56F8322 datasheet(29/136 Pages) FREESCALE

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External Clock Operation

56F8322 Technical Data, Rev. 16

Freescale Semiconductor

Preliminary

3.2.2

Ceramic Resonator (Default)

It is also possible to drive the internal oscillator with a ceramic resonator, assuming the overall system

design can tolerate the reduced signal integrity. A typical ceramic resonator circuit is shown in Figure 3-2.

Refer to the supplier’s recommendations when selecting a ceramic resonator and associated components.

The resonator and components should be mounted as near as possible to the EXTAL and XTAL pins.

Figure 3-2 Connecting a Ceramic Resonator

Note:

The OCCS_COHL bit must be set to 0 when a crystal resonator 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.

3.2.3

External Clock Source

The recommended method of connecting an external clock is illustrated in Figure 3-3. The external clock

source is connected to XTAL and the EXTAL pin is grounded.

Figure 3-3 Connecting an External Clock Register

EXTAL XTAL

Sample External Ceramic Resonator Parameters:

Rz = 750 KΩ

EXTAL XTAL

CL1

CL2

Resonator Frequency = 4 - 8MHz (optimized for 8MHz)

3 Terminal

2 Terminal

CLKMODE = 0

XTAL

EXTAL

External

VSS

Clock

Note: when using an external clocking

source with this configuration, the

CLKMODE and COHL bits

of the OSCTL register should be set to 1.

or GPIO

56F8322 Datasheet(PDF) 29 Page - Freescale Semiconductor, Inc