MC68HC908RF2 Datasheet, PDF (116/254 Pages) Motorola, Inc – HCMOS Microcontroller Unit

Internal Clock Generator Module (ICG)
8.4.3 External Clock Generator
The ICG also provides for an external oscillator or clock source, if
desired. The external clock generator, shown in Figure 8-3, contains an
external oscillator amplifier and an external clock input path.
STOP
ECGON
EXTSLOW
EXTERNAL
CLOCK
GENERATOR
INPUT PATH
AMPLIFIER
ECLK
INTERNAL TO MCU
EXTERNAL
NAME
NAME
NAME
NAME
CONFIGURATION REGISTER BIT
TOP LEVEL SIGNAL
REGISTER BIT
MODULE SIGNAL
OSC1
OSC2
R_B
R_S*
X₁
*R_S can be 0 (shorted) when used
with higher-frequency crystals.
Refer to manufacturer’s data.
C₁
C₂
COMPONENTS REQUIRED FOR
EXTERNAL CRYSTAL USE ONLY
Figure 8-3. External Clock Generator Block Diagram
8.4.3.1 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 configuration register. 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.
Advance Information
116
Internal Clock Generator Module (ICG)
MC68HC908RF2 — Rev. 1
MOTOROLA