MC68HC908LB8_05 Datasheet, PDF (128/234 Pages) Freescale Semiconductor, Inc – M68HC08 Microcontrollers

Oscillator Module (OSC)
13.4 Oscillator Module Signals
The following paragraphs describe the signals that are inputs to and outputs from the oscillator module.
13.4.1 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 13-1.
13.4.2 Crystal Amplifier Output Pin (OSC2/PTC1/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 PTC1 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 13-1, or the output of the oscillator clock (BUSCLKX4).
Table 13-1. OSC2 Pin Function
Option
XTAL oscillator
External clock
Internal oscillator
or
RC oscillator
OSC2 Pin Function
Inverting OSC1
PTC1 I/O
Controlled by OSC2EN bit in PTCPUE register
OSC2EN = 0: PTC1 I/O
OSC2EN = 1: BUSCLKX4 output
13.4.3 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.
13.4.4 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 13-2 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.
13.4.5 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 13-3 shows only the logical relation of RCCLK to OSC1 and may not
represent the actual circuitry.
MC68HC908LB8 Data Sheet, Rev. 1
128
Freescale Semiconductor