Freescale Semiconductor 56F8122 - Manual (Page 26)

56F8322 Techncial Data, Rev. 10.0
26 Freescale Semiconductor
Preliminary
Part 3 On-Chip Clock Synthesis (OCCS)
3.1 Introduction
Refer to the OCCS chapter of the 56F8300 Peripheral User Manual for a full description of the OCCS.
The material contained here identifies the specific features of the OCCS design.
3.2 External Clock Operation
The system clock can be derived from an external crystal, ceramic resonator or an external system clock
signal. To generate a reference frequency using the internal oscillator, a reference crystal or ceramic
resonator must be connected between the EXTAL and XTAL pins.
3.2.1 Crystal Oscillator
The internal oscillator is designed to interface with a parallel-resonant crystal resonator in the frequency
range specified for the external crystal in
Table 10-15 . A recommended crystal oscillator circuit is shown
in Figure 3-1 . Follow the crystal supplier’s recommendations when selecting a crystal, since crystal
parameters determine the component values required to provide maximum stability and reliable start-up.
The crystal and associated components should be mounted as near as possible to the EXTAL and XTAL
pins to minimize output distortion and start-up stabilization time.
Figure 3-1 Connecting to a Crystal Oscillator
Note: The OCCS_COHL bit should be set to 1 when a crystal oscillator is used. The reset condition on the
OCCS_COHL bit is 0. Please see the COHL bit in the Oscillator Control (OSCTL) register, discussed
in the 56F8300 Peripheral User Manual.
Sample External Crystal Parameters:
R
z
= 750 K
Ω
Note: If the operating temperature range is limited to
below 85
o C (105 o C junction), then R
z
= 10 Meg
Ω
CLKMODE = 0
EXTAL XTAL
R
z
CL1 CL2
Crystal Frequency = 4 - 8MHz (optimized for 8MHz)
EXTAL XTAL
R
z