MC68HC908QC16_07 Datasheet, PDF (103/274 Pages) Freescale Semiconductor, Inc – Microcontrollers

I/O Signals
To protect status bits during the break state, write a 0 to BCFE. With BCFE cleared (its default state),
software can read and write registers during the break state without affecting status bits. Some status bits
have a two-step read/write clearing procedure. If software does the first step on such a bit before the
break, the bit cannot change during the break state as long as BCFE is cleared. After the break, doing the
second step clears the status bit.
10.7 I/O Signals
The OSC shares its pins with general-purpose input/output (I/O) port pins. See Figure 10-1 for port
location of these shared pins.
10.7.1 Oscillator Input Pin (OSC1)
The OSC1 pin is an input to the crystal oscillator amplifier, an input to the RC oscillator circuit, or an input
from an external clock source.
When the OSC is configured for internal oscillator, the OSC1 pin can be used as a general-purpose
input/output (I/O) port pin or other alternative pin function.
10.7.2 Oscillator Output Pin (OSC2)
For the XTAL oscillator option, the OSC2 pin is the output of the crystal oscillator amplifier.
When the OSC is configured for internal oscillator, external clock, or RC, the OSC2 pin can be used as a
general-purpose I/O port pin or other alternative pin function. When the oscillator is configured for internal
or RC, the OSC2 pin can be used to output BUSCLKX4.
Table 10-1. OSC2 Pin Function
Option
XTAL oscillator
External clock
Internal oscillator
or
RC oscillator
OSC2 Pin Function
Inverting OSC1
General-purpose I/O or alternative pin function
Controlled by OSC2EN bit
OSC2EN = 0: General-purpose I/O or alternative pin function
OSC2EN = 1: BUSCLKX4 output
10.8 Registers
The oscillator module contains two registers:
• Oscillator status and control register (OSCSC)
• Oscillator trim register (OSCTRIM)
10.8.1 Oscillator Status and Control Register
The oscillator status and control register (OSCSC) contains the bits for switching between internal and
external clock sources. If the application uses an external crystal, bits in this register are used to select
the crystal oscillator amplifier necessary for the desired crystal. While running off the internal clock source,
the user can use bits in this register to select the internal clock source frequency.
MC68HC908QC16 • MC68HC908QC8 • MC68HC908QC4 Data Sheet, Rev. 3
Freescale Semiconductor
103