NXP Semiconductors K22F series, Reference Manual, Page: 839 / 1407

to reflect an input change or a configuration change to one of the components
involved in the data path.
• When programmed for filtering modes, COUT will initially be equal to 0, until
sufficient clock cycles have elapsed to fill all stages of the filter. This occurs even if
COUTA is at a logic 1.
35.3.4 Low-pass filter
The low-pass filter operates on the unfiltered and unsynchronized and optionally inverted
comparator output COUTA and generates the filtered and synchronized output COUT.
Both COUTA and COUT can be configured as module outputs and are used for different
purposes within the system.
Synchronization and edge detection are always used to determine status register bit
values. They also apply to COUT for all sampling and windowed modes. Filtering can be
performed using an internal timebase defined by FPR[FILT_PER], or using an external
SAMPLE input to determine sample time.
The need for digital filtering and the amount of filtering is dependent on user
requirements. Filtering can become more useful in the absence of an external hysteresis
circuit. Without external hysteresis, high-frequency oscillations can be generated at
COUTA when the selected INM and INP input voltages differ by less than the offset
voltage of the differential comparator.
35.3.4.1 Enabling filter modes
Filter modes can be enabled by:
• Setting CR0[FILTER_CNT] > 0x01 and
• Setting FPR[FILT_PER] to a nonzero value or setting CR1[SE]=1
If using the divided bus clock to drive the filter, it will take samples of COUTA every
FPR[FILT_PER] bus clock cycles.
The filter output will be at logic 0 when first initalized, and will subsequently change
when all the consecutive CR0[FILTER_CNT] samples agree that the output value has
changed. In other words, SCR[COUT] will be 0 for some initial period, even when
COUTA is at logic 1.
Setting both CR1[SE] and FPR[FILT_PER] to 0 disables the filter and eliminates
switching current associated with the filtering process.
Chapter 35 Comparator (CMP)
K22F Sub-Family Reference Manual, Rev. 4, 08/2016
NXP Semiconductors 839