PLL Control (CTL) Register
481
SLAU367P – October 2012 – Revised April 2020
Copyright © 2012–2020, Texas Instruments Incorporated
High-Speed PLL (HSPLL)
20.3 PLL Control (CTL) Register
The PLL block takes its input from the oscillator (4 MHz to 8 MHz) and generates the output clock in the
range of 68 MHz to 80 MHz. The PLL block does not have a separate power control. It is fully controlled
by the Power Sequencer (PSQ). When the USS module is in OFF state (UUPSCTL.UPSTATE = 0) or
STANDBY state (UUPSCTL.UPSTATE = 1), the PLL block is turned off and does not consume power.
See
for details. The output of the PLL block is divided by half to keep a 50-50 duty cycle at the
final output. The maximum frequency at the final output must be limited to 80 MHz.
20.3.1 PLLM[5:0] Bits
The PLL output frequency is determined as :
•
PLL output clock frequency = input clock frequency × (PLLM +1)
•
The final output clock frequency = PLL output clock frequency / 2
The input clock to the PLL block must be 4 MHz to 8 MHz. Choose a HSPLLCTL.PLLM[5:0] value so that
the final output clock is in the range of 68 MHz to 80 MHz. Set HSPLLCTL.PLLM[5:0] to the desired value
before powering up the USS module and do not change the value while the USS module is on.
20.3.2 PLLINFREQ Bit
The PLL can be optimized for the best performance based on its input frequency. The
HSPLLCTL.PLLINFREQ bit divides the input frequency range into two categories (
≤
6 MHz and >6 MHz)
and optimizes the PLL block for the specified input range and output frequency.
20.3.3 PLL_LOCK Bit
The HSPLLCTL.PLL_LOCK bit indicates whether or not the PLL output clock is stable.
HSPLLCTL.PLL_LOCK is set to 1 when the PLL output frequency reaches the desired frequency and
becomes stable. The lock signal is also used by the PSQ during its power-up sequence. When the PLL
output is changed from locked to unlocked, the PLL unlock interrupt bit (HSPLLRIS.PLLUNLOCK) is set.
When the PLL unlock interrupt occurs, TI recommends turning off the USS module, and then checking the
USSXT oscillator to make sure it is operating correctly before enabling the USS module again.
20.3.4 USSXT Control Register
show that the PLL has one input clock source, the USSXT oscillator. The oscillator supports
both crystal resonators and ceramic resonators with the range of 4 MHz to 8 MHz. The oscillator must be
enabled and fully stable before the PLL is enabled.
20.4 Start-up Sequence of the USSXT Oscillator
The PLL block is automatically enabled and disabled by the Power Sequencer (PSQ) during the power-up
and power-down sequences of the USS module. The USSXT oscillator must be enabled and stabled
before the application enables the USS module. The application must start the USS XT oscillator and wait
until it is stable before powering up the USS module, as described in the following sequence:
1. Configure HSPLLUSSXTLCTL.OSCTYPE bit correctly based on the resonator type (0 for a crystal
resonator, 1 for a ceramic resonator).
2. Write 1 to the HSPLLUSSXTLCTL.USSXTEN bit.
3. Wait for the start-up time. The device can enter a low-power mode while waiting for a TIMER interrupt.
4. Read the HSPLLUSSXTLCTL.OSCSTATE bit to check if the USSXT started.
5. Now the USSXT is running. The USS module can be powered up.
NOTE:
All of the USS submodules must be configured properly before powering up the USS
module.