56F8335 datasheet(33/160 Pages) FREESCALE

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External Clock Operation

56F8335 Technical Data, Rev. 1

Freescale Semiconductor

Preliminary

Figure 3-2 Connecting to a Crystal Oscillator

Note:

The OCCS_COHL bit must 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’s Manual.

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 3-3.

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 3-3 Connecting a Ceramic Resonator

Note:

The OCCS_COHL bit must be set to 0 when a ceramic 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’s Manual.

3.2.3

External Clock Source

The recommended method of connecting an external clock is illustrated in Figure 3-4. The external clock

source is connected to XTAL and the EXTAL pin is grounded. Set OCCS_COHL bit high when using an

external clock source as well.

Sample External Crystal Parameters:

Rz = 750 KΩ

Note: If the operating temperature range is limited to

below 85oC (105oC junction), then Rz = 10 Meg Ω

CLKMODE = 0

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

EXTAL

XTAL

CL1

CL2

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

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