MC68HC908GR8 Datasheet, PDF (131/286 Pages) Freescale Semiconductor, Inc – M68HC08 Microcontrollers

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, V_TST, as long as vector addresses $FFFE
and $FFFF are blank, thus reducing the hardware requirements for in-circuit programming.
15.2 Features
Features of the monitor ROM include:
• Normal user-mode pin functionality
• One pin dedicated to serial communication between monitor ROM and host computer
• Standard mark/space non-return-to-zero (NRZ) communication with host computer
• Execution of code in RAM or FLASH
• FLASH memory security feature⁽¹⁾
• FLASH memory programming interface
• Enhanced PLL (phase-locked loop) option to allow use of external 32.768-kHz crystal to generate
internal frequency of 2.4576 MHz
• 310 byte monitor ROM code size ($FE20 to $FF55)
• Monitor mode entry without high voltage, V_TST, if reset vector is blank ($FFFE and $FFFF contain
$FF)
• Standard monitor mode entry if high voltage, V_TST, is applied to IRQ
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.
MC68HC908GR8 • MC68HC908GR4 Data Sheet, Rev. 7
Freescale Semiconductor
131