MC68HC908GP32_08 Datasheet, PDF (220/266 Pages) Freescale Semiconductor, Inc – M68HC08 Microcontrollers

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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.
MC68HC908GP32 Data Sheet, Rev. 10
220
Freescale Semiconductor