MC68HC908GT16 Datasheet, PDF (213/412 Pages) Motorola, Inc – Microcontrollers

Monitor ROM (MON)
Functional Description
15.4.1 Entering Monitor Mode
Table 15-1 shows the pin conditions for entering monitor mode. As
specified in the table, monitor mode may be entered after a power-on
reset (POR) and will allow communication at 9600 baud provided one of
the following sets of conditions is met:
1. If $FFFE and $FFFF does not contain $FF (programmed state):
– The external clock is 4.9152 MHz with PTC3 low or
9.8304 MHz with PTC3 high
– IRQ = V_TST
2. If $FFFE and $FFFF contain $FF (erased state):
– The external clock is 9.8304 MHz
– IRQ = V_DD (this can be implemented through the internal IRQ
pullup)
3. If $FFFE and $FFFF contain $FF (erased state):
– IRQ = V_SS (ICG is selected, no external clock required)
If V_TST is applied to IRQ and PTC3 is low upon monitor mode entry
(above condition set 1), the bus frequency is a divide-by-two of the input
clock. If PTC3 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
PTC3 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.
If entering monitor mode without high voltage on IRQ (above condition
set 2 or 3, where applied voltage is either V_DD or V_SS), then all port C pin
requirements and conditions, including the PTC3 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 POR reset. Once the part has
been programmed, the traditional method of applying a voltage, V_TST, to
IRQ must be used to enter monitor mode.
MC68HC908GT16 • MC68HC908GT8 — Rev. 2
MOTOROLA
Monitor ROM (MON)
Technical Data
213