Oscillator Module (OSC)
Oscillator Module Signals
MC68HC908QY/QT Family — Rev. 3
Data Sheet
MOTOROLA
Oscillator Module (OSC)
101
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
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 again in the SIM and results in the internal
Table 11-1. OSC2 Pin Function
Option
OSC2 Pin Function
XTAL oscillator
Inverting OSC1
External clock
PTA4 I/O
Internal oscillator
or
RC oscillator
Controlled by OSC2EN bit in PTAPUE register
OSC2EN = 0: PTA4 I/O
OSC2EN = 1: BUSCLKX4 output