mc68hc908qf4 Freescale Semiconductor, Inc, mc68hc908qf4 Datasheet - Page 173

• Manufacturer Part Number: mc68hc908qf4
• Description: M68hc08 Microcontrollers Microcontroller
• Manufacturer: Freescale Semiconductor, Inc

16.3.2 Security
MC68HC908QF4 — Rev. 1.0
MOTOROLA
NOTE:
NOTE:
The MCU executes the SWI and PSHH instructions when it enters monitor mode.
The RUN command tells the MCU to execute the PULH and RTI instructions.
Before sending the RUN command, the host can modify the stacked CPU registers
to prepare to run the host program. The READSP command returns the
incremented stack pointer value, SP + 1. The high and low bytes of the program
counter are at addresses SP + 5 and SP + 6.
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.
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 PTA0. 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
Upon power-on reset, if the received bytes of the security code do not match the
data at locations $FFF6–$FFFD, the host fails to bypass the security feature. The
MCU remains in monitor mode, but reading a FLASH location returns an invalid
value and trying to execute code from FLASH causes an illegal address reset. After
receiving the eight security bytes from the host, the MCU transmits a break
character, signifying that it is ready to receive a command.
The MCU does not transmit a break character until after the host sends the eight
security bytes.
Freescale Semiconductor, Inc.
For More Information On This Product,
Figure 16-18. Stack Pointer at Monitor Mode Entry
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HIGH BYTE OF PROGRAM COUNTER
LOW BYTE OF PROGRAM COUNTER
HIGH BYTE OF INDEX REGISTER
LOW BYTE OF INDEX REGISTER
CONDITION CODE REGISTER
ACCUMULATOR
SP
SP + 1
SP + 2
SP + 3
SP + 4
SP + 5
SP + 6
SP + 7
Figure
Monitor Module (MON)
Development Support
16-19.
Data Sheet
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