Low Power Oscillator and Clock Detect (LPOCLKDET)
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SPNU563A – March 2018
Copyright © 2018, Texas Instruments Incorporated
Oscillator and PLL
14.4 Low Power Oscillator and Clock Detect (LPOCLKDET)
The Low Power Oscillator (LPO) is comprised of two oscillators -- HF LPO and LF LPO -- in a single
macro. The low power oscillator and clock detect (LPOCLKDET) uses a relaxation oscillator to generate
an internal clock whose frequency is NOT tightly controlled. This frequency is used to monitor the
oscillator input frequency and is also available as an independent clock source in the GCM.
The LPO produces two frequencies:
•
High-frequency low-power oscillator (HF LPO) with a nominal frequency of 9.6MHz and a range from
5.5MHz to 19.5MHz; the HF LPO generates clock source 5 in the GCM.
•
Low-frequency low-power oscillator (LF LPO) with a nominal frequency of 85kHz; the LF LPO
generates clock source 4 in the GCM.
A single current source drives current onto a capacitor; when the voltage on the capacitor exceeds some
threshold, the clock toggles. The LPO uses a single current source and the two different comparators to
generate the HF LPO and LF LPO frequencies. The LPO is controlled by 4 different bit fields --
CSDIS.(5:4), HFTRIM(4:0), LFTRIM(4:0), and BIASEN.
•
CSDIS.5 enables/disables the comparator that generates HF LPO.
•
CSDIS.4 enables/disables the comparator that generates LF LPO.
•
The HF TRIM and LF TRIM bit fields vary the current into the comparator to independently trim the HF
LPO and LF LPO frequencies.
•
BIAS ENABLE (LPOMONCTL.24) enables/disables the current source which drives the LPO.
14.4.1 Clock Detect
The LPO HF clock frequency is typically near 9.6MHz, but ranges from 5.5MHz to 19.5MHz. The clock
detect establishes a window for the oscillator by:
OSCIN > HF LPO
min
/ 4
OSCIN / 4 < HF LPO
max
OSCIN > 5.5[MHz] / 4 = 1.375[MHz]
OSCIN < 4 × 19.5 = 78[MHz]
The clock detect circuit works by checking for a rising edge on one clock (oscillator or HF LPO) between
rising edges of the other clock. The result is that in addition to flagging incorrect, repeating frequencies,
the circuit also fails due to transient conditions.
The low end of the clock detect window ignores a transient low phase of at least 12 HF LPO cycles.
NOTE:
Clock Detection of Oscillator MUST be Disabled Before Disabling HF LPO
The HF LPO frequency is the comparison frequency for the oscillator. The clock detection
must be disabled prior to disabling the HF LPO frequency.
If the clock detection is NOT disabled prior to disabling the HF LPO, the clock detect circuitry
will fail the oscillator as too fast (compared to the non-existent HF LPO). The clock detect
circuitry will switch to the non-existent clock, leaving the device without a valid clock.
14.4.2 Behavior on Oscillator Failure
If the oscillator frequency fails, the clock detects supplies:
•
the HF LPO clock to GCM clock source 0 instead of the oscillator
•
the HF LPO clock to GCM clock source 1 instead of the PLL
The HF LPO signal will be available as three different clock sources:
•
GCM clock source 0 (replacing the oscillator)
•
GCM clock source 1 (replacing the PLL)
•
GCM clock source 5 as HF LPO
