AM1808_1004 Datasheet, PDF (242/264 Pages) Texas Instruments – AM1808 ARM Microprocessor

AM1808
SPRS653A – FEBRUARY 2010 – REVISED APRIL 2010
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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.
+1.2V
Switch for Device
Core Power
CV_DD
Real Time Clock
RTC_CV_DD
C2
RTC_XI
XTAL
32.768
kHz
C1
RTC_XO
32K
OSC
Real
Time
Clock
(RTC)
Module
RTC_VSS
Isolated RTC
Power Domain
Figure 6-83. Clock Source
242 Peripheral Information and Electrical Specifications
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