MC908KX2MDWE Datasheet, PDF (174/210 Pages) Freescale Semiconductor, Inc – High-performance M68HC08 architecture, Fully upward-compatible object code with M6805, M146805, and M68HC05 Families

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16.3 Monitor ROM (MON)
The monitor ROM allows complete testing 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 $FFFF are blank, thus reducing hardware requirements for
in-circuit programming.
Features include:
• Normal user-mode pin functionality
• One pin dedicated to serial communication between monitor ROM and host computer
• Standard mark/space non-return-to-zero (NRZ) communication with host computer
• Execution of code in random-access memory (RAM) or FLASH
• FLASH memory security⁽¹⁾
• FLASH memory programming interface
• Monitor mode entry without high voltage, V_TST, if reset vector is blank ($FFFE and $FFFF contain
$FF)
• Standard monitor mode entry if high voltage, V_TST, is applied to IRQ
16.3.1 Functional Description
The monitor ROM receives and executes commands from a host computer via a standard RS-232
interface. Simple monitor commands can access any memory address. In monitor mode, the
microcontroller unit (MCU) can execute host-computer code in RAM while all MCU pins retain normal
operating mode functions. All communication between the host computer and the MCU is through the
PTA0 pin. A level-shifting and multiplexing interface is required between PTA0 and the host computer.
PTA0 is used in a wired-OR configuration and requires a pullup resistor.
16.3.1.1 Monitor Mode Entry
There are two methods for entering monitor mode. The first is the traditional M68HC08 method where
V_TST is applied to IRQ1 and the mode pins are configured appropriately. A second method, intended for
in-circuit programming applications, will force entry into monitor mode without requiring high voltage on
the IRQ1 pin when the reset vector locations of the FLASH are erased ($FF).
Both of these methods require that the PTA1 pin be pulled low for the first 24 CGMXCLK cycles after the
part comes out of reset. This check is used by the monitor code to configure the MCU for serial
communication.
16.3.1.2 Normal Monitor Mode
Normal monitor mode is useful for MCU evaluation, factory testing, and development tool programming
operation. Figure 16-9 shows an example circuit used for normal monitor mode. Table 16-1 shows the pin
conditions for entering this mode.
1. No security feature is absolutely secure. However, Freescale’s strategy is to make reading or copying the FLASH difficult for
unauthorized users.
MC68HC908KX8 • MC68HC908KX2 • MC68HC08KX8 Data Sheet, Rev. 2.1
174
Freescale Semiconductor