Frequency Flexibility
119
SPRUH82C – April 2013 – Revised September 2016
Copyright © 2013–2016, Texas Instruments Incorporated
Device Clocking
6.2
Frequency Flexibility
There are two PLLs on the device with similar architecture and behavior. Each PLL has two clocking
modes:
•
PLL Bypass
•
PLL Active
When the PLL is in Bypass mode, the reference clock supplied on OSCIN serves as the clock source from
which all of the system clocks (SYSCLK1 to SYSCLK7) are derived. This means that when the PLL is in
Bypass mode, the reference clock supplied on OSCIN passes directly to the system of PLLDIV blocks that
creates each of the system clocks. For PLL0 only, the EXTCLKSRC bit in PLLCTL can be configured to
use PLL1_SYSCLK3 as the Bypass mode reference clock.
When the PLL operates in Active mode, the PLL is enabled and the PLL multiplier setting is used to
multiply the input clock frequency supplied on the OSCIN pin up to the desired frequency. It is this
multiplied frequency that all system clocks are derived from in PLL Active mode.
The output of the PLL multiplier passes through a post divider (POSTDIV) block and then is applied to the
system of PLLDIV blocks that creates each of the system clock domains (SYSCLK1 to SYSCLK7). Each
SYSCLK
n
has a PLLDIV
n
block associated with it. See the
Phase-Locked Loop Controller (PLLC)
chapter
for more details on the PLL.
The combination of the PLL multiplier, POSTDIV, and PLLDIV blocks provides flexibility in the frequencies
that the system clock domains support. This flexibility does have limitations, as follows:
•
OSCIN input frequency is limited to a supported range.
•
The output of the PLL Multiplier must be within the range specified in the device-specific data manual.
•
The output of each PLLDIV block must be less than or equal to the maximum device frequency
specified in the device-specific data manual.
NOTE:
The above limitations are provided here as an example and are used to illustrate the
recommended configuration of the PLL controller. These limitations may vary based on core
voltage and between devices. See the device-specific data manual for more details.
shows examples of possible PLL multiplier settings, along with the available PLL post-divider
modes. The PLL post-divider modes are defined by the value programmed in the RATIO field of the PLL
post-divider control register (POSTDIV). For Div1, Div2, Div3, and Div4 modes, the RATIO field would be
programmed to 0, 1, 2, and 3, respectively. The Div1, Div2, Div3, and Div4 modes are shown here as an
example. Additional post-divider modes are supported and are documented in the
Phase-Locked Loop
Controller (PLLC)
chapter.
NOTE:
PLL power consumption increases as the output frequency of the PLL multiplier increases.
To decrease PLL power consumption, the lowest PLL multiplier (PLLM) setting should be
chosen that achieves the desired frequency. For example, if 200 MHz is the desired CPU
operating frequency and the OSCIN frequency is 25 MHz; lower power consumption is
achieved by choosing a PLLM setting of ×16 and a post-divider (POSTDIV) setting of /2
instead of a PLLM setting of ×24 and a POSTDIV setting of /3, even though both of these
modes would result in a CPU frequency of 200 MHz.
