Clocks
117
SPNU503C – March 2018
Copyright © 2018, Texas Instruments Incorporated
Architecture
2.4.2.1
Enabling / Disabling Clock Domains
Each clock domain can be independently enabled or disabled using the set of Clock Domain Disable
registers – CDDIS, CDDISSET, and CDDISCLR.
Each bit in these registers corresponds to the clock domain number indicated in
. For example,
setting bit 1 in the CDDIS or CDDISSET registers disables the HCLK clock domain. The clock domain will
be turned off only when every module that uses the HCLK domain gives the “permission” for HCLK to be
turned off.
All clock domains are enabled by default, or upon a system reset, or whenever a wake up condition is
detected.
2.4.2.2
Mapping Clock Sources to Clock Domains
Each clock domain needs to be mapped to a valid clock source. There are control registers that allow an
application to choose the clock sources for each clock domain.
•
Selecting clock source for GCLK, HCLK and VCLKx domains
The CPU clock (GCLK), the system module clock (HCLK), and the peripheral bus clocks (VCLKx) all use
the same clock source. This clock source is selected via the GHVSRC register. The default source for the
GCLK, HCLK and VCLKx is the main oscillator. That is, after power up, the GCLK and HCLK are running
at the OSCIN frequency, while the VCLKx frequency is the OSCIN frequency divided by 2.
•
Selecting clock source for VCLKA1 domain
The clock source for VCLKA1 domain is selected via the VCLKASRC register. The default source for the
domain is VCLK.
•
Selecting clock source for VCLKA3 domain
The clock source for VCLKA3 domain is selected via the VCLKACON1 register. The default source for the
VCLKA3 domain is VCLK.
•
Selecting clock source for VCLKA4 domain
The clock source for VCLKA4 domain is selected via the VCLKACON1 register. The default source for the
VCLKA4 domain is VCLK.
•
Selecting clock source for RTICLK domain
The clock source for RTICLK domain is selected via the RCLKSRC register. The default source for the
RTICLK domain is VCLK.
NOTE:
Selecting a clock source for RTICLK that is not VCLK
When the application chooses a clock source for RTICLK domain that is not VCLK, then the
application must ensure that the resulting RTICLK frequency must be less than or equal to
VCLK frequency divided by 3. The application can configure the RTI1DIV field of the
RCLKSRC register for dividing the selected clock source frequency by 1, 2, 4 or 8 to meet
this requirement.
2.4.3 Low Power Modes
All clock domains are active in the normal operating mode. This is the default mode of operation. As
described in
and
, the application can choose to disable any particular clock
source and domain that it does not plan to use. Also, the peripheral central resource controller (PCR) has
control registers to enable / disable the peripheral clock (VCLK) for each peripheral select. This offers the
application a large number of choices for enabling / disabling clock sources, or clock domains, or clocks to
specific peripherals.
This section describes three particular low-power modes and their typical characteristics. They are not the
only low-power modes configurable by the application, as just described.