Freescale Semiconductor, Inc.
Op e ra ting Mod e s
6.3 Osc illa tor Sourc e
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.
$000D
Read:
Write:
Reset:
Bit 7
IRQE
1
6
OM2
1
5
OM1
0
4
0
3
2
0
1
0
Bit 0
0
IRQF
R
0
IRQR
0
0
0
0
= Unimplemented
= Reserved
R
Figure 6-1. IRQ Status and Control Register (ISCR)
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.
.
Table 6-1. Oscillator Selection
External
Internal
Oscillator
Selected
Pin
Power
OM2 OM1
Low-Power
Oscillator Consumption
(EPO)
Oscillator (LPO)
0
0
1
1
0
1
0
1
Internal
External
Internal
Internal
Enabled
Disabled
Enabled
Enabled
Disabled
Enabled
Disabled
Enabled
Lowest
Normal
Lowest
Normal
General Release Specification
MC68HC705JJ7/MC68HC705JP7 — Rev. 3.0
Operating Modes
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