Datasheet

MC56F825x/MC56F824x Digital Signal Controller, Rev. 3
General System Control Information
Freescale Semiconductor38
when selecting a crystal, because crystal parameters determine the component values required to provide maximum stability
and reliable startup. 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 startup stabilization time. When using low-frequency, low-power mode, the only external component is the
crystal itself. In the other oscillator modes, load capacitors (C
x
, C
y
) and feedback resistor (R
F
) are required. In addition, a series
resistor (R
S
) may be used in high-gain modes. Recommended component values appear in Table 27.
Figure 9. Typical Crystal Oscillator Circuit without Frequency Compensation Capacitor
Figure 10. Typical Crystal or Ceramic Resonator Circuit
5.4.3 Alternate External Clock Input
The recommended method of connecting an external clock appears in Figure 11. The external clock source is connected to the
CLKIN pin while:
• both the EXT_SEL bit and the CLK_MODE bit in the OSCTL register are set, and
• corresponding bits in the GPIOB_PER register in the GPIO module and the GPS_C0 bit in the GPS0 register in the
system integration module (SIM) are set to the correct values.
The external clock input must be generated using a relatively low-impedance driver with a maximum frequency not greater than
120 MHz.
EXTALXTAL
MC56F825x/MC56F824x
Crystal Frequency = 4–16 MHz
OSC_DIV2 = 1 if 16 MHz is chosen
R
F
EXTALXTAL
C
1
C
2
MC56F825x/MC56F824x
Crystal Frequency = 4–16 MHz
OSC_DIV2 = 1 if 16 MHz is chosen