dsp56858 Freescale Semiconductor, Inc, dsp56858 Datasheet - Page 28

• Manufacturer Part Number: dsp56858
• Description: 16-bit Digital Signal Controllers
• Manufacturer: Freescale Semiconductor, Inc

Figure 4-5
4.5 External Clock Operation
The 56858 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.
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Active state, when a bus or signal is driven, and enters a low impedance state
Tri-stated, when a bus or signal is placed in a high impedance state
Data Valid state, when a signal level has reached V
Data Invalid state, when a signal level is in transition between V
Data Invalid State
Crystal Oscillator
shows the definitions of the following signal states:
Data1 Valid
Data1
Data Active
EXTAL XTAL
Crystal Frequency = 2–4MHz (optimized for 4MHz)
R
f
C
z
Figure 4-6 Crystal Oscillator
Figure 4-5 Signal States
Data2 Valid
56858 Technical Data, Rev. 6
Data2
Table
Sample External Crystal Parameters:
R
TOD_SEL bit in CGM must be set to 0
f
c
z
= 4MHz
= 10MΩ
4-6. In
OL
Figure 4-6
or V
Tri-stated
Data
OH
OL
a typical crystal oscillator circuit is
and V
OH
Data3 Valid
Data Active
Data3
Freescale Semiconductor