M68HC08 datasheet(113/288 Pages) FREESCALE

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Oscillator (OSC)

MC68HC908BD48 — Rev. 2.1

Data Sheet

Freescale Semiconductor

Oscillator (OSC)

113

8.4 I/O Signals

The following paragraphs describe the oscillator I/O signals.

8.4.1 Crystal Amplifier Input Pin (OSC1)

The OSC1 pin is an input to the crystal oscillator amplifier.

An externally generated clock also can feed the OSC1 pin of the crystal

oscillator circuit. Connect the external clock to the OSC1 pin and let the

OSC2 pin float.

8.4.2 Crystal Amplifier Output Pin (OSC2)

The OSC2 pin is the output of the crystal oscillator inverting amplifier.

8.4.3 Oscillator Enable Signal (SIMOSCEN)

The SIMOSCEN signal comes from the SIM and enables the oscillator.

8.4.4 External Clock Source (OSCXCLK)

OSCXCLK is the crystal oscillator output signal. It runs at the full speed

of the crystal (f

XCLK) and comes directly from the crystal oscillator circuit.

Figure 8-1 shows only the logical relation of OSCXCLK to OSC1 and

OSC2 and may not represent the actual circuitry. The duty cycle of

OSCXCLK is unknown and may depend on the crystal and other

external factors. Also, the frequency and amplitude of OSCXCLK can be

unstable at start-up.

8.4.5 Oscillator Out (OSCOUT)

The clock driven to the SIM is the crystal frequency divided by two. This

signal is driven to the SIM for generation of the bus clocks used by the

CPU and other modules on the MCU. OSCOUT will be divided again in

the SIM and results in the internal bus frequency being one fourth of the

OSCXCLK frequency.

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