flash section from a software update by the MCU.
Allow software update in the entire Flash.
See
Table 24-2 and
Table 24-3 for further details. The Boot Lock Bits can be set in software and in Serial or Paral-
lel Programming mode, but they can be cleared by a chip erase command only. The general Write Lock (Lock bit
mode 2) does not control the programming of the Flash memory by SPM instruction. Similarly, the general
Read/Write Lock (Lock bit mode 3) does not control reading nor writing by LPM/SPM, if it is attempted.
Note:
1. “1” means unprogrammed, “0” means programmed
Note:
1. “1” means unprogrammed, “0” means programmed
Table 24-2.
Boot Lock Bit0 Protection Modes (Application Sectio
n)BLB0
Mode
BLB02
BLB01
Protection
1
No restrictions for SPM or LPM accessing the Application section.
2
1
0
SPM is not allowed to write to the Application section.
30
0
SPM is not allowed to write to the Application section, and LPM
executing from the Boot Loader section is not allowed to read from the
Application section. If Interrupt Vectors are placed in the Boot Loader
section, interrupts are disabled while executing from the Application
section.
40
1
LPM executing from the Boot Loader section is not allowed to read
from the Application section. If Interrupt Vectors are placed in the Boot
Loader section, interrupts are disabled while executing from the
Application section.
Table 24-3.
Boot Lock Bit1 Protection Modes (Boot Loader Section)
BLB1
Mode
BLB12
BLB11
Protection
1
No restrictions for SPM or LPM accessing the Boot Loader section.
2
1
0
SPM is not allowed to write to the Boot Loader section.
30
0
SPM is not allowed to write to the Boot Loader section, and LPM
executing from the Application section is not allowed to read from the
Boot Loader section. If Interrupt Vectors are placed in the Application
section, interrupts are disabled while executing from the Boot Loader
section.
40
1
LPM executing from the Application section is not allowed to read from
the Boot Loader section. If Interrupt Vectors are placed in the
Application section, interrupts are disabled while executing from the
Boot Loader section.