DSP56F803 datasheet(32/57 Pages) FREESCALE

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External Clock Operation

56F803 Technical Data, Rev. 16

Freescale Semiconductor

3.5 External Clock Operation

The 56F803 system clock can be derived from an external crystal or an external system clock signal. To

generate a reference frequency using the internal oscillator, a reference crystal must be connected between

the EXTAL and XTAL pins.

3.5.1

Crystal Oscillator

The internal oscillator is also designed to interface with a parallel-resonant crystal resonator in the

frequency range specified for the external crystal in Table 3-9. In Figure 3-7 a recommended crystal

oscillator circuit is shown. Follow the crystal supplier’s recommendations when selecting a crystal,

because crystal parameters determine the component values required to provide maximum stability and

reliable start-up. The crystal and associated components should be mounted as close as possible to the

EXTAL and XTAL pins to minimize output distortion and start-up stabilization time. The internal

56F80x oscillator circuitry is designed to have no external load capacitors present. As shown in

Figure 3-8 no external load capacitors should be used.

The 56F80x components internally are modeled as a parallel resonant oscillator circuit to provide a

capacitive load on each of the oscillator pins (XTAL and EXTAL) of 10pF to 13pF over temperature and

process variations. Using a typical value of internal capacitance on these pins of 12pF and a value of 3pF

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 3-7 Connecting to a Crystal Oscillator

CL =

CL1 * CL2

CL1 + CL2

+ Cs =

+ 3 = 6 + 3 = 9pF

12 * 12

12 + 12

Recommended External Crystal

Parameters:

Rz = 1 to 3 MΩ

fc = 8MHz (optimized for 8MHz)

EXTAL XTAL

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