mc68hc908gt8 Freescale Semiconductor, Inc, mc68hc908gt8 Datasheet - Page 83

• Manufacturer Part Number: mc68hc908gt8
• Description: M68hc08 Microcontrollers
• Manufacturer: Freescale Semiconductor, Inc

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
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
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.
Freescale Semiconductor
NAME
NAME
EXTXTALEN
EXTSLOW
MC68HC908GT16 • MC68HC908GT8 • MC68HC08GT16 Data Sheet, Rev. 5.0
NAME
NAME
ECGEN
Figure
INTERNAL TO MCU
EXTERNAL
Figure 7-4. External Clock Generator Block Diagram
7.3.1 Clock Enable
MODULE SIGNAL
REGISTER BIT
7-4, contains an external oscillator amplifier and an external clock input path.
CONFIG2 BIT
TOP LEVEL SIGNAL
GENERATOR
EXTERNAL
CLOCK
Circuit) and indicates that the external clock function is
OSC1
PTE4
C
1
R
X
B
1
C
R
AMPLIFIER
2
S
*
These components are required
for external crystal use only.
INPUT PATH
OSC2
PTE3
*R
when used with higher-
frequency crystals. Refer
to manufacturer’s data.
S
can be 0 (shorted)
Functional Description
ECLK
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