RTIDIV
f
f
RTISRC
VCLK
´
³
3
PLL
529
SPNU563A – March 2018
Copyright © 2018, Texas Instruments Incorporated
Oscillator and PLL
14.5.2.2 PLL Disable
The clock sources (for example, OSC, PLL) are disabled by setting the appropriate bit in the Clock Source
Disable Register (CSDIS) or setting the appropriate bit in the Clock Source Disable Set Register
(CSDISSET) of the System and Peripheral Control Registers. These bit allow the clock to disable but do
not force the behavior until the clock is no longer used as the source for a clock domain (for example,
GCLK1, VCLK, VCLK2, RTICLK1).
The PLL receives a signal to disable after the clock is no longer used by any clock domain. Within the
PLL, the clock is disabled and the appropriate CLKSRnV bit for the PLL in the Clock Source Valid Status
Register (CSVSTAT), of the System and Peripheral Control Registers, becomes inactive. Then the PLL is
placed into a low power state after the following length of time: T
Enable
= 150 × T
OSCIN
14.5.2.3 OD-Divider Change
The PLL gates the clock if the ODPLL bit-field is changed while the PLL is active. The output clock from
the PLL is gated for 3 or 12 OSCIN clock cycles. As the post-ODCLK is gated in the low phase, the output
clock to the device -- PLL CLK -- may be gated in a high or low phase though the transition is always
glitchless: T
ODPLL
= 3 × T
OSCIN
NOTE:
ODPLL change should occur prior to enabling asynchronous clock domains
Since changing the ODPLL bit-field causes the PLL CLK to be gated, these changes to
ODPLL should be completed before configuring a clock domain for an asynchronous clock
source. Some clock domains (RTICLK1, VCLK2) require a frequency relationship to the
VCLK.
If the PLL is clocking VCLK and it is stopped for some cycles, then the frequency relationship
is temporarily violated.
Many asynchronous domains require frequency relationships between VCLK and the
asynchronous domain. Therefore, if the PLL clock is the source for GCLK1, HCLK, and
VCLK, then the gating produces a short-term change in the PLL clock frequency (and hence
also the VCLK frequency). As such, this frequency change could violate the requirements for
an asynchronous clock domain.
14.5.2.4 Changing the PLL Operating Point While the PLL is Active
Once the valid bit (CLKSRnV bit in the Clock Source Valid Status Register (CSVSTAT) of the System and
Peripheral Control Registers) is set, software may change values to the PLL. If the change of values
results in a small percentage change to the VCO frequency (
∆
f
OutputCLK
< 0.1 × f
OutputCLK
), then these
changes can be done on-the-fly. In this mode, the values are updated into the PLL synchronously, and the
PLL re-locks to the new value without gating the clocks or the slip bits. If the operating point change is too
large, then the slip bits will be set.
Conversely, if the changes to the VCO frequency are large, then the PLL should be disabled prior to
changing the values. Typically, any change to the REFCLKDIV field or large changes to the PLLMUL field
in the PLL Control Register 1 (PLLCTL1) of the System and Peripheral Control Registers requires a
complete disable-and-relock strategy.