MC68HC908GR8A_07 Datasheet, PDF (225/260 Pages) Freescale Semiconductor, Inc – Microcontrollers

Monitor Module (MON)
Enter monitor mode with pin configuration shown in Table 18-1 with a power-on reset. The rising edge of
RST latches monitor mode. Once monitor mode is latched, the levels on the port pins except PTA0 can
change.
Once out of reset, the MCU waits for the host to send eight security bytes (see 18.3.2 Security). After the
security bytes, the MCU sends a break signal (10 consecutive 0s) to the host, indicating that it is ready to
receive a command.
18.3.1.1 Normal Monitor Mode
When monitor mode is entered with V_TST on IRQ, the computer operating properly (COP) is disabled as
long as V_TST is applied to either IRQ or RST.
This condition states that as long as V_TST is maintained on the IRQ pin after entering monitor mode, or if
V_TST is applied to RST after the initial reset to get into monitor mode (when V_TST was applied to IRQ),
then the COP will be disabled. In the latter situation, after V_TST is applied to the RST pin, V_TST can be
removed from the IRQ pin in the interest of freeing the IRQ for normal functionality in monitor mode.
18.3.1.2 Forced Monitor Mode
If entering monitor mode without high voltage on IRQ, all port B pin requirements and conditions are not
in effect. This is to reduce circuit requirements when performing in-circuit programming.
NOTE
If the reset vector is blank and monitor mode is entered, the chip will see an
additional reset cycle after the initial power-on reset (POR). Once the reset
vector has been programmed, the traditional method of applying a voltage,
V_TST, to IRQ must be used to enter monitor mode.
An external oscillator of 9.8304 MHz is required for a baud rate of 9600, as the internal bus frequency is
automatically set to the external frequency divided by four.
When the forced monitor mode is entered the COP is always disabled regardless of the state of IRQ or
RST.
18.3.1.3 Monitor Vectors
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.
Table 18-2 summarizes the differences between user mode and monitor mode.
Table 18-2. Mode Differences
Modes
User
Monitor
Reset
Vector High
$FFFE
$FEFE
Reset
Vector Low
$FFFF
$FEFF
Functions
Break
Break
Vector High Vector Low
$FFFC
$FFFD
$FEFC
$FEFD
SWI
Vector High
$FFFC
$FEFC
SWI
Vector Low
$FFFD
$FEFD
MC68HC908GR8A • MC68HC908GR4A Data Sheet, Rev. 5
Freescale Semiconductor
225