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Clock System Operation
The XT1 pins are shared with general-purpose I/O ports. At power up, the default operation is XT1, LF
mode of operation. However, XT1 remains disabled until the ports shared with XT1 are configured for XT1
operation. The configuration of the shared I/O is determined by the PSEL bit associated with XIN and the
XT1BYPASS bit. Setting the PSEL bit causes the XIN and XOUT ports to be configured for XT1 operation.
If XT1BYPASS is also set, XT1 is configured for bypass mode of operation, and the oscillator associated
with XT1 is powered down. In bypass mode of operation, XIN can accept an external clock input signal
and XOUT is configured as a general-purpose I/O. The PSEL bit associated with XOUT is a don't care.
If the PSEL bit associated with XIN is cleared, both XIN and XOUT ports are configured as general-
purpose I/Os, and XT1 is disabled.
XT1 is enabled under any of the following conditions:
•
XT1 is a source for ACLK (SELA = {0}) and in active mode (AM) through LPM3 (OSCOFF = 0)
•
XT1 is a source for MCLK (SELM = {0}) and in active mode (AM) (CPUOFF = 0)
•
XT1 is a source for SMCLK (SELS = {0}) and in active mode (AM) through LPM1 (SMCLKOFF = 0)
•
XT1OFF = 0. XT1 enabled in active mode (AM) through LPM4.
3.2.4 XT2 Oscillator
Some devices have a second crystal oscillator, XT2. XT2 sources XT2CLK, and its characteristics are
identical to XT1 in HF mode. The XT2DRIVE bits select the frequency range of operation of XT2. Devices
that support XT2 may or may not support XT1 in HF mode; see the device-specific data sheet for
availability.
XT2 may be used with external clock signals on the XT2IN pin by setting XT2BYPASS = 1. When used
with an external signal, the external frequency must meet the data-sheet parameters for XT2. XT2 is
powered down when used in bypass mode.
The XT2 pins are shared with general-purpose I/O ports. At power up, the default operation is XT2.
However, XT2 remains disabled until the ports shared with XT2 are configured for XT2 operation. The
configuration of the shared I/O is determined by the PSEL bit associated with XT2IN and the XT2BYPASS
bit. Setting the PSEL bit causes the XT2IN and XT2OUT ports to be configured for XT2 operation. If
XT2BYPASS is also set, XT2 is configured for bypass mode of operation, and the oscillator associated
with XT2 is powered down. In bypass mode of operation, XT2IN can accept an external clock input signal
and XT2OUT is configured as a general-purpose I/O. The PSEL bit associated with XT2OUT is a don't
care.
If the PSEL bit associated with XT2IN is cleared, both XT2IN and XT2OUT ports are configured as
general-purpose I/Os, and XT2 is disabled.
XT2 is enabled under any of the following conditions:
•
XT2 is a source for ACLK (SELA = {5, 6, 7}) and in active mode (AM) through LPM3 (OSCOFF = 0)
•
XT2 is a source for MCLK (SELM = {5, 6, 7}) and in active mode (AM) (CPUOFF = 0)
•
XT2 is a source for SMCLK (SELS = {5, 6, 7}) and in active mode (AM) through LPM1 (SMCLKOFF =
0)
•
XT2OFF = 0. XT2 enabled in active mode (AM) through LPM4.
3.2.5 Digitally Controlled Oscillator (DCO)
The DCO is an integrated digitally controlled oscillator. The DCO has three frequency settings determined
by the DCOFSEL bits. Each frequency is trimmed at the factory. The DCO can be used as a source for
ACLK, MCLK, or SMCLK. See the device-specific data sheet for DCO characteristics.
The DCO frequency can be changed at any time, but care should be taken to ensure no other system
clock frequency constraints are exceeded with the new frequency selection. Any change in the DCOFSEL
or DCORSEL bits causes the DCOCLK to be held for four clock cycles before releasing the new value into
the system. This allows for the DCO to settle properly.
74
Clock System (CS)
SLAU272C – May 2011 – Revised November 2013
Copyright © 2011–2013, Texas Instruments Incorporated