Datasheet

Security
MC68HC908JL3E Family Data Sheet, Rev. 4
Freescale Semiconductor 79
7.4 Security
A security feature discourages unauthorized reading of Flash locations while in monitor mode. The host
can bypass the security feature at monitor mode entry by sending eight security bytes that match the
bytes at locations $FFF6–$FFFD. Locations $FFF6–$FFFD contain user-defined data.
NOTE
Do not leave locations $FFF6–$FFFD blank. For security reasons, program
locations $FFF6–$FFFD even if they are not used for vectors.
During monitor mode entry, the MCU waits after the power-on reset for the host to send the eight security
bytes on pin PTB0. If the received bytes match those at locations $FFF6–$FFFD, the host bypasses the
security feature and can read all Flash locations and execute code from Flash. Security remains bypassed
until a power-on reset occurs. If the reset was not a power-on reset, security remains bypassed and
security code entry is not required. (See Figure 7-7.)
Table 7-8. READSP (Read Stack Pointer) Command
Description Reads stack pointer
Operand None
Data Returned Returns stack pointer in high byte:low byte order
Opcode $0C
Command Sequence
Table 7-9. RUN (Run User Program) Command
Description Executes RTI instruction
Operand None
Data Returned None
Opcode $28
Command Sequence
SP HIGHREADSPREADSP SP LOW
ECHO
SENT TO
MONITOR
RESULT
RUNRUN
ECHO
SENT TO
MONITOR