AM1808AZCED3 datasheet(242/264 Pages) TI

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XTAL

32.768

kHz

RTC_XI

RTC_XO

RTC_VSS

32K

OSC

Real

Time

Clock

(RTC)

Module

Isolated RTC

Power Domain

Switch for Device

Core Power

Real Time Clock

+1.2V

CVDD

RTC_CVDD

AM1808

SPRS653A – FEBRUARY 2010 – REVISED APRIL 2010

www.ti.com

6.31.1 Clock Source

The clock reference for the RTC is an external 32.768-kHz crystal or an external clock source of the same

frequency. The RTC also has a separate power supply that is isolated from the rest of the system. When

the CPU and other peripherals are without power, the RTC can remain powered to preserve the current

time and calendar information. Even if the RTC is not used, it must remain powered when the rest of the

device is powered.

The source for the RTC reference clock may be provided by a crystal or by an external clock source. The

RTC has an internal oscillator buffer to support direct operation with a crystal. The crystal is connected

between pins RTC_XI and RTC_XO. RTC_XI is the input to the on-chip oscillator and RTC_XO is the

output from the oscillator back to the crystal.

An external 32.768-kHz clock source may be used instead of a crystal. In such a case, the clock source is

connected to RTC_XI, and RTC_XO is left unconnected.

If the RTC is not used, the RTC_XI pin should be held either low or high, RTC_XO should be left

unconnected, RTC_CVDD should be connected to the device CVDD and RTC_VSS should remain

grounded.

Figure 6-83. Clock Source

242

Peripheral Information and Electrical Specifications

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