CC2430 datasheet(69/212 Pages) TAOS

69 / 212 page

CC2430

Peripherals : Power Management and clocks

CC2430 Data Sheet (rev. 2.1) SWRS036F

Page 69 of 211

•

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

disabled

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

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.

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

kHz

performed

with

the

CLKCON.OSC32K

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 Datasheet(PDF) 69 Page - TEXAS ADVANCED OPTOELECTRONIC SOLUTIONS

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