Datasheet

56F8035/56F8025 Data Sheet, Rev. 6
34 Freescale Semiconductor
The 56F8000 family devices’ on-chip clock synthesis module has the following registers:
• Control Register (OCCS_CTRL)
• Divide-by Register (OCCS_DIVBY)
• Status Register (OCCS_STAT)
• Shutdown Register (OCCS_SHUTDN)
• Oscillator Control Register (OCCS_OCTRL)
For more information on these registers, please refer to the 56F802x and 56F803x Peripheral Reference
Manual.
3.4 Internal Clock Source
An internal relaxation oscillator can supply the reference frequency when an external frequency source or
crystal is not used. It is optimized for accuracy and programmability while providing several power-saving
configurations which accommodate different operating conditions. The internal relaxation oscillator has
very little temperature and voltage variability. To optimize power, the architecture supports a standby state
and a power-down state.
During a boot or reset sequence, the relaxation oscillator is enabled by default (the PRECS bit in the
PLLCR word is set to 0). Application code can then also switch to the external clock source and power
down the internal oscillator, if desired. If a changeover between internal and external clock sources is
required at power-on, the user must ensure that the clock source is not switched until the desired external
clock source is enabled and stable.
To compensate for variances in the device manufacturing process, the accuracy of the relaxation oscillator
can be incrementally adjusted to within + 0.078% of 8MHz by trimming an internal capacitor. Bits 0-9 of
the OSCTL (oscillator control) register allow the user to set in an additional offset (trim) to this preset
value to increase or decrease capacitance. Each unit added or subtracted changes the output frequency by
about 0.078% of 8MHz, allowing incremental adjustment until the desired frequency accuracy is achieved.
The center frequency of the internal oscillator is calibrated at the factory to 8MHz and the TRIM value is
stored in the Flash information block and loaded to the FMOPT1 register at reset. When using the
relaxation oscillator, the boot code should read the FMOPT1 register and set this value as OSCTL TRIM.
For further information, see the 56F802x and 56F803x Peripheral Reference Manual.
3.5 Crystal Oscillator
The internal crystal oscillator circuit is designed to interface with a parallel-resonant crystal resonator in a
frequency range of 4-8MHz, specified for the external crystal. Figure 3-1 shows a typical crystal oscillator
circuit. 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 load
capacitance values used in the oscillator circuit design should include all stray layout capacitances. The
crystal and associated components should be mounted as near as possible to the EXTAL and XTAL pins
to minimize output distortion and start-up stabilization time.