Low Pass Filter; Enabling Filter Modes - NXP Semiconductors freescale KV4 Series Reference Manual

Table of Contents

Functional description

35.4.4 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.

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.

Always switch to this setting prior to making any changes in

filter parameters. This resets the filter to a known state.

Switching CR0[FILTER_CNT] on the fly without this

intermediate step can result in unexpected behavior.

KV4x Reference Manual, Rev. 2, 02/2015

Freescale Semiconductor, Inc.