OSC Control Register (HSPLLUSSXTCTL)
480
SLAU367P – October 2012 – Revised April 2020
Copyright © 2012–2020, Texas Instruments Incorporated
High-Speed PLL (HSPLL)
The control bits of the OSC are grouped in the HSPLLUSSXTCTL register, and the control bits of the PLL
are grouped in the HSPLLCTL register.
NOTE:
Naming conventions in this document for register names and bit fields:
•
HSPLL registers: RegisterName or RegisterName.BitField
•
Other module registers: ModuleNameRegisterName or
ModuleNameRegisterName.BitField
20.2 OSC Control Register (HSPLLUSSXTCTL)
As shown in
, the PLL takes the input clock from OSC block. The OSC block supports both
crystal resonators and ceramic resonators with a frequency range of 4 MHz to 8 MHz. The OSC does not
support a bypass mode, so do not attempt to feed an external clock to the USSXTIN pin. Ensure that the
oscillator is enabled and fully stable before enabling the USS module.
20.2.1 OSCEN Bit
The HSPLLUSSXTCTL.OSCEN bit enables or disables the oscillator. The output of the oscillator is gated
by default. When the oscillator is enabled, it drives the external resonator and waits for a predefined
counter value before enabling the output of the oscillator to feed a stable clock to the PLL. One of the two
predefined counter values can be selected by HSPLLUSSXTLCTL.OSCTYPE.
20.2.2 OSCTYPE Bit
The oscillator supports both crystal resonators and ceramic resonators. The resonators have different
start-up times, so the correct setting must be applied before enabling the OSC (by writing 1 to
HSPLLUSSXTCTL.OSCEN). Ceramic resonators have faster wake-up time than crystal resonators. For a
crystal resonator, TI recommends setting HSPLLUSSXTCTL.OSCTYPE to 0 (gating counter = 4096). For
a ceramic resonator, TI recommends setting HSPLLUSSXTCTL.OSCTYPE to 1 (gating counter = 512).
20.2.3 OSCSTATE Bit
The HSPLLUSSXTLCTL.OSCSTATE bit is set after the predefined cycle count has been reached after the
oscillator is enabled. This bit can be used to check whether the oscillator has started. The power-up
sequence of the USS module is fully controlled by the PSQ (see
). However, the oscillator must
be enabled and stable before powering up the USS module. The application must turn on the USSXT
oscillator (HSPLLUSSXTLCTL.OSCEN = 1) and wait until the oscillator output is available
(HSPLLUSSXTLCTL.OSCSTATE = 1) before powering up the USS module. The OSCSTATE bit indicates
sufficient signal strength, not signal quality of the oscillator output.
CAUTION
The
application
must
turn
on
the
USSXT
oscillator
(HSPLLUSSXTLCTL.OSCEN = 1) and wait for a sufficient time to let the
oscillator start (this depends on the crystal and resonator characteristics) before
powering up the USS module (see
). The USSXT oscillator is not
controlled by the PSQ. The PSQ assumes that the oscillator output is already
available (see
20.2.4 XTOUTOFF Bit
An application may be required to monitor the clock from the oscillator or to use the clock as the source of
another subsystem. To meet these requirements, the buffered output clock from the oscillator can be
enabled on the USSXT_BOUT pin by setting HSPLLUSSXTLCTL.XTOUTOFF = 0. The clock on the
USSXT_BOUT pin can be monitored or used as a clock source. Never use the USSXTOUT pin for
monitoring or for a clock source.