MC68HC908QY4_10 Datasheet, PDF (94/168 Pages) Freescale Semiconductor, Inc – M68HC08 Microcontrollers

Oscillator Module (OSC)
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).
Table 11-1. OSC2 Pin Function
Option
XTAL oscillator
External clock
Internal oscillator or
RC oscillator
OSC2 Pin Function
Inverting OSC1
PTA4 I/O
Controlled by OSC2EN bit in PTAPUE register
OSC2EN = 0: PTA4 I/O
OSC2EN = 1: BUSCLKX4 output
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 (f_XCLK) 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
MC68HC908QY/QT Family Data Sheet, Rev. 6
94
Freescale Semiconductor