MC68HC05L16 Datasheet, PDF (18/146 Pages) Freescale Semiconductor, Inc – Microcontrollers

General Description
1.5.2.2 External Clock
An external clock from another CMOS-compatible device can be connected to the OSC1 input, with the
OSC2 input not connected, as shown in Figure 1-3. This configuration is possible regardless of how the
oscillator is set up.
1.5.3 XOSC1 and XOSC2
The XOSC1 and XOSC2 pins are the connections for the 2-pin on-chip oscillator. The XOSC1 and
XOSC2 pins can accept:
• A crystal as shown in Figure 1-4(a)
• An external clock signal as shown in Figure 1-4(b)
The frequency, f_OSC, of the oscillator or external clock source is divided by two to produce the internal
operating frequency, f_OP, if selected by SYS1–SYS0 bits.
When XOSC is not used, the XOSC1 pin must be connected to the RESET pin to ensure proper
initialization of the clock circuitry. XOSC2 pin should remain unconnected.
1.5.3.1 Crystal Resonator
The circuit in Figure 1-4(a) shows a typical 2-pin oscillator circuit for an AT-cut, parallel resonant crystal.
The crystal manufacturer’s recommendations should be followed, as the crystal parameters determine
the external component values required to provide maximum stability and reliable startup. The load
capacitance values used in the oscillator circuit design should include all stray capacitances. The crystal
and components should be mounted as close as possible to the pins for startup stabilization and to
minimize output distortion. An internal startup feedback resistor of R_XOF between XOSC1 and XOSC2
and a damping resistor of R_XOD in series to XOSC2 may be selected as a mask option. Typical R_XOF
resistor value is 15 MΩ, and R_XOD resistor value is 1 MΩ.
MASK OPTIONS
MCU
R_XOF
R_XOD
XOSC1 XOSC2
32.768 kHz (TYP)
MCU
XOSC1
XOSC2
C_XO1 C_XO2
UNCONNECTED
EXTERNAL CLOCK
(a) Crystal Connections
(b) External Clock
Source Connection
Figure 1-4. Oscillator Connections
MC68HC05L16 • MC68HC705L16 Data Sheet, Rev. 4.1
18
Freescale Semiconductor