MC68HC08AS20 Datasheet, PDF (154/386 Pages) Freescale Semiconductor, Inc – M68HC08 Microcontrollers

12.3 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
• 4800 Baud–28.8 kBaud Communication with Host Computer
• Execution of Code in RAM or ROM
12.4 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 RS-232 interface.
While simple monitor commands can access any memory address, the
MC68HC08AS20 has a ROM security feature that requires proper
procedures to be followed before the ROM can be accessed. Access to
the ROM 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.
Advance Information
154
MC68HC08AS20 —Rev. 4.1
Freescale Semiconductor