Information

During operation, the propagation delay of the selected data paths must always be
considered. It may take many bus clock cycles for COUT and SCR[CFR]/SCR[CFF] 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.
32.8.4 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.
32.8.4.1 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.
CMP functional description
K20 Sub-Family Reference Manual, Rev. 2, Feb 2012
656 Freescale Semiconductor, Inc.