AM3874 datasheet(190/361 Pages) TI

190 / 361 page

AM3874, AM3871

SPRS695C – SEPTEMBER 2011 – REVISED DECEMBER 2013

www.ti.com

7.4.1

Device (DEV) and Auxiliary (AUX) Clock Inputs

The

device

provides

two

clock

inputs,

Device

(DEVOSC_MXI/DEV_CLKIN)

and

Auxiliary

(AUXOSC_MXI/AUX_CLKIN). The Device (DEV) clock is used to generate the majority of the internal

reference clocks, while the Auxiliary (AUX) clock can optionally be used as a source for the Audio and/or

Video PLLs.

The DEV and AUX clocks can be sourced in two ways:

1. Using an external crystal in conjunction with the internal oscillator or

2. Using an external 1.8-V LVCMOS-compatible clock input

Note: The external crystals used with the internal oscillators must operate in fundamental parallel

resonant mode only. There is no overtone support.

The DEV Clock should in most cases be 20 MHz. However, it can optionally range anywhere from 20 - 30

MHz if the following are true:

•

The DEV Clock is not used to source the SATA reference clock

•

A precise 32768-Hz clock is not needed for Real-Time Clock functionality

•

If the boot mode is FAST XIP

The AUX Clock is optional and can range from 20-30 MHz. AUX Clock can be used to source the Audio

and/or Video PLLs when a very precise audio or video frequency is required.

7.4.1.1

Using the Internal Oscillators

When the internal oscillators are used to generate the DEV and AUX clocks, external crystals are required

to be connected across the DEVOSC or AUXOSC oscillator MXI and MXO pins, along with two load

capacitors (see Figure 7-7 and Figure 7-8). The external crystal load capacitors should also be connected

to the associated oscillator ground pin (VSSA_DEVOSC or VSSA_AUXOSC). The capacitors should not

be connected to board ground (VSS).

Figure 7-7. Device Oscillator

190

Power, Reset, Clocking, and Interrupts

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