MC68HC908GZ8 Datasheet, PDF (154/344 Pages) Motorola, Inc – Microcontrollers

Freescale Semiconductor, Inc.
Monitor ROM (MON)
15.3.1 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.
15.3.2 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.
15.3.3 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 15-2 summarizes the differences between user mode and monitor mode.
Data Sheet
154
Monitor ROM (MON)
For More Information On This Product,
Go to: www.freescale.com
MC68HC908GZ8
MOTOROLA