Texas Instruments MSP430x4xx - Manual (Page 115)

FLL+ Clock Module Operation
4-6 FLL+ Clock Module
4.2.2 LFXT1 Oscillator
The LFXT1 oscillator supports ultralow-current consumption using a
32,768-Hz watch crystal in LF mode (XTS_FLL = 0). A watch crystal connects
to XIN and XOUT without any external components.
The LFXT1 oscillator supports high-speed crystals or resonators when in HF
mode (XTS_FLL = 1). The high-speed crystal or resonator connects to XIN
and XOUT.
LFXT1 may be used with an external clock signal on the XIN pin when
XTS_FLL = 1. The input frequency range is ~1 Hz − 8 MHz. When the input
frequency is below 450 KHz, the XT1OF bit may be set preventing the CPU
from being clocked from the external frequency.
The software-selectable XCAPxPF bits configure the internally provided load
capacitance for the LFXT1 crystal. The internal pin capacitance plus the
parasitic 2 pF pin capacitance combine serially to form the load capacitance.
The load capacitance can be selected as 1, 6, 8, or 10 pF. Additional external
capacitors can be added if necessary.
Software can disable LFXT1 by setting OSCOFF if this signal does not source
MCLK (SELM
≠
3 or CPUOFF = 1 ).
Note: LFXT1 Oscillator Characteristics
Low-frequency crystals often require hundreds of milliseconds to start up,
depending on the crystal.
Ultralow-power oscillators such as the LFXT1 in LF mode should be guarded
from noise coupling from other sources. The crystal should be placed as
close as possible to the MSP430 with the crystal housing grounded and the
crystal traces guarded with ground traces.
The default value of XCAPxPF is 0, providing a crystal load capacitance of
~1 pF. Reliable crystal operation may not be achieved unless the crystal is
provided with the proper load capacitance-either by selection of XCAPxPF
values or by external capacitors.
4.2.3 XT2 Oscillator
Some devices have a second crystal oscillator, XT2. XT2 sources XT2CLK
and its characteristics are identical to LFXT1 in HF mode, except XT2 does not
have internal load capacitors. The required load capacitance for the high
frequency crystal or resonator must be provided externally.
The XT2OFF bit disables the XT2 oscillator if XT2CLK is unused for MCLK
(SELMx
≠
2 or CPUOFF = 1) and SMCLK (SELS = 0 or SMCLKOFF = 1).
XT2 may be used with external clock signals on the XT2IN pin. When used with
an external signal, the external frequency must meet the datasheet
parameters for XT2.