R
OSCIN
OSCOUT
Oscillator
521
SPNU563A – March 2018
Copyright © 2018, Texas Instruments Incorporated
Oscillator and PLL
14.3.1 Oscillator Implementation
The oscillator operates at 3.3V and uses a constant current source to drive current onto the OSCOUT
node. An internal transistor shunts the current (and current from the external circuitry) to GND. This
current steering drives the voltage waveform on OSCOUT.
Figure 14-3. Oscillator Implementation
14.3.2 Oscillator Enable
The oscillator is enabled asynchronously when nPORRST is low.
The oscillator is enabled by clearing bit 0 in the Clock Source Disable Register (CSDIS) or setting bit 0 in
the Clock Source Disable Clear Register (CSDISCLR) of the System and Peripheral Control Registers.
The bit sends a start signal to the oscillator. Bit 0 of CSDIS is cleared to 0 by default on a system or
power-on reset so that the oscillator starts-up by default. After the oscillator swings at a high-enough
amplitude to pass an input clock into the core domain and nPORRST is released, 1024 oscillator periods
are counted before setting the CLKSR0V bit in the Clock Source Valid Status Register (CSVSTAT) of the
System and Peripheral Control Registers. The oscillator generates clock source 0 in the global clock
module (GCM).
14.3.3 Oscillator 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 bits
allow
the clock source 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 CLKSR0V bit in the Clock Source Valid Status Register
(CSVSTAT), of the System and Peripheral Control Registers, is cleared after clock disable is asserted
(which occurs after all clock domains are stopped).
The oscillator disable signal places the oscillator into a low-power state, disconnects the feedback (bias)
resistor between OSCIN and OSCOUT, and OSCIN is grounded.