Datasheet

The external oscillator circuit is designed for use with an external crystal or ceramic
resonator to provide an accurate clock source. In its typical configuration, the oscillator is
connected in a Pierce oscillator configuration, as shown in the above figure. This figure
shows only the logical representation of the internal components and may not represent
actual circuitry. The oscillator configuration uses five components:
• Crystal or ceramic, X1
• Fixed capacitor, C1
• Tuning capacitor, C2, which can also be a fixed capacitor
• Feedback resistor, R
B
• Series resistor, R
S
(optional)
8.3.5.1 Bypass mode
In bypass mode (ICS_OSCSC[OSCEN] = 0, ICS_OSCSC[OSCOS] = 0), external clock
module is disabled. EXTAL can be used as the input of external clock source. When
external clock source is not used in this mode, the EXTAL can be used as GPIO or other
function muxed with this pinout. XTAL can be used as GPIO or other function muxed
with its pinout even EXTAL is used as external clock source. The following figure shows
the typical OSC bypass mode connection.
MCU
EXTAL
GPIO
XTAL
External Clock Input
Figure 8-5. OSC bypass mode connection
8.3.5.2 Low-power configuration
In low-power mode, when ICS_OSCSC[OSCEN] = 1, ICS_OSCSC[OSCOS] = 1, and
ICS_OSCSC[HGO] = 0, the series resistor R
S
is not used. The feedback resistor R
F
must
be carefully selected to get best performance. The figure below shows the typical OSC
low-gain mode connection.
Chapter 8 Clock management
MC9S08PA60 Reference Manual, Rev. 1, 9/2012
Freescale Semiconductor, Inc. 207