MC68HC908EY16 Datasheet, PDF (83/278 Pages) Motorola, Inc – Microcontrollers

Functional Description
8.3.3.1 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
CONFIG. 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 is set. ECGEN is controlled by the clock enable
circuit (see 8.3.1 Clock Enable Circuit) and indicates that the external clock function is desired. When
enabled, the amplifier will be connected between the PTC4/OSC1 and PTC3/OSC2 pins. Otherwise, the
PTC3/OSC2 pin reverts to its port function.
ECGEN
EXTXTALEN
EXTSLOW
EXTERNAL
CLOCK
GENERATOR
INTERNAL TO MCU
EXTERNAL
INPUT PATH
AMPLIFIER
OSC1
PTC4
R_B
OSC2
PTC3
ECLK
NAME
NAME
NAME
NAME
CONFIGURATION 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 8-4. External Clock Generator Block Diagram
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 8-4 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.)
MC68HC908EY16 • MC68HC908EY8 Data Sheet, Rev. 10
Freescale Semiconductor
83