
Monitor ROM (MON)
MC68HC908GR8 MC68HC908GR4 Data Sheet, Rev. 7
134
Freescale Semiconductor
Figure 15-2 shows a simplified diagram of the monitor mode entry when the reset vector is blank and just
1 x VDD voltage is applied to the IRQ pin. 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.
Enter monitor mode with pin configuration shown in
Figure 15-1 by pulling RST low and then high. The
rising edge of RST latches monitor mode. Once monitor mode is latched, the values on the specified pins
can change.
Once out of reset, the MCU waits for the host to send eight security bytes. (See
Security.) After the
security bytes, the MCU sends a break signal (10 consecutive logic 0s) to the host, indicating that it is
ready to receive a command.
NOTE
The PTA1 pin must remain at logic 0 for 24 bus cycles after the RST pin
goes high to enter monitor mode properly.
Table 15-1. Monitor Mode Signal Requirements and Options
IRQ
RESET
$FFFE/
$FFFF
PLL
PTB0 PTB1
External
Clock(1)
1. External clock is derived by a 32.768 kHz crystal or a 9.8304 MHz off-chip oscillator
CGMOUT
Bus
Freq
COP
For Serial
Communication
Comment
PTA0 PTA1
Baud
Rate(2) (3)
2. PTA0 = 1 if serial communication; PTA0 = X if parallel communication
3. PTA1 = 0
→ serial, PTA1 = 1 → parallel communication for security code entry
4. DNA = does not apply, X = don’t care
XGND
X
0
Disabled
X
0
No operation
until reset goes
high
VTST
VDD
or
VTST
XOFF
1
0
9.8304
MHz
4.9152
MHz
2.4576
MHz
Disabled
1
0
9600
PTB0 and PTB1
voltages only
required if
IRQ =VTST
X1
DNA
VDD
$FFFF
OFF
X
9.8304
MHz
4.9152
MHz
2.4576
MHz
Disabled
1
0
9600
External
frequency always
divided by 4
X1
DNA
GND
VDD
$FFFF
ON
X
32.768
kHz
4.9152
MHz
2.4576
MHz
Disabled
1
0
9600
PLL enabled
(BCS set) in
monitor code
X1
DNA
VDD
or
GND
VTST
$FFFF
OFF
X
—
Enabled
X
—
Enters user
mode — will
encounter an
illegal address
reset
VDD
or
GND
VDD
or
VTST
Not
$FFFF
OFF
X
—
Enabled
X
—
Enters
user mode