Monitor ROM (MON)
Technical Data MC68HC908AZ60A — Rev 2.0
220 Monitor ROM (MON) MOTOROLA
14.4.8 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.
If FLASH is unprogrammed, the eight security byte values to be sent are
$FF, the unprogrammed state of FLASH.
During monitor mode entry, the MCU waits after the power-on reset for
the host to send the eight security bytes on pin PA0.
Figure 14-6. Monitor Mode Entry Timing
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. After the host bypasses security, any reset other than a
power-on reset requires the host to send another eight bytes. If the reset
Byte 1
Byte 1 Echo
Byte 2
Byte 2 Echo
Byte 8
Byte 8 Echo
Command
Command Echo
PA0
RST
V
DD
4096 + 32 CGMXCLK CYCLES
24 BUS CYCLES (MINIMUM)
1
4
1
1
2
1
Break
NOTE: 1 = Echo delay (2 bit times)
2 = Data return delay (2 bit times)
4 = Wait 1 bit time before sending next byte.
4
FROM HOST
FROM MCU
