M68ICS08GR datasheet(192/408 Pages) FREESCALE

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Monitor ROM (MON)

Technical Data

MC68HC908GR8 — Rev 4.0

192

Monitor ROM (MON)

MOTOROLA

The monitor code has been updated from previous versions to allow

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 addition, which is enabled when IRQ is held low out of rest, 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 not blank

because entry into monitor mode in this case requires VTST on IRQ.

15.4.1 Entering Monitor Mode

Table 15-1 shows the pin conditions for entering monitor mode. As

specified in the table, monitor mode may be entered after a power-on

reset (POR) and will allow communication at 9600 baud provided one of

the following sets of conditions is met:

1. If $FFFE and $FFFF contain values not cared:

–

The external clock is 9.8304 MHz

–

IRQ = VTST (PLL off)

2. If $FFFE and $FFFF contain $FF, blank state:

–

The external clock is 9.8304 MHz

–

IRQ = VDD (this can be implemented through the internal IRQ

pullup; PLL off)

3. If $FFFE and $FFFF contain $FF, blank state:

–

The external clock is 32.768 kHz (crystal)

–

IRQ = VSS (this setting initiates the PLL to boost the external

32.768 kHz to an internal bus frequency of 2.4576 MHz)

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