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:
CL1 * CL2
CL1 + CL2
12 * 12
12 + 12
CL =
+ Cs =
+ 3 = 6 + 3 = 9pF
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.
Recommended External Crystal
EXTAL XTAL
Parameters:
Rz = 1 to 3 MΩ
Rz
fc = 8MHz (optimized for 8MHz)
fc
Figure 3-7 External Crystal Oscillator Circuit
3.5.2
Ceramic Resonator
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 Figure 3-8, a typical ceramic resonator circuit is
shown. 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 3-7 no external load capacitors should be used.
Recommended Ceramic Resonator
Parameters:
Rz = 1 to 3 MΩ
EXTAL XTAL
Rz
fc = 8MHz (optimized for 8MHz)
fc
Figure 3-8 Connecting a Ceramic Resonator
Note: Freescale recommends only two terminal ceramic resonators vs. three terminal resonators
(which contain an internal bypass capacitor to ground).
56F801 Technical Data, Rev. 17
24
Freescale Semiconductor
FREESCALE [ Freescale ]
