SWCU195A December 2024 – May 2025 CC2744R7-Q1 , CC2745P10-Q1 , CC2745R10-Q1 , CC2745R7-Q1 , CC2755R10
LFINC is filtered using a proportional filter with automatic gearing. The filter is enabled only when the real LFCLK is available and the tracking loop is running (to provide a precise HFOSC).
If CKMD.LFINCCTL.PREVENTSTBY is set, and the filter is not settled because the tracking is not active; the filter can continue to run until it is settled.
Configuration of "too large of an error" for increasing K:
The filter uses a "decaying error accumulator" with a decay rate configured in CKMD.LFINCCTL.ERRTHR
LARGE means "fastest decay" (actually no history at all)—Large instantaneous errors (approximately 125ns) are needed to make the error accumulator cross the threshold
MIDLARGE means "fast decay"—Subsequent large errors in the same direction make the error accumulator cross the threshold.
MIDSMALL means "slower decay"—Subsequent smaller errors in the same direction make the error accumulator cross the threshold
SMALL means "slowest decay"—Even smaller errors in the same direction required to make the error accumulator cross the threshold.
CKMD.LFINCCTL.GEARRSTRT decides when to increase K:
NEVER – used for "bad" clocks (high frequency variation), when the automatic gearing restarts, prevents settling at all
ONETHR – increase K when the threshold is crossed once
TWOTHR – increase K when the threshold is crossed twice in a row in the same direction (positive/negative)
CKMD.LFINCCTL.SOFTRSTRT decides if K can also be increased for the first update after wakup / reenabling the filter (to account for potential frequency changes during STANDBY / periods without tracking loop).
Comparison of the filter transfer function for different values of K.
The transfer function of the filter can be written as:
The filter bandwidth for different values of K is mentioned below :
Higher values of Loop Bandwidth result in higher sensitivity to variations in LFOSC frequency. To ensure the filter reduces the effects of noise from the LFOSC frequency, the gearing mechanism is designed such that it starts with a larger value of 'k' to help the filter settle faster, and eventually settles on a small value of 'k', which reduces the noise of LFOSC.