MC68HC908LJ12 Datasheet, PDF (156/413 Pages) Freescale Semiconductor, Inc – 8-bit microcontroller units

Monitor ROM (MON)
10.4 Functional Description
The monitor ROM receives and executes commands from a host
computer. Figure 10-1 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.
The monitor code allows enabling the PLL to generate the internal clock,
provided the reset vector is blank, when the device is being clocked by
a low-frequency crystal. This entry method, which is enabled when IRQ
is held low out of reset, is intended to support serial communication/
programming at 9600 baud in monitor mode by stepping up the external
frequency (assumed to be 32.768 kHz) by a fixed amount to generate
the desired internal frequency (2.4576 MHz). Since this feature is
enabled only when IRQ is held low out of reset, it cannot be used when
the reset vector is non-zero because entry into monitor mode in this case
requires V_TST on IRQ.
MC68HC908LJ12 — Rev. 2.1
Freescale Semiconductor
Monitor ROM (MON)
Technical Data
157