Start-up Sequence of the USSXT Oscillator
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SLAU367P – October 2012 – Revised April 2020
Copyright © 2012–2020, Texas Instruments Incorporated
High-Speed PLL (HSPLL)
20.4.1 USSXT Start-up Behavior
The USSXT oscillator is designed to generate an even running output clock in frequency and amplitude
while still allowing fast start-up. Therefore, the crystal or resonator is given more degrees of freedom
during start-up. Some crystals and resonators use the distributed capacitance and inductance of the PCB
traces and package as tank circuits and tend to oscillate on its harmonic frequencies during power up.
Increasing the value of the serial resistance between USSXTOUT and crystal or resonator prevents such
oscillation conditions at those higher frequencies. The maximum time for crystals and resonators is give in
the data sheet. USSXT_BOUT can be used to monitor USSXT start-up and operation. Use USSXT_BOUT
as system clock (for example, through HFXTIN) only after USSXT is completely started and has settled.
20.5 Interrupts
The HSPLL module supports one interrupt:
•
PLLUNLOCK interrupt
: When the PLL output status changes from locked to unlocked,
HSPLLRIS.PLLUNLOCK is set to 1.