MC68HC08GZ32 Datasheet, PDF (290/320 Pages) Freescale Semiconductor, Inc – Microcontrollers

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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 20-15. Stack Pointer at Monitor Mode Entry
20.3.2 Security
A security feature discourages unauthorized reading of ROM 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 ROM locations and execute code from ROM. 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 20-16.
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 ROM location returns an invalid value and trying to execute code from ROM 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.
NOTE
The MCU does not transmit a break character until after the host sends the
eight security bytes.
To determine whether the security code entered is correct, check to see if bit 6 of RAM address $40 is
set. If it is, then the correct security code has been entered and ROM can be accessed.
If the security sequence fails, the device should be reset by a power-on reset and brought up in monitor
mode to attempt another entry.
MC68HC08GZ32 Data Sheet, Rev. 3
290
Freescale Semiconductor