Datasheet

RC Oscillator
MC68HC908AP Family Data Sheet, Rev. 4
Freescale Semiconductor 75
5.4 RC Oscillator
The RC oscillator circuit is designed for use with an external resistor and a capacitor.
In its typical configuration, the RC oscillator requires two external components, one R and one C.
Component values should have a tolerance of 1% or less, to obtain a clock source with less than 10%
tolerance. The oscillator configuration uses two components:
•C
EXT
•R
EXT
Figure 5-5. RC Oscillator
5.5 X-tal Oscillator
The crystal (x-tal) oscillator circuit is designed for use with an external 32.768kHz crystal to provide an
accurate clock source.
In its typical configuration, the x-tal oscillator is connected in a Pierce oscillator configuration, as shown
in Figure 5-6. This figure shows only the logical representation of the internal components and may not
represent actual circuitry. The oscillator configuration uses five components:
• Crystal, X
1
(32.768kHz)
• Fixed capacitor, C
1
• Tuning capacitor, C
2
(can also be a fixed capacitor)
• Feedback resistor, R
B
• Series resistor, R
S
(optional)
R
EXT
C
EXT
V
REG
MCU
OSC1
SIMOSCEN
From SIM
STOP_RCLKEN
CONFIG2
RC OSCILLATOR
EN
See Chapter 22 for component value requirements.
BUS CLOCK
From SIM
OSC2
RCCLK
To Clock Selection MUX