68HC705SP7 datasheet(70/230 Pages) FREESCALE

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Operating Modes

General Release Specification

MC68HC705JJ7/MC68HC705JP7 — Rev. 3.0

Operating Modes

6.3 Oscillator Source

The MCU can be clocked by either an internal low-power oscillator

(LPO) without external components or by an external pin oscillator

(EPO) which uses external components. The enable and selection of the

clock source is determined by the state of the oscillator select bits (OM1

and OM2) in the interrupt status and control register (ISCR) as shown in

Figure 6-1.

IRQE — External Interrupt Request Enable

This read/write bit enables external interrupts. Refer to Section 4.

Interrupts for more details.

OM1 and OM2 — Oscillator Selects

These bits control the selection and enabling of the oscillator source

for the MCU. One choice is the internal LPO and the other oscillator

is the EPO which is common to most MC68HC05 MCU devices. The

EPO uses external components like filter capacitors and a crystal or

ceramic resonator and consumes more power than the LPO. The

selection and enable conditions for these two oscillators are shown in

Table 6-1. Reset clears OM1 and sets OM2, which selects the LPO

and disables the EPO.

$000D

Bit 7

654321

Bit 0

Read:

IRQE

OM2

OM1

IRQF

Write:

IRQR

Reset:

11000000

= Unimplemented

= Reserved

Figure 6-1. IRQ Status and Control Register (ISCR)

Table 6-1. Oscillator Selection

OM2

OM1

Oscillator

Selected

Internal

Low-Power

Oscillator (LPO)

External

Oscillator

(EPO)

Power

Consumption

Internal

Enabled

Disabled

Lowest

External

Disabled

Enabled

Normal

Internal

Enabled

Disabled

Lowest

Internal

Enabled

Normal

Freescale Semiconductor, Inc.

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