MC68HC908AT32 Datasheet, PDF (117/378 Pages) Freescale Semiconductor, Inc – Microcontrollers

Chapter 12
Monitor ROM (MON)
12.1 Introduction
This section describes the monitor ROM (MON). The monitor ROM allows complete testing of the
microcontroller unit (MCU) through a single-wire interface with a host computer.
12.2 Features
Features of the MON include:
• Normal user-mode pin functionality
• One pin dedicated to serial communication between MON and a host computer
• Standard mark/space non-return-to-zero (NRZ) communication with host computer
• 4800 baud–28.8 Kbaud communication with host computer
• Execution of code in random-access memory (RAM) or read-only memory (ROM)
12.3 Functional Description
Monitor ROM receives and executes commands from a host computer. Figure 12-1 shows a sample
circuit used to enter monitor mode and communicate with a host computer via a standard RS232
interface.
While simple monitor commands can access any memory address, the MC68HC908AT32 has 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.
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.
MC68HC908AT32 Data Sheet, Rev. 3.1
Freescale Semiconductor
117