Monitor ROM (MON)
Technical Data MC68HC908AB32 — Rev. 1.0
168 Monitor ROM (MON) MOTOROLA
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.
To determine whether the security code entered is correct, check to see
if bit 6 of RAM address $50 is set. If it is, then the correct security code
has been entered and FLASH can be accessed.
NOTE:
The MCU does not transmit a break character until after the host sends
the eight security bits.
If the host fails the security bypass as described in 10.5 Security, 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. The MCU monitor commands are still valid and user
software can execute from RAM. A bulk-erase operation is possible,
erasing the entire FLASH memory, including the security bytes at
$FFF6–$FFFD.
10.6 Extended Security
To further disable monitor mode functions, an extended security
command keyword can be programmed at FLASH locations
$FFC0–$FFC7. The keyword is eight bytes long with a 7-byte ASCII
string and 1-byte $00 delimiter. The keyword for the MC68HC908AB32
MCU is "PSWDOPT" + $00.
Entry to monitor mode with extended security command keyword
programmed, the MCU stops communicating with the host after
transmitting a break character if the host fails the security bypass as
described in 10.5 Security.
NOTE:
Once the extended security command keyword is programmed, the
FLASH memory cannot be erased without a valid security code
(matching $FFF6–$FFFD). Therefore, the extended security command
keyword should not be programmed during software development.
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