External Clock Operation
IFREN
XADR
XE
MAS1
YE=SE=OE=0
ERASE
NVSTR
Tnvs
Tnvh1
Trcv
Tme
Figure 3-6 Flash Mass Erase Cycle
3.4 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.
3.4.1
Crystal Oscillator
The internal oscillator is also designed to interface with a parallel-resonant crystal resonator in the
frequency range specified for the external crystal in Table 3-9. In Figure 3-7 a recommended crystal
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 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
56F807 Technical Data Technical Data, Rev. 16
Freescale Semiconductor
29
FREESCALE [ Freescale ]
