MC68HC705JP7 Datasheet, PDF (76/242 Pages) Motorola, Inc – HCMOS Microcontroller Unit

Operating Modes
Freescale Semiconductor, Inc.
6.3 Oscillator Source
The microcontroller unit (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.
Address: $000D
Bit 7
6
5
4
3
2
1
Bit 0
Read:
0
IRQF
0
0
0
IRQE OM2 OM1
Write:
R
IRQR
Reset: 1
1
0
0
0
0
0
0
= Unimplemented
R = Reserved
Figure 6-1. IRQ Status and Control Register (ISCR)
IRQE — External Interrupt Request Enable Bit
This read/write bit enables external interrupts. Refer to Section 4.
Interrupts for more details.
OM1 and OM2 — Oscillator Select Bits
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 M68HC05 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.
Therefore, the lowest power is consumed when OM1 is cleared. The
state with both OM1 and OM2 set is provided so that the EPO can be
started up and allowed to stabilize while the LPO still clocks the MCU.
Advance Information
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MC68HC705JJ7 • MC68HC705JP7 — REV 4
Operating Modes
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