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DCOCLK
ACLK
MCLK
ACLK
DCOCLK
Select
ACLK
Wait for
ACLK
Module Oscillator (MODOSC)
NOTE:
The XT1 startup includes a counter that ensures that 4096 valid clock cycles have passed
before XT1_LF_OscFault and XT1_HF_OscFault signals are cleared. A valid cycle is any
cycle that meets the frequency requirement (f
Fault,LF
or f
Fault,HF
) as outlined in the device-specific
data sheet. Any crystal fault restarts the counter. It is recommended that the counter always
be enabled; however, the counter can be disabled by clearing ENSTFCNT1. Similarly, XT2
startup includes a counter. It can be disabled by clearing ENSTFCNT2. The disabling of the
counters is valid for bypass and normal modes of operation.
3.2.8 Synchronization of Clock Signals
When switching ACLK, MCLK, or SMCLK from one clock source to the another, the switch is
synchronized to avoid critical race conditions as shown in
•
The current clock cycle continues until the next rising edge.
•
The clock remains high until the next rising edge of the new clock.
•
The new clock source is selected and continues with a full high period.
Figure 3-4. Switch MCLK from DCOCLK to XT1CLK
3.3
Module Oscillator (MODOSC)
The CS module also supports an internal oscillator, MODOSC, that is used by the power management
module and, optionally, by other modules in the system. It is also used as a fail-safe clock source as
described in
. The MODOSC sources MODCLK.
3.3.1 MODOSC Operation
To conserve power, MODOSC is powered down when not needed and enabled only when required. When
the MODOSC source is required, the respective module requests it. MODOSC is enabled based on
unconditional and conditional requests. Setting MODOSCREQEN enables conditional requests.
Unconditional requests are always enabled. It is not necessary to set MODOSCREQEN for modules that
use unconditional requests; for example, PMM, ADC, and fail-safe.
The ADC10_A may optionally use MODOSC as a clock source for its conversion clock. The user chooses
the ADC10OSC as the conversion clock source. During a conversion, the ADC10_A module issues an
unconditional request for the ADC10OSC clock source. Upon doing so, the MODOSC source is enabled, if
not already enabled from other modules' previous requests.
78
Clock System (CS)
SLAU272C – May 2011 – Revised November 2013
Copyright © 2011–2013, Texas Instruments Incorporated