Datasheet

Basic Clock Module+ Operation www.ti.com
5.2.1 Basic Clock Module+ Features for Low-Power Applications
Conflicting requirements typically exist in battery-powered applications:
• Low clock frequency for energy conservation and time keeping
• High clock frequency for fast reaction to events and fast burst processing capability
• Clock stability over operating temperature and supply voltage
The basic clock module+ addresses the above conflicting requirements by allowing the user to select from
the three available clock signals: ACLK, MCLK, and SMCLK. For optimal low-power performance, ACLK
can be sourced from a low-power 32768-Hz watch crystal (if available), providing a stable time base for
the system and low-power standby operation, or from the internal low-frequency oscillator when crystal-
accurate time keeping is not required. The MCLK can be configured to operate from the on-chip DCO that
can be activated when requested by interrupt-driven events. The SMCLK can be configured to operate
from a crystal or the DCO, depending on peripheral requirements. A flexible clock distribution and divider
system is provided to fine tune the individual clock requirements.
5.2.2 Internal Very-Low-Power Low-Frequency Oscillator (VLO)
The internal very-low-power low-frequency oscillator (VLO) provides a typical frequency of 12 kHz (see
device-specific data sheet for parameters) without requiring a crystal. VLOCLK source is selected by
setting LFXT1Sx = 10 when XTS = 0. The OSCOFF bit disables the VLO for LPM4. The LFXT1 crystal
oscillators are disabled when the VLO is selected reducing current consumption. The VLO consumes no
power when not being used.
Devices without LFXT1 (for example, the MSP430G22x0) should be configured to use the VLO as ACLK.
5.2.3 LFXT1 Oscillator
The LFXT1 oscillator is not implemented in the MSP430G22x0 device family.
The LFXT1 oscillator supports ultra-low current consumption using a 32768-Hz watch crystal in LF mode
(XTS = 0). A watch crystal connects to XIN and XOUT without any other external components. The
software-selectable XCAPx bits configure the internally provided load capacitance for the LFXT1 crystal in
LF mode. This capacitance can be selected as 1 pF, 6 pF, 10 pF, or 12.5 pF typical. Additional external
capacitors can be added if necessary.
The LFXT1 oscillator also supports high-speed crystals or resonators when in HF mode (XTS = 1,
XCAPx = 00). The high-speed crystal or resonator connects to XIN and XOUT and requires external
capacitors on both terminals. These capacitors should be sized according to the crystal or resonator
specifications. When LFXT1 is in HF mode, the LFXT1Sx bits select the range of operation.
LFXT1 may be used with an external clock signal on the XIN pin in either LF or HF mode when
LFXT1Sx = 11, OSCOFF = 0, and XCAPx = 00. When used with an external signal, the external
frequency must meet the data sheet parameters for the chosen mode. When the input frequency is below
the specified lower limit, the LFXT1OF bit may be set preventing the CPU from being clocked with
LFXT1CLK.
Software can disable LFXT1 by setting OSCOFF, if LFXT1CLK does not source SMCLK or MCLK, as
shown in Figure 5-3.
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Basic Clock Module+ SLAU144J–December 2004–Revised July 2013
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