Belkin XM Commander Especificaciones descargar pdf (Pagina 71)

REF: BB_SRM_xM BeagleBoard-xM System

Reference Manual

Revision A2

Page 71 of 164

8.12.1 32KHz Clock

The 32KHz clock is needed for the TPS65950 and the processoe and is provided by the

TPS65950 via the external 32KHz crystal, Y2. The TPS65950 has a separate output from

the crystal to drive the processor that buffers the resulting 32-kHz signal and provides it

as 32KCLKOUT, which is provided to the processor on ball AE25. The default mode of

the 32KCLKOUT signal is active, but it can be disabled if desired under SW control.

The 32.768-kHz clock drives the RTC embedded in the TPS65950. The RTC is not

enabled by default; the host processor must set the correct date and time to enable the

RTC.

8.12.2 26MHz Clock

This section describes the 26MHz clock section of the BeagleBoard.

8.12.2.1 26MHz Source

The BeagleBoard is designed to support two suppliers of the 26MHz oscillator. The

26MHz clock is provided by an onboard oscillator, Y1. The TPS65950 receives the

external HFCLKIN signal on ball A14 and uses it to synchronize or generate the clocks

required to operate the TPS65950 subsystems. The TPS65950 must have this clock in

order to function to the point where it can power up the BeagleBoard. This is the reason

the 26MHz clock is routed through the TPS65950.

8.12.2.2 TPS65950 Setup

When the TPS65950 enters an active state, the Processor must immediately indicate the

HFCLKIN frequency (26 MHz) by setting the HFCLK_FREQ bit field (bits [1:0]) in the

CFG_BOOT register of the TPS65950. HFCLK_FREQ has a default of being not

programmed, and in that condition, the USB subsection does not work. The three DCDC

switching supplies (VIO, VDD1, and VDD2) operate from their free-running 3-MHz

(RC) oscillators, and the PWR registers are accessed at a default 1.5-M byte.

HFCLK_FREQ must be set by the processor during the initial power-up sequence. On the

BeagleBoard, this is done by the internal boot ROM on startup.

8.12.2.3 Processor 26MHz

The 26MHz clock for the processor is provided by the TPS65950 on ball R12 through

R38, a 33 ohm resistor is providing to minimize any reflections on the clock line. The

clock signal enters via ball AE17 on the PROCESSOR.

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Belkin XM Commander Especificaciones Pagina 71