DSP56F807 Motorola Inc, DSP56F807 Datasheet - Page 25

• Manufacturer Part Number: DSP56F807
• Description: 56F807 16-bit Hybrid Processor
• Manufacturer: Motorola Inc

3.5 External Clock Operation
The 56F807 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
The internal oscillator is also designed to interface with a parallel-resonant crystal resonator in the
frequency range specified for the external crystal in
oscillator circuit is shown. Follow the crystal supplier’s recommendations when selecting a crystal, since
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
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.
3.5.2
It is also possible to drive the internal oscillator with a ceramic resonator, assuming the overall system
design can tolerate the reduced signal integrity. In
Refer to supplier’s recommendations when selecting a ceramic resonator and associated components. The
resonator and components should be mounted as close as possible to the EXTAL and XTAL pins. The
internal 56F80x oscillator circuitry is designed to have no external load capacitors present. As shown in
Figure 9
56F807 Technical Data
no external load capacitors should be used.
Crystal Oscillator
Ceramic Resonator
CL =
Figure 9. Connecting to a Crystal Oscillator
CL1 * CL2
CL1 + CL2
Freescale Semiconductor, Inc.
EXTAL XTAL
For More Information On This Product,
R
f
c
z
Go to: www.freescale.com
+ Cs =
Recommended External Crystal
Parameters:
R
f
c
z
= 8MHz (optimized for 8MHz)
= 1 to 3 M
Figure
12 * 12
12 + 12
Table
10, a typical ceramic resonator circuit is shown.
+ 3 = 6 + 3 = 9pF
28. In
Figure 9
Figure 10
External Clock Operation
a recommended crystal
no external load
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