Oscillators And Clocks; Oscillators; System Clock - Texas Instruments CC253x User Manual

Oscillators and Clocks

4.4 Oscillators and Clocks
The device has one internal system clock, or main clock. The source for the system clock can be either
the 16-MHz RC oscillator or the 32-MHz crystal oscillator. Clock control is performed using the
CLKCONCMD SFR register.
There is also one 32-kHz clock source that can either be an RC oscillator or a crystal oscillator, also
controlled by the CLKCONCMD register.
The CLKCONSTA register is a read-only register used for getting the current clock status.
The choice of oscillator allows a trade-off between high accuracy in the case of the crystal oscillator and
low power consumption when the RC oscillator is used. Note that operation of the RF transceiver requires
that the 32-MHz crystal oscillator is used.
In the CC2533, CC2540 and CC2541, an additional module for detection of 32-MHz XOSC stability is
available. This amplitude detector can be useful in environments with significant noise on the power
supply, to ensure that the clock source is not used until the clock signal is stable. In the CC2533, this
module can be enabled by setting the SRCRC.XOSC_AMP_DET_EN bit, and this adds around 20 μs to the
32-MHz XOSC startup time. In the CC2540 and CC2541, the module is always enabled.

4.4.1 Oscillators

Figure 4-1 gives an overview of the clock system with available clock sources.
Two high-frequency oscillators are present in the device:
• 32-MHz crystal oscillator
• 16-MHz RC oscillator
The 32-MHz crystal-oscillator start-up time may be too long for some applications; therefore, the device
can run on the 16-MHz RC oscillator until the crystal oscillator is stable. The 16-MHz RC oscillator
consumes less power than the crystal oscillator, but because it is not as accurate as the crystal oscillator it
cannot 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 XOSC is designed to operate at 32.768 kHz and provide a stable clock signal for systems
requiring time accuracy. The 32-kHz RCOSC runs at 32.753 kHz when calibrated. The calibration can only
take place when the 32-MHz XOSC is enabled, and this calibration can be disabled by enabling the
SLEEPCMD.OSC32K_CALDIS bit. The 32-kHz RCOSC should be used to reduce cost and power
consumption compared to the 32-kHz XOSC solution. The two 32-kHz oscillators cannot be operated
simultaneously.

4.4.2 System Clock

The system clock is derived from the selected system clock source, which is the 32-MHz XOSC or the 16-
MHz RCOSC. The CLKCONCMD.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 CLKCONCMD.OSC bit does not cause the system clock to change instantly. The
clock source change first takes effect when CLKCONSTA.OSC = CLKCONCMD.OSC. This is due to the
requirement to have stable clocks prior to actually changing the clock source. Also note that the
CLKCONCMD.CLKSPD bit reflects the frequency of the system clock and thus is a mirror of the
CLKCONCMD.OSC bit.
The 16 MHz RC oscillator is calibrated once after the 32-MHz XOSC has been selected and is stable, that
is, when the CLKCONSTA.OSC bit switches from 1 to 0.
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Power Management and Clocks
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SWRU191F – April 2009 – Revised April 2014
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