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Introduction
367
SPNU503C – March 2018
Copyright © 2018, Texas Instruments Incorporated
Oscillator and PLL
10.1 Introduction
This chapter provides an overview of the oscillator and PLL clock source paths for the device.
The oscillator macro will pass a signal driven into the OSCIN pin to clock source 0 that is the device
default clock source on reset. When a crystal or resonator with appropriate load circuitry is connected to
OSCIN and OSCOUT, the oscillator macro drives the crystal/resonator to generate the input waveform. In
addition to being directly usable as clock source 0, the oscillator clock is the input to the PLL.
The oscillator frequency is continuously monitored by a dedicated clock detect circuit. If the frequency falls
out of a fixed range, the clock detect switches the clock from the oscillator to an internally generated, free-
running frequency (generated by the low-power oscillator (LPO)).
The phase lock loop (PLL), a circuit in the microcontroller, is used to multiply the input frequency to some
higher (device operation) frequency. This frequency synthesis is useful for generating higher frequencies
than can be conveniently achieved with an external crystal or resonator. Additionally, the PLL allows the
flexibility to be able to synthesize one of multiple frequency options from a given crystal or resonator.
Frequency modulation can be superimposed on the synthesized frequency. The modulation provides a
means to reduce the impact of electromagnetic radiation from the device; this reduction in measured
radiation can be useful in sensitive applications.
10.1.1 Features
The main features of the source clock path are:
•
The oscillator may drive a crystal/resonator or be driven from an external source
•
The clock detect provides continuous monitoring of the oscillator frequency and provides an automatic
switch over to a free-running clock in case of oscillator failure.
•
The FM-PLL module can be operated in either modulation or non-modulation mode.
•
The phase-frequency detector assures lock to the fundamental reference frequency.
•
(1)
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Configurable prescale divider (NR) for the input clock
–
Configurable multiplier (NF)
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Configurable postscale dividers (OD, R)
•
The PLL may be used with modulation enabled.
–
Configurable modulation frequency (NS)
–
Configurable modulation depth (NV)
•
The slip control circuitry provides flexible response to a PLL failure (slip) including reset or automatic
switch over to oscillator.