Texas Instruments MSP430x1xx - Manual (Page 115)

Features for Low-Power Applications
7-15
Basic Clock Module
7.4.3 Basic Clock Features for Low-Power Applications
Conflicting requirements typically exist in battery powered MSP430x1xx
applications:
- Low clock frequency for energy conservation and time keeping
- High clock frequency for fast reaction to events and fast burst processing
capability
The Basic Clock Module addresses the above conflicting requirements by
allowing the design engineer to select from the three available clock signals:
ACLK, MCLK and SMCLK. For optimal low-power performance, the ACLK can
be configured to oscillate with a low 32,786-Hz watch-crystal frequency,
providing a stable time base for the system and low power stand-by operation.
The MCLK can be configured to operate from the on-chip DCO which is only
activated when requested by events. The SMCLK can be configured to
operate from either the watch-crystal or the DCO, depending on peripheral
requirements. A flexible clock distribution and divider system is provided to
fine tune the individual clock requirements. All basic clock-module
configurations are under full software control.
7.4.4 Selecting a Crystal Clock for MCLK
After power up, the basic clock module uses the DCO clock for the system
clock MCLK. The LFXT1 oscillator starts in the low-frequency mode (XTS=0).
Regardless of the configuration of the clock system the application uses, if all
initial conditions are set, the software execution is ensured by the integrated
DCO. Finally, the application may use a crystal for further software execution.
Figure 7–14. Select Crystal Oscillator for MCLK, Example Uses LFXT1 for MCLK
MCLK
Main System Clock
SELM
2
DIVM
2
CPUOff
MCLKGEN
LFXT1CLK
/1, /2, /4, /8, off
DCOCLk
XT2CLk
3
0,1
2
The sequence to switch the MCLK source from the DCO clock to the crystal
clock (LFXT1CLK or XT2CLK) should be:
1) Switch on the crystal oscillator
a) OscOff = 0, XTS is reset (LF mode is selected) or set (HF mode is
selected)
BIS.B #XTS,&BCSCTL1 ; Set XTS bit to select LFXT1
; oscillator
BIC #OSCOFF,SR ; Turn on LFXT1 oscillator