MC68HC908GT16 Datasheet, PDF (210/412 Pages) Motorola, Inc – Microcontrollers

Monitor ROM (MON)
15.3 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
• Execution of code in random-access memory (RAM) or FLASH
• FLASH memory security feature⁽¹⁾
• FLASH memory programming interface
• External 4.92MHz or 9.83MHz clock used to generate internal
frequency of 2.4576 MHz
• Optional ICG mode of operation (no external clock or high voltage)
• 304 bytes monitor ROM code size ($FE20 to $FF4F)
• Monitor mode entry without high voltage, V_TST, if reset vector is
blank ($FFFE and $FFFF contain $FF)
• Standard monitor mode entry if high voltage is applied to IRQ
15.4 Functional Description
The monitor ROM receives and executes commands from a host
computer. Figure 15-1, 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.
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, Motorola’s strategy is to make reading or
copying the FLASH difficult for unauthorized users.
Technical Data
210
MC68HC908GT16 • MC68HC908GT8 — Rev. 2
Monitor ROM (MON)
MOTOROLA