Datasheet
Development Support
MC68HC908GP32 Data Sheet, Rev. 10
220 Freescale Semiconductor
18.3 Monitor Module (MON)
The monitor module allows debugging and programming of the microcontroller unit (MCU) through a
single-wire interface with a host computer. Monitor mode entry can be achieved without use of the higher
test voltage, V
TST
, as long as vector addresses $FFFE and $FFFF are blank, thus reducing the hardware
requirements for in-circuit programming.
Features of the monitor module include:
• Normal user-mode pin functionality
• One pin dedicated to serial communication between MCU and host computer
• Standard mark/space non-return-to-zero (NRZ) communication with host computer
• Standard communication baud rate
• Execution of code in random-access memory (RAM) or Flash
• Flash memory security feature
(1)
• Flash memory programming interface
• External 4.92 MHz or 9.83 MHz clock used to generate internal frequency of 2.4576 MHz
• Enhanced PLL option to allow use of 32.768 kHz crystal to generate internal bus frequency of
2.4576 MHz
• Monitor mode entry without high voltage, V
TST
, if reset vector is blank ($FFFE and $FFFF contain
$FF)
• Normal monitor mode entry if high voltage, V
TST
, is applied to IRQ
18.3.1 Functional Description
Figure 18-8 shows a simplified monitor mode entry flowchart.
The monitor ROM receives and executes commands from a host computer. Figure 18-9, Figure 18-10,
and Figure 18-11 show example circuits used to enter monitor mode and communicate with a host
computer via a standard RS-232 interface.
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.
1. No security feature is absolutely secure. However, Freescale’s strategy is to make reading or copying the Flash difficult for
unauthorized users.