Chapter 8
Monitor ROM (MON)
8.1 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
, as long as vector addresses $FFFE
TST
and $FFFF are blank, thus reducing the hardware requirements for in-circuit programming.
In addition, to simplify user coding, routines are also stored in the monitor ROM area for FLASH memory
program /erase.
8.2 Features
Features of the monitor ROM include:
•
•
•
•
•
•
Normal user-mode pin functionality
One pin dedicated to serial communication between monitor ROM and host computer
Standard mark/space non-return-to-zero (NRZ) communication with host computer
Execution of code in RAM or FLASH
(1)
FLASH memory security feature
959 bytes monitor ROM code size
($FC00–$FDFF and $FE10–$FFCE)
•
Monitor mode entry without high voltage, V
$FF)
, if reset vector is blank ($FFFE and $FFFF contain
TST
•
•
Standard monitor mode entry if high voltage, V
Resident routines for in-circuit programming
, is applied to IRQ1
TST
8.3 Functional Description
The monitor ROM receives and executes commands from a host computer. Figure 8-1 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.
MC68HC908AP Family Data Sheet, Rev. 4
Freescale Semiconductor
115
FREESCALE [ Freescale ]
