AM1808 datasheet(73/262 Pages) TI

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OSCOUT

OSCIN

OSCVSS

ClockInput

toPLL

AM1808

www.ti.com

SPRS653C

– FEBRUARY 2010 – REVISED DECEMBER 2011

5.5

Crystal Oscillator or External Clock Input

The device includes two choices to provide an external clock input, which is fed to the on-chip PLLs to

generate high-frequency system clocks. These options are illustrated in Figure 5-6 and Figure 5-7. For

input clock frequencies between 12 and 20 MHz, a crystal with 80 ohm max ESR is recommended. For

input clock frequencies between 20 and 30 MHz, a crystal with 60 ohm max ESR is recommended.

Typical load capacitance values are 10-20 pF, where the load capacitance is the series combination of C1

and C2.

The CLKMODE bit in the PLLCTL register must be 0 to use the on-chip oscillator. If CLKMODE is set to 1,

the internal oscillator is disabled.

Figure 5-6 illustrates the option that uses on-chip 1.2V oscillator with external crystal circuit. Figure 5-7

illustrates the option that uses an external 1.2V clock input.

Figure 5-6. On-Chip Oscillator

Table 5-2. Oscillator Timing Requirements

PARAMETER

MIN

MAX

UNIT

fosc

Oscillator frequency range (OSCIN/OSCOUT)

MHz

Peripheral Information and Electrical Specifications

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