MC908GZ60CFUE Datasheet, PDF (309/352 Pages) Freescale Semiconductor, Inc – Standard features, Features of the CPU08

Monitor Module (MON)
20.3.1.1 Normal Monitor Mode
If V_TST is applied to IRQ and PTB4 is low upon monitor mode entry, the bus frequency is a divide-by-two
of the input clock. If PTB4 is high with V_TST applied to IRQ upon monitor mode entry, the bus frequency
will be a divide-by-four of the input clock. Holding the PTB4 pin low when entering monitor mode causes
a bypass of a divide-by-two stage at the oscillator only if V_TST is applied to IRQ. In this event, the
CGMOUT frequency is equal to the CGMXCLK frequency, and the OSC1 input directly generates internal
bus clocks. In this case, the OSC1 signal must have a 50% duty cycle at maximum bus frequency.
When monitor mode was 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.
20.3.1.2 Forced Monitor Mode
If entering monitor mode without high voltage on IRQ, then all port B pin requirements and conditions,
including the PTB4 frequency divisor selection, 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 8 MHz is required for a baud rate of 7200, 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.
20.3.1.3 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 20-2 summarizes the differences between user mode and monitor mode.
Table 20-2. Mode Differences
Modes
Reset
Vector High
Reset
Vector Low
Functions
Break
Break
Vector High Vector Low
SWI
Vector High
SWI
Vector Low
User
$FFFE
$FFFF
$FFFC
$FFFD
$FFFC
$FFFD
Monitor $FEFE
$FEFF
$FEFC
$FEFD
$FEFC
$FEFD
MC68HC908GZ60 • MC68HC908GZ48 • MC68HC908GZ32 Data Sheet, Rev. 6
Freescale Semiconductor
309