MC68HC908GT16 Datasheet, PDF (223/412 Pages) Motorola, Inc – Microcontrollers

Monitor ROM (MON)
Security
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.
HIGH BYTE OF INDEX REGISTER
CONDITION CODE REGISTER
ACCUMULATOR
LOW BYTE OF INDEX REGISTER
HIGH BYTE OF PROGRAM COUNTER
LOW BYTE OF PROGRAM COUNTER
SP
SP + 1
SP + 2
SP + 3
SP + 4
SP + 5
SP + 6
SP + 7
Figure 15-9. Stack Pointer at Monitor Mode Entry
15.5 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 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 Figure 15-10.)
MC68HC908GT16 • MC68HC908GT8 — Rev. 2
MOTOROLA
Monitor ROM (MON)
Technical Data
223