MC68HC908GZ8 Datasheet, PDF (149/344 Pages) Motorola, Inc – Microcontrollers

Data Sheet — MC68HC908GZ8
Section 15. Monitor ROM (MON)
15.1 Introduction
This section describes the monitor ROM (MON) and the monitor mode entry
methods. 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 $FFFF are blank, thus reducing the hardware requirements
for in-circuit programming.
15.2 Features
Features of the monitor ROM include:
• Normal user-mode pin functionality
• One pin dedicated to serial communication between monitor read-only
memory (ROM) and host computer
• Standard mark/space non-return-to-zero (NRZ) communication with host
computer
• Standard communication baud rate (7200 @ 8-MHz bus frequency)
• Execution of code in random-access memory (RAM) or FLASH
• FLASH memory security feature⁽¹⁾
• FLASH memory programming interface
• 350 bytes monitor ROM code size ($FE20 to $FF7D)
• Monitor mode entry without high voltage, V_TST, if reset vector is blank
($FFFE and $FFFF contain $FF)
• Normal monitor mode entry if high voltage is applied to IRQ
15.3 Functional Description
Figure 15-1 shows a simplified diagram of the monitor mode.
The monitor ROM receives and executes commands from a host computer.
Figure 15-2 and Figure 15-3 show example circuits used to enter monitor mode
and communicate with a host computer via a standard RS-232 interface.
1. No security feature is absolutely secure. However, Freescale’s strategy is to make reading or
copying the FLASH difficult for unauthorized users.
MC68HC908GZ8
Freescale Semiconductor
Monitor ROM (MON)
Data Sheet
149