MC68HC908GT16_07 Datasheet, PDF (83/292 Pages) Freescale Semiconductor, Inc – Microcontrollers

Functional Description
7.3.3 External Clock Generator
The ICG also provides for an external oscillator or external clock source, if desired. The external clock
generator, shown in Figure 7-4, 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 7-4. External Clock Generator Block Diagram
7.3.3.1 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.
The amplifier is enabled when the external clock generator enable (ECGEN) signal is set and when the
external crystal enable (EXTXTALEN) bit in the CONFIG2 register is set. ECGEN is controlled by the
clock enable circuit (see 7.3.1 Clock Enable Circuit) and indicates that the external clock function is
desired. When enabled, the amplifier will be connected between the PTE4/OSC1 and PTE3/OSC2 pins.
Otherwise, the PTE3/OSC2 pin reverts to its port function.
MC68HC908GT16 • MC68HC908GT8 • MC68HC08GT16 Data Sheet, Rev. 5.0
Freescale Semiconductor
83