MC68HC908MR8 Datasheet, PDF (197/372 Pages) Motorola, Inc – Microcontrollers

Monitor ROM (MON)
Security
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 satisfy 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 $60 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 can be reset (via power-pin
reset only) and brought up in monitor mode to attempt another entry.
After failing the security sequence, the FLASH memory can also be bulk
erased by executing an erase routine that was downloaded into internal
RAM. The bulk erase operation clears the security code locations so that
all eight security bytes become $FF.
MC68HC908MR8 — Rev 4.1
Freescale Semiconductor
Monitor ROM (MON)
Technical Data
197