Datasheet

Monitor ROM (MON)
MC68HC908AB32 — Rev. 1.1 Technical Data
Freescale Semiconductor Monitor ROM (MON)
167
10.5 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 10-6.)
Figure 10-6. Monitor Mode Entry Timing
BYTE 1
BYTE 1 ECHO
BYTE 2
BYTE 2 ECHO
BYTE 8
BYTE 8 ECHO
COMMAND
COMMAND ECHO
PTA0
RST
V
DD
4096 + 32 CGMXCLK CYCLES
256 BUS CYCLES (MINIMUM)
1
4
1
1
2
1
BREAK
NOTES:
2 = Data return delay, 2 bit times
4 = Wait 1 bit time before sending next byte.
4
FROM HOST
FROM MCU
1 = Echo delay, 2 bit times