MC68HC908KX8 Datasheet, PDF (108/310 Pages) Motorola, Inc – Microcontrollers

Internal Clock Generator Module (ICG)
7.4.3.1 External Oscillator Amplifier
The external oscillator amplifier provides the gain required by an
external crystal connected in a Pierce oscillator configuration. The
amount of this gain is controlled by the slow external (EXTSLOW) bit in
the CONFIG or MOR. When EXTSLOW is set, the amplifier gain is
reduced for operating low-frequency crystals (32 kHz to 100 kHz). When
EXTSLOW is clear, the amplifier gain will be sufficient for 1-MHz to 8-
MHz crystals. EXTSLOW must be configured correctly for the given
crystal or the circuit may not operate.
The amplifier is enabled when the external clock generator enable
(ECGEN) signal is set and when the external crystal enable
(EXTXTALEN) bit in the CONFIG or MOR is set. ECGEN is controlled by
the clock enable circuit (see 7.4.1 Clock Enable Circuit) and indicates
that the external clock function is desired. When enabled, the amplifier
will be connected between the PTB6/(OSC1) and PTB7/(OSC2) pins.
Otherwise, the PTB7/(OSC2) pin reverts to its port function.
In its typical configuration, the external oscillator requires five external
components:
1. Crystal, X₁
2. Fixed capacitor, C₁
3. Tuning capacitor, C₂ (can also be a fixed capacitor)
4. Feedback resistor, RB
5. Series resistor, R_S (included in Figure 7-3 to follow strict Pierce
oscillator guidelines and may not be required for all ranges of
operation, especially with high frequency crystals. Refer to the
crystal manufacturer’s data for more information.)
7.4.3.2 External Clock Input Path
The external clock input path is the means by which the microcontroller
uses an external clock source. The input to the path is the PTB6/(OSC1)
pin and the output is the external clock (ECLK). The path, which contains
input buffering, is enabled when the external clock generator enable
signal (ECGEN) is set. When not enabled, the PTB6/(OSC1) pin reverts
to its port function.
Technical Data
108
MC68HC908KX8 • MC68HC908KX2 • MC68HC08KX8 — Rev. 1.0
Internal Clock Generator Module (ICG)
MOTOROLA