dsp56857 Freescale Semiconductor, Inc, dsp56857 Datasheet - Page 27

• Manufacturer Part Number: dsp56857
• Description: Dsp56857 Digital Signal Controller
• Manufacturer: Freescale Semiconductor, Inc

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
The internal oscillator is designed to interface with a parallel-resonant crystal resonator in the frequency
range specified for the external crystal in
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.
4.5.2
The recommended method of connecting an external clock is given in
source is connected to XTAL and the EXTAL pin is held at ground, V
in CGM must be set to 0.
4.5.3
The recommended method of connecting an external clock is given in
source is connected to XTAL and the EXTAL pin is held at V
set to 0.
Freescale Semiconductor
Figure 4-7 Connecting a High Speed External Clock Signal using XTAL
Crystal Oscillator
High Speed External Clock Source (> 4MHz)
Low Speed External Clock Source (2-4MHz)
EXTAL XTAL
Crystal Frequency = 2–4MHz (optimized for 4MHz)
R
Figure 4-6 Crystal Oscillator
z
(up to 240MHz)
56857 Technical Data, Rev. 6
External
Table
Clock
XTAL
Sample External Crystal Parameters:
R
TOD_SEL bit in CGM must be set to 0
z
= 10MΩ
56857
4-6. In
EXTAL
GND,
or V
Figure 4-6
V
DDA
DDA
DDA
/2
,
/2. The TOD_SEL bit in CGM must be
a typical crystal oscillator circuit is
DDA
Figure
Figure
, or V
DDA
4-7. The external clock
4-8. The external clock
/2. The TOD_SEL bit
External Clock Operation
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