
Clock System Operation
The function of the ACLKREQEN, MCLKREQEN, and SMCLKREQEN bits depends on which power mode
is selected; that is, they do not have an effect across all power modes. For example, ACLKREQEN
enables or disables ACLK requests. It is only effective in LPM4, because in all other modes (AM, LPM0,
LPM1, LPM2, LPM3), ACLK is always active. SMCLKREQEN enables or disables SMCLK requests.
When SMCLKOFF = 0 and in AM, LPM0, or LPM1, it is a don't care because SMCLK is always on in
these cases. For SMCLKOFF = 0 and in LPM2, LPM3, and LPM4, SMCLKREQEN can enable or disable
SMCLK requests, because in these modes, SMCLK is normally off. When SMCLKOFF = 1,
SMCLKREQEN can enable or disable SMCLK requests, because under this condition SMCLK is normally
off in all power modes. This is summarized in
Table 3-1. System Clocks vs Power Modes and Clock Requests
System Clocks
SMCLK
MCLK
ACLK
SMCLKOFF = 0
SMCLKOFF = 1
MCLKREQEN =
MCLKREQEN =
ACLKREQEN = 0
ACLKREQEN = 1
SMCLKREQEN =
SMCLKREQEN =
SMCLKREQEN =
SMCLKREQEN =
Mode
0 and clock
1 and clock
and clock
and clock
0 and clock
1 and clock
0 and clock
1 and clock
requested
requested
requested
requested
requested
requested
requested
requested
AM
Active
Active
Active
Active
Active
Active
Disabled
Active
LPM0
Disabled
Active
Active
Active
Active
Active
Disabled
Active
LPM1
Disabled
Active
Active
Active
Active
Active
Disabled
Active
LPM2
Disabled
Active
Active
Active
Disabled
Active
Disabled
Active
LPM3
Disabled
Active
Active
Active
Disabled
Active
Disabled
Active
LPM4
Disabled
Active
Disabled
Active
Disabled
Active
Disabled
Active
LPM3.5
Disabled
Disabled
Disabled
(1)
Disabled
Disabled
Disabled
Disabled
Disabled
LPM4.5
Disabled
Disabled
Disabled
Disabled
Disabled
Disabled
Disabled
Disabled
(1)
LFXTCLK is available directly as the clock source to the RTC module.
3.2.7 CS Module Fail-Safe Operation
The CS module incorporates an oscillator-fault fail-safe feature. This feature detects an oscillator fault for
XT1 and XT2 as shown in
. The available fault conditions are:
•
Low-frequency oscillator fault (XT1OFFG) for XT1 in LF mode
•
High-frequency oscillator fault (XT1OFFG) for XT1 in HF mode
•
High-frequency oscillator fault (XT2OFFG) for XT2
•
External clock signal faults for all bypass modes; that is, XT1BYPASS = 1 or XT2BYPASS = 1
The crystal oscillator fault bits XT1OFFG and XT2OFFG are set if the corresponding crystal oscillator is
turned on and not operating properly. Once set, the fault bits remain set until reset in software, even if the
fault condition no longer exists. If the user clears the fault bits and the fault condition still exists, the fault
bits are automatically set again, otherwise they remain cleared.
The OFIFG oscillator-fault interrupt flag is set and latched at POR or when any oscillator fault (XT1OFFG
or XT2OFFG) is detected. When OFIFG is set and OFIE is set, the OFIFG requests a user NMI. When the
interrupt is granted, the OFIE is not reset automatically as it is in previous MSP430 families. It is no longer
required to reset the OFIE. NMI entry and exit circuitry removes this requirement. The OFIFG flag must be
cleared by software. The source of the fault can be identified by checking the individual fault bits.
If XT1 in LF mode is sourcing any system clock (ACLK, MCLK, or SMCLK), and a fault is detected, the
system clock is automatically switched to the VLO for its clock source (VLOCLK). Similarly, if XT1 in HF
mode is sourcing any system clock and a fault is detected, the system clock is automatically switched to
MODOSC for its clock source (MODCLK). When XT2 (if available) is sourcing any system clock and a
fault is detected, the system clock is automatically switched to MODOSC for its clock source (MODCLK).
The fail-safe logic does not change the respective SELA, SELM, and SELS bit settings. The fail-safe
mechanism behaves the same in normal and bypass modes.
76
Clock System (CS)
SLAU272C – May 2011 – Revised November 2013
Copyright © 2011–2013, Texas Instruments Incorporated