MC68HC908GT16 Datasheet, PDF (117/412 Pages) Motorola, Inc – Microcontrollers

Internal Clock Generator (ICG) Module
Functional Description
7.4.3 External Clock Generator
The ICG also provides for an external oscillator or external clock source,
if desired. The external clock generator, shown in Figure 7-3, contains
an external oscillator amplifier and an external clock input path.
ECGEN
EXTXTALEN
EXTSLOW
EXTERNAL
CLOCK
GENERATOR
INTERNAL TO MCU
EXTERNAL
INPUT PATH
AMPLIFIER
OSC1
PTE4
R_B
OSC2
PTE3
ECLK
NAME
NAME
NAME
NAME
CONFIG2 BIT
TOP LEVEL SIGNAL
REGISTER BIT
MODULE SIGNAL
R_S
X₁
*R_S can be 0 (shorted)
when used with higher-
frequency crystals. Refer
to manufacturer’s data.
C₁
C₂
These components are required
for external crystal use only.
Figure 7-3. External Clock Generator Block Diagram
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 CONFIG2 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.
MC68HC908GT16 • MC68HC908GT8 — Rev. 2
MOTOROLA
Internal Clock Generator (ICG) Module
Technical Data
117