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Texas Instruments CC2541 User Manual

Texas Instruments CC2541
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Oscillators and Clocks
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NOTE: The change from the 16-MHz clock source to the 32-MHz clock source (and vice versa)
aligns with the CLKCONCMD.TICKSPD setting. A slow CLKCONCMD.TICKSPD setting
when CLKCONCMD.OSC is changed results in a longer time before the actual source
change takes effect. The fastest switching is obtained when CLKCONCMD.TICKSPD equals
000.
NOTE: After coming up from PM1, PM2, or PM3, the CPU must wait for CLKCONSTA.OSC to be 0
before operations requiring the system to run on the 32-MHz XOSC (such as the radio) are
started.
4.4.3 32 kHz Oscillators
Two 32 kHz oscillators are present in the device as clock sources for the 32-kHz clock:
32-kHz XOSC
32-kHz RCOSC
By default, after a reset, the 32-kHz RCOSC is enabled and selected as the 32-kHz clock source. The
RCOSC consumes less power, but is less accurate compared to the 32-kHz XOSC. The chosen 32-kHz
clock source drives the Sleep Timer, generates the tick for the Watchdog Timer, and is used as a strobe in
Timer 2 to calculate the Sleep Timer sleep time. The CLKCONCMD.OSC32K register bit selects the
oscillator to be used as the 32-kHz clock source.
The CLKCONCMD.OSC32K register bit can be written at any time, but does not take effect before the
16-MHz RCOSC is the active system clock source. When system clock is changed from the 16-MHz
RCOSC to the 32-MHz XOSC (CLKCONCMD.OSC from 1 to 0), calibration of the 32-kHz RCOSC starts up
and is performed once if the 32-kHz RCOSC is selected. During calibration, a divided version of the
32-MHz XOSC is used. The result of the calibration is that the 32-kHz RSOSC is running at 32.753 kHz.
The 32-kHz RCOSC calibration may take up to 2 ms to complete. Calibration can be disabled by setting
SLEEPCMD.OSC32K_CALDIS to 1. At the end of the calibration, an extra pulse may occur on the 32-kHz
clock source, which causes the sleep timer to be incremented by 1.
Note that after having switched to the 32-kHz XOSC and when coming up from PM3 with the 32-kHz
XOSC enabled, the oscillator requires up to 500 ms to stabilize on the correct frequency. The Sleep
Timer, Watchdog Timer and clock-loss detector should not be used before the 32-kHz XOSC is stable.
4.4.4 Oscillator and Clock Registers
This section describes the oscillator and clock registers. All register bits retain their previous values when
entering PM2 or PM3.
70
Power Management and Clocks SWRU191C April 2009 Revised January 2012
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Copyright © 20092012, Texas Instruments Incorporated

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Texas Instruments CC2541 Specifications

General IconGeneral
BrandTexas Instruments
ModelCC2541
CategoryMicrocontrollers
LanguageEnglish

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