DSP56F826 datasheet(31/60 Pages) FREESCALE

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External Clock Operation

56F827 Technical Data, Rev. 12

Freescale Semiconductor

as a typical circuit board trace capacitance the parallel load capacitance presented to the crystal is 9pF as

determined by the following equation:

This is the value load capacitance that should be used when selecting a crystal and determining the actual

frequency of operation of the crystal oscillator circuit.

Figure 3-9 Connecting to a Crystal Oscillator Circuit

3.6.2

Ceramic Resonator

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 3-10. The resonator and components should be mounted as close as possible to the EXTAL and

XTAL pins. The internal 56F82x oscillator circuitry is designed to have no external load capacitors

present. As shown in Figure 3-9, no external load capacitors should be used.

Figure 3-10 Connecting a Ceramic Resonator

Note: Freescale recommends only two terminal ceramic resonators vs. three terminal resonators

(which contain an internal bypass capacitor to ground).

CL =

CL1 * CL2

CL1 + CL2

+ Cs =

+ 3 = 6 + 3 = 9pF

12 * 12

12 + 12

Recommended External Crystal

Parameters:

Rz = 1 to 3MΩ

fc = 4MHz (optimized for 4MHz)

EXTAL XTAL

Recommended Ceramic Resonator

Parameters:

Rz = 1 to 3 MΩ

fc = 4MHz (optimized for 4MHz)

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