MC68HC908GR4MP datasheet(131/286 Pages) FREESCALE

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MC68HC908GR8 • MC68HC908GR4 Data Sheet, Rev. 7

Freescale Semiconductor

131

Chapter 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 MCU through a single-wire interface with a host computer. Monitor mode

entry can be achieved without use of the higher test voltage, VTST, 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 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 feature(1)

•

FLASH memory programming interface

•

Enhanced PLL (phase-locked loop) option to allow use of external 32.768-kHz crystal to generate

internal frequency of 2.4576 MHz

•

310 byte monitor ROM code size ($FE20 to $FF55)

•

Monitor mode entry without high voltage, VTST, if reset vector is blank ($FFFE and $FFFF contain

$FF)

•

Standard monitor mode entry if high voltage, VTST, is applied to IRQ

15.3 Functional Description

The monitor ROM receives and executes commands from a host computer. Figure 15-1 shows an

example 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 pullup resistor.

1. No security feature is absolutely secure. However, Freescale’s strategy is to make reading or copying the FLASH difficult for

unauthorized users.

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