Development Support
Data Sheet
MC68HC908GR60A MC68HC908GR48A MC68HC908GR32A
306
Development Support
MOTOROLA
Enter monitor mode with pin configuration shown in Table 19-1 by pulling RST low
and then high. 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
consecutive 0s) to the host, indicating that it is ready to receive a command.
19.3.1.1 Normal Monitor Mode
If VTST 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 VTST 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 VTST 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 VTST on IRQ, the computer operating
properly (COP) is disabled as long as VTST is applied to either IRQ or RST.
This condition states that as long as VTST is maintained on the IRQ pin after
entering monitor mode, or if VTST is applied to RST after the initial reset to get into
monitor mode (when VTST was applied to IRQ), then the COP will be disabled. In
the latter situation, after VTST is applied to the RST pin, VTST can be removed from
the IRQ pin in the interest of freeing the IRQ for normal functionality in monitor
mode.
19.3.1.2 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, VTST, 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.