MC68HC908RK2 Datasheet, PDF (64/158 Pages) Motorola, Inc – Microcontroller Unit

Internal Clock Generator Module (ICG)
6.3.3 External Clock Generator
The ICG also provides for an external oscillator or clock source, if desired. The external clock generator,
shown in Figure 6-4, 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 6-4. External Clock Generator Block Diagram
6.3.3.1 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
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.
The amplifier is enabled when the ECGON bit is set and stop mode is not enabled. When the amplifier is
enabled, it will be connected between the OSC1 and OSC2 pins. 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, R_B
5. Series resistor, R_S (included in the diagram 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.
MC68HC908RK2 Data Sheet, Rev. 5.1
64
Freescale Semiconductor