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Chapter 14
Monitor ROM (MON)
14.1 Introduction
This chapter describes the monitor ROM (MON). The monitor ROM allows complete testing of the MCU
through a single-wire interface with a host computer.
14.2 Features
Features of the monitor ROM 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
• Up to 28.8 kBaud Communication with Host Computer
• Execution of Code in RAM or FLASH
• FLASH Security
• FLASH Programming
14.3 Functional Description
Monitor ROM receives and executes commands from a host computer. Figure 14-1 shows a sample
circuit used to enter monitor mode and communicate with a host computer via a standard RS-232
interface.
While simple monitor commands can access any memory address, the MC68HC908AS60A and
MC68HC908AZ60A have a FLASH security feature to prevent external viewing of the contents of FLASH.
Proper procedures must be followed to verify FLASH content. Access to the FLASH is denied to
unauthorized users of customer specified software (see 14.3.8 Security).
In monitor mode, the MCU can execute host-computer code in RAM while all MCU pins except PTA0
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.
MC68HC908AZ60A • MC68HC908AS60A • MC68HC908AS60E Data Sheet, Rev. 6
Freescale Semiconductor
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