CC2430 Datasheet, PDF (69/212 Pages) Texas Instruments – A True System-on-Chip solution for 2.4 GHz IEEE 802.15.4 / ZigBee-TM

CC2430
Peripherals : Power Management and clocks
• 32 MHz crystal oscillator.
• 16 MHz RC oscillator.
The 32 MHz crystal oscillator startup time may
be too long for some applications, therefore
the device can run on the 16 MHz RC
oscillator until crystal oscillator is stable. The
16 MHz RC oscillator consumes less power
than the crystal oscillator, but since it is not as
accurate as the crystal oscillator it can not be
used for RF transceiver operation.
Two low frequency oscillators are present in
the device:
• 32 kHz crystal oscillator
• 32 kHz RC oscillator
The 32 kHz crystal oscillator is designed to
operate at 32.768 kHz and provide a stable
clock signal for systems requiring time
accuracy. The 32 kHz RC oscillator run at
32.753 kHz when calibrated. The calibration
can only take place when 32 MHz crystal
oscillator is enabled, and this calibration can
be disabled by enabling the
SLEEP.OSC32K_CALDIS bit. The 32 kHz RC
oscillator should be used to reduce cost and
power consumption compared to the 32 kHz
crystal oscillator solution. The two low
frequency oscillators can not be operated
simultaneously.
13.1.4.2
System clock
The system clock is derived from the selected
system clock source, which is the 32 MHz
crystal oscillator or the 16 MHz RC oscillator.
The CLKCON.OSC bit selects the source of the
system clock. Note that to use the RF
transceiver the 32 MHz crystal oscillator must
be selected and stable.
Note that changing the CLKCON.OSC bit does
not happen instantaneously. This is caused by
the requirement to have stable clocks prior to
actually changing the clock source. Also note
that CLKCON.CLKSPD bit reflect the frequency
of the system clock and thus is a mirror of the
CLKCON.OSC bit.
When the SLEEP.XOSC_STB is 1, the 32 MHz
crystal oscillator is reported stable by the
system. This may however not be the case
and a safety time of additional 64 µs should be
used prior to selecting 32 MHz clock as source
for the system clock. Failure to do so may lead
13.1.4.3 32 kHz oscillators
Two 32 kHz oscillators are present in the
device as clock sources for the 32 kHz clock:
• 32.768 kHz crystal oscillator
• 32 kHz RC oscillator
By default, after a reset, the 32 kHz RC
oscillator is enabled and selected as the 32
kHz clock source. The RC oscillator consumes
less power, but is less accurate than the
32.768 kHz crystal oscillator. Refer to Table 9
and Table 10 on page 15 for characteristics of
these oscillators. The 32 kHz clock runs the
Sleep Timer and Watchdog Timer and used as
a strobe in Timer2 (MAC timer) for when to
calculate Sleep Timer sleep time. Selecting
which oscillator source to use as source for
the 32 kHz is performed with the
CLKCON.OSC32K register bit.
to system crash. E.g. a loop of CPU NOP
instructions should be used to suspend further
system operation prior to selecting XOSC as
clock source.
The oscillator not selected as the system clock
source, will be set in power-down mode by
setting SLEEP.OSC_PD to 1 (the default state).
Thus the 16MHz RC oscillator may be turned
off when the 32 MHz crystal oscillator has
been selected as system clock source and
vice versa. When SLEEP.OSC_PD is 0, both
oscillators are powered up and running.
When the 32 MHz crystal oscillator is selected
as system clock source and the 16 MHz RC
oscillator is also powered up, the 16 MHz RC
oscillator will be continuously calibrated to
ensure clock stability over supply voltage and
operating temperature. This calibration is not
performed when the 16 MHz RC oscillator
itself is chosen as system clock source.
The CLKCON.OSC32K register bit must only be
changed while using the 16 MHz RC oscillator
as the system clock source. When the 32 MHz
crystal oscillator is selected and it is stable, i.e.
SLEEP.XOSC_STB is 1, calibration of the 32
kHz RC oscillator is continuously performed
and 32kHz clock is derived from 32 MHz clock.
This calibration is not performed in other
power modes than PM0. The result of the
calibration is a RC clock running at 32.753
kHz.
The 32 kHz RC oscillator calibration may take
up to 2 ms to complete. When entering low
power modes PM1 or PM2 an ongoing
calibration must be completed before the low
power mode is entered. In some applications
this extra delay may be unacceptable and
CC2430 Data Sheet (rev. 2.1) SWRS036F
Page 69 of 211