M68HC08 datasheet(83/172 Pages) FREESCALE

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Security

MC68HC908JL3E Family Data Sheet, Rev. 3

Freescale Semiconductor

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 HIGH

READSP

SP LOW

ECHO

SENT TO

MONITOR

RESULT

RUN

ECHO

SENT TO

MONITOR

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