MC68HC08GZ32 Datasheet, PDF (284/320 Pages) Freescale Semiconductor, Inc – Microcontrollers

Development Support
The monitor module receives and executes commands from a host computer. Figure 20-10 shows an
example circuit used to enter monitor mode and communicate with a host computer via a standard
RS-232 interface.
MC68HC08GZ32
V_DD
1 μF
1 μF
DB9
2
3
1 _C1+
+
3 _C1–
4 _C2+
+
5 _C2–
7
8
MAX232
V_CC ¹⁶
GND ¹⁵
V+ ²
V– ⁶
10
9
N.C.
47 pF
V_DD
+
1 μF
1 μF
+
27 pF
8 MHz
10 MΩ
1 kΩ
1 μF
+
74HC125
6
5
74HC125
2
3
4
V_DD
9.1 V
10 kΩ
RST
OSC2
OSC1
IRQ
PTA0
5
1
V_DD
V_DDA
0.1 μF
V_DD
10 k
PTB4
10 k
PTB0
10 k
PTB1
10 k
PTA1
V_SSA
V_SS
Figure 20-10. Monitor Mode Circuit
Simple monitor commands can access any memory address. In monitor mode, the MCU can execute
code downloaded into RAM by a host computer while most MCU pins retain normal operating mode
functions. All communication between the host computer and the MCU is through the PTA0 pin. A
level-shifting and multiplexing interface is required between PTA0 and the host computer. PTA0 is used
in a wired-OR configuration and requires a pullup resistor.
Table 20-1 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 7200 baud provided one
of the following sets of conditions is met:
• The external clock is 4.0 MHz (7200 baud)
– PTB4 = low
– IRQ = V_TST
• The external clock is 8.0 MHz (7200 baud)
– PTB4 = high
– IRQ = V_TST
Enter monitor mode with pin configuration shown in Table 20-1 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. 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.
MC68HC08GZ32 Data Sheet, Rev. 3
284
Freescale Semiconductor