RC Oscillator
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
R
EXT
EXT
To Clock Selection MUX
RCCLK
From SIM
SIMOSCEN
From SIM
BUS CLOCK
CONFIG2
EN
STOP_RCLKEN
RC OSCILLATOR
MCU
OSC1
OSC2
See Chapter 22 for component value requirements.
V
REG
R
C
EXT
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 (32.768kHz)
1
Fixed capacitor, C
1
Tuning capacitor, C (can also be a fixed capacitor)
2
Feedback resistor, RB
Series resistor, RS (optional)
MC68HC908AP Family Data Sheet, Rev. 4
Freescale Semiconductor
75
FREESCALE [ Freescale ]
