MC68HC908LD64 Datasheet, PDF (138/362 Pages) Freescale Semiconductor, Inc – Microcontrollers

Monitor ROM (MON)
10.3 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
• Execution of code in RAM or FLASH
• FLASH memory security feature¹
• FLASH memory programming interface
• 1024 bytes monitor ROM code size ($FA00 to $FDFF)
• Monitor mode entry without high voltage, V_TST, if reset vector is
blank ($FFFE and $FFFF contain $FF)
• Standard monitor mode entry if high voltage, V_TST, is applied to
IRQ
10.4 Functional Description
The monitor ROM receives and executes commands from a host
computer. Figure 10-1 shows a sample circuit 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 pull-up resistor.
Data Sheet
138
1. No security feature is absolutely secure. However, Freescale’s strategy is to make reading or
copying the FLASH difficult for unauthorized users.
Monitor ROM (MON)
MC68HC908LD64 — Rev. 3.0
Freescale Semiconductor