Functional Description
Enter monitor mode with pin configuration shown in Figure 8-1 by pulling RST low and then high. The
rising edge of RST latches monitor mode. Once monitor mode is latched, the values on the specified pins
can change.
Once out of reset, the MCU waits for the host to send eight security bytes. (See 8.4 Security.) After the
security bytes, the MCU sends a break signal (10 consecutive logic 0’s) to the host, indicating that it is
ready to receive a command.
POR RESET
NO
NORMAL USER
MODE
IS VECTOR
BLANK?
YES
MONITOR MODE
EXECUTE
MONITOR
CODE
POR
TRIGGERED?
NO
YES
Figure 8-2. Low-Voltage Monitor Mode Entry Flowchart
In monitor mode, the MCU uses different vectors for reset, SWI (software interrupt), and break interrupt
than those for user mode. The alternate vectors are in the $FE page instead of the $FF page and allow
code execution from the internal monitor firmware instead of user code.
NOTE
Exiting monitor mode after it has been initiated by having a blank reset
vector requires a power-on reset (POR). Pulling RST low will not exit
monitor mode in this situation.
Table 8-2 summarizes the differences between user mode and monitor mode vectors.
Table 8-2. Mode Differences (Vectors)
Functions
Reset
Vector
High
Reset
Vector
Low
Break
Vector
High
Break
Vector
Low
SWI
Vector
High
SWI
Vector
Low
Modes
User
$FFFE
$FEFE
$FFFF
$FEFF
$FFFC
$FEFC
$FFFD
$FEFD
$FFFC
$FEFC
$FFFD
$FEFD
Monitor
MC68HC908AP Family Data Sheet, Rev. 4
Freescale Semiconductor
119
FREESCALE [ Freescale ]
