ACLK_REQ
MCLK_REQ
SMCLK_REQ
Watch Dog Timer Module
CS
Module n−1
Module n
WDTACLKON
WDTSMCLKON
ACLK_REQ
MCLK_REQ
SMCLK_REQ
ACLK_REQ
MCLK_REQ
SMCLK_REQ
ACLK
MCLK
SMCLK
Direct clock request
in Watchdog mode
Module n−2
ACLK_REQ
MCLK_REQ
SMCLK_REQ
0
0
0
0
Clock System Operation
3.2.6 Operation From Low-Power Modes, Requested by Peripheral Modules
A peripheral module automatically requests its clock source from the CS module if that clock is required
for the peripheral's operation, regardless of the power mode of operation that is selected, as shown in
Figure 3-2. Module Request Clock System
A peripheral module asserts one of three possible clock request signals based on its control bits:
ACLK_REQ, MCLK_REQ, or SMCLK_REQ. These request signals are based on the configuration and
clock selection of the respective module. For example, if a timer selects ACLK as its clock source and the
timer is enabled, the timer generates an ACLK_REQ signal to the CS system. The CS, in turn, enables
ACLK regardless of the power mode settings.
Any clock request from a peripheral module causes its respective clock off signal to be overridden, but
does not change the setting of clock off control bit. For example, a peripheral module may require ACLK
that is currently disabled by the OSCOFF bit (OSCOFF = 1). The module can request ACLK by generating
an ACLK_REQ. This causes the OSCOFF bit to have no effect, thereby allowing ACLK to be available to
the requesting peripheral module. The OSCOFF bit remains at its current setting (OSCOFF = 1).
If the requested source is not active, the software NMI handler must manage the required actions. For the
previous example, if ACLK was sourced by XT1, and XT1 was not enabled, an oscillator fault condition
occurs and the software must handle the event. The watchdog, due to its security requirement, actively
selects the VLOCLK source if the originally selected clock source is not available.
Due to the clock request feature, care must be taken in the application when entering low-power modes to
save power. Although the device enters the selected low-power mode, a clock request causes more
current consumption than the specified values in the data sheet. By default, the clock request feature is
enabled. The feature can be disabled for each system clock by clearing ACLKREQEN, MCLKREQEN, or
SMCLKREQEN for the respective clocks. This does not disable fail-safe clock requests; for example,
those of the watchdog timer or the clock system itself.
75
SLAU272C – May 2011 – Revised November 2013
Clock System (CS)
Copyright © 2011–2013, Texas Instruments Incorporated