MC68HC908QY2CDW Datasheet, PDF (93/184 Pages) Freescale Semiconductor, Inc – Microcontrollers

Oscillator Module Signals
11.3.4 RC Oscillator
The RC oscillator circuit is designed for use with an external resistor (R_EXT) to provide a clock source with
a tolerance within 25% of the expected frequency. See Figure 11-3.
The capacitor (C) for the RC oscillator is internal to the MCU. The R_EXT value must have a tolerance of
1% or less to minimize its effect on the frequency.
In this configuration, the OSC2 pin can be left in the reset state as PTA4. Or, the OSC2EN bit in the port
A pullup enable register can be set to enable the OSC2 output function on the pin. Enabling the OSC2
output slightly increases the external RC oscillator frequency, f_RCCLK.
FROM SIM
INTCLK
0
1
SIMOSCEN
EN
EXTERNAL RC
OSCILLATOR
RCCLK
OSCRCOPT
TO SIM
TO SIM
BUSCLKX4
BUSCLKX2
÷2
MCU
OSC1
1
0
PTA4
I/O
PTA4/BUSCLKX4 (OSC2)
PTA4
OSC2EN
V_DD
R_EXT
See Chapter 16 Electrical Specifications for component value requirements.
Figure 11-3. RC Oscillator External Connections
11.4 Oscillator Module Signals
The following paragraphs describe the signals that are inputs to and outputs from the oscillator module.
11.4.1 Crystal Amplifier Input Pin (OSC1)
The OSC1 pin is either an input to the crystal oscillator amplifier, an input to the RC oscillator circuit, or
an external clock source.
For the internal oscillator configuration, the OSC1 pin can assume other functions according to Table 1-3.
Function Priority in Shared Pins.
MC68HC908QY/QT Family Data Sheet, Rev. 6
Freescale Semiconductor
93