MC908QT4MPE datasheet(94/184 Pages) FREESCALE

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Oscillator Module (OSC)

MC68HC908QY/QT Family Data Sheet, Rev. 5

Freescale Semiconductor

11.4.2 Crystal Amplifier Output Pin (OSC2/PTA4/BUSCLKX4)

For the XTAL oscillator device, the OSC2 pin is the crystal oscillator inverting amplifier output.

For the external clock option, the OSC2 pin is dedicated to the PTA4 I/O function. The OSC2EN bit has

no effect.

For the internal oscillator or RC oscillator options, the OSC2 pin can assume other functions according to

Table 1-3. Function Priority in Shared Pins, or the output of the oscillator clock (BUSCLKX4).

11.4.3 Oscillator Enable Signal (SIMOSCEN)

The SIMOSCEN signal comes from the system integration module (SIM) and enables/disables either the

XTAL oscillator circuit, the RC oscillator, or the internal oscillator.

11.4.4 XTAL Oscillator Clock (XTALCLK)

XTALCLK is the XTAL oscillator output signal. It runs at the full speed of the crystal (fXCLK) and comes

directly from the crystal oscillator circuit. Figure 11-2 shows only the logical relation of XTALCLK to OSC1

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

depend on the crystal and other external factors. Also, the frequency and amplitude of XTALCLK can be

unstable at start up.

11.4.5 RC Oscillator Clock (RCCLK)

RCCLK is the RC oscillator output signal. Its frequency is directly proportional to the time constant of

external R and internal C. Figure 11-3 shows only the logical relation of RCCLK to OSC1 and may not

represent the actual circuitry.

11.4.6 Internal Oscillator Clock (INTCLK)

INTCLK is the internal oscillator output signal. Its nominal frequency is fixed to 12.8 MHz, but it can be

also trimmed using the oscillator trimming feature of the OSCTRIM register (see 11.3.1.1 Internal

Oscillator Trimming).

11.4.7 Oscillator Out 2 (BUSCLKX4)

BUSCLKX4 is the same as the input clock (XTALCLK, RCCLK, or INTCLK). This signal is driven to the

SIM module and is used to determine the COP cycles.

11.4.8 Oscillator Out (BUSCLKX2)

The frequency of this signal is equal to half of the BUSCLKX4, this signal is driven to the SIM for

generation of the bus clocks used by the CPU and other modules on the MCU. BUSCLKX2 will be divided

Table 11-1. OSC2 Pin Function

Option

OSC2 Pin Function

XTAL oscillator

Inverting OSC1

External clock

PTA4 I/O

Internal oscillator

RC oscillator

Controlled by OSC2EN bit in PTAPUE register

OSC2EN = 0: PTA4 I/O

OSC2EN = 1: BUSCLKX4 output

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