DCOCLK
LFXTCLK
MCLK
LFXTCLK
DCOCLK
Select
LFXTCLK
Wait for
LFXTCLK
Clock System Operation
392
SLAU356I – March 2015 – Revised June 2019
Copyright © 2015–2019, Texas Instruments Incorporated
Clock System (CS)
Figure 6-4. Switch MCLK From DCOCLK to LFXTCLK
6.2.13 Clock Status
The CSSTAT is a read-only register that can used by the application to determine what system clocks and
system resources are currently active. It is possible that a system resource could be active, yet a
corresponding system clock that has selected the resource is not. For example, the clock resource may be
REFO and ACLK has selected REFO as its source. In addition, the user has set REFO_EN = 1. If there
are no modules actively using ACLK, REFO is shown as active (REFO_ON = 1), but ACLK is shown as
inactive (ACLK_ON = 0).
In addition, there are READY bits corresponding to each clock tree. Before the change in frequency or the
source oscillator for a particular clock tree, the application must ensure that the current clock settings are
stable by polling on the READY bits. For example, if the application intends to switch the ACLK source
from REFOCLK to LFXTCLK, it must ensure that the current setting for ACLK (out of REFO) is ready by
polling the ACLK_READY in the CSSTAT register and then initiate the SELA change to LFXT. Then
ACLK_READY bit can be polled again to ascertain that ACLK is sourced out of LFXT. This is to ensure
that the clock system does not go into an nondeterministic state. If the application changes the clock
settings for ACLK from REFO to LFXT (assume ACLK_READY = 1) and then immediately changes the
SELA to use VLO while the REFO to LFXT change was ongoing (ACLK_READY = 0), then the clock
system can get into an nondeterministic state.
NOTE:
Before entering into any of the low-power modes, the user software must ensure that the
system clocks have been set to the correct frequency of operation. The application must
check the clock ready bits in the CSSTAT register. Failure to check the clocks being ready
before entering into low-power modes may result in the system being in an nondeterministic
state.
After a change in clock source or clock tree or clock frequency or divider settings,
10 CPU
bus clock cycles
are required until the correct status is captured in the CSSTAT register.
Application must account for this latency while checking the clock configuration status in the
CSSTAT register.
NOTE:
If any outstanding power-mode transition is ongoing, then the PCM locks the clock system
registers until the power-mode transition is complete. This is indicated in the PCM module
through the PMR_BUSY bit in the PCMCTL1 register.