Datasheet

2.2.3 Oscillator (XTAL, EXTAL)
The XTAL and EXTAL pins are used to provide the connections for the on-chip
oscillator. The oscillator (XOSC) in this MCU is a Pierce oscillator that can
accommodate a crystal or ceramic resonator. Optionally, an external clock source can be
connected to the EXTAL input pin. The oscillator can be configured to run in stop3
mode.
Refer to the following figure, R
S
(when used) and R
F
must be low-inductance resistors
such as carbon composition resistors. Wire-wound resistors, and some metal film
resistors, have too much inductance. C1 and C2 normally must be high-quality ceramic
capacitors that are specifically designed for high-frequency applications.
MCU
EXTAL
XTAL
R
s
R
F
C1
C2
X1
Figure 2-7. Typical crystal or resonator circuit
R
F
is used to provide a bias path to keep the EXTAL input in its linear range during
crystal startup; its value is not generally critical. Typical systems use 1 M to 10 M.
Higher values are sensitive to humidity and lower values reduce gain and (in extreme
cases) could prevent startup.
C1 and C2 are typically in the 5 pF to 25 pF range and are chosen to match the
requirements of a specific crystal or resonator. Take into account printed circuit board
(PCB) capacitance and MCU pin capacitance when selecting C1 and C2. The crystal
manufacturer typically specifies a load capacitance, which is the series combination of
C1 and C2 (which are usually the same size). As a first-order approximation, use 10 pF as
an estimate of combined pin and PCB capacitance for each oscillator pin (EXTAL and
XTAL).
Chapter 2 Pins and connections
MC9S08PA60 Reference Manual, Rev. 1, 9/2012
Freescale Semiconductor, Inc. 45