DSP56857E datasheet(27/53 Pages) FREESCALE

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External Clock Operation

56857 Technical Data, Rev. 6

Freescale Semiconductor

4.5 External Clock Operation

The 56857 system clock can be derived from a 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.

4.5.1

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 4-6. In Figure 4-6 a typical 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.

Figure 4-6 Crystal Oscillator

4.5.2

High Speed External Clock Source (> 4MHz)

The recommended method of connecting an external clock is given in Figure 4-7. The external clock

source is connected to XTAL and the EXTAL pin is held at ground, VDDA, or VDDA/2. The TOD_SEL bit

in CGM must be set to 0.

Figure 4-7 Connecting a High Speed External Clock Signal using XTAL

4.5.3

Low Speed External Clock Source (2-4MHz)

The recommended method of connecting an external clock is given in Figure 4-8. The external clock

source is connected to XTAL and the EXTAL pin is held at VDDA/2. The TOD_SEL bit in CGM must be

set to 0.

Sample External Crystal Parameters:

Rz = 10MΩ

TOD_SEL bit in CGM must be set to 0

Crystal Frequency = 2–4MHz (optimized for 4MHz)

EXTAL XTAL

56857

XTAL

EXTAL

External

GND,

VDDA,

Clock

(up to 240MHz)

or VDDA/2

DSP56857E Datasheet(PDF) 27 Page - Freescale Semiconductor, Inc