• Setting C0[FILTER_CNT] > 0x01 and
• Setting C0[FPR] to a nonzero value or setting C0[SE]=1
If using the divided bus clock to drive the filter, it samples COUTA every C0[FPR] bus
clock cycles.
The filter output is at logic 0 when first initalized, and subsequently changes when all the
consecutive C0[FILTER_CNT] samples agree that the output value has changed. In other
words, C0[COUT] is 0 for some initial period, even when COUTA is at logic 1.
Setting all of C0[SE], C0[FPR] and C0[FILTER_CNT] to 0 disables the filter and
eliminates switching current associated with the filtering process.
Note
Always switch to this setting prior to making any changes in
filter parameters. This resets the filter to a known state.
Switching C0[FILTER_CNT] on the fly without this
intermediate step can result in unexpected behavior.
If C0[SE]=1, the filter samples COUTA on each positive transition of the sample input.
The output state of the filter changes when all the consecutive C0[FILTER_CNT]
samples agree that the output value has changed.
37.9.2.2 Latency issues
The value of C0[FPR] or SAMPLE period must be set such that the sampling period is
just longer than the period of the expected noise. This way a noise spike will corrupt only
one sample. The value of C0[FILTER_CNT] must be chosen to reduce the probability of
noisy samples causing an incorrect transition to be recognized. The probability of an
incorrect transition is defined as the probability of an incorrect sample raised to the power
of C0[FILTER_CNT].
The values of C0[FPR] or SAMPLE period and C0[FILTER_CNT] must also be traded
off against the desire for minimal latency in recognizing actual comparator output
transitions. The probability of detecting an actual output change within the nominal
latency is the probability of a correct sample raised to the power of C0[FILTER_CNT].
The following table summarizes maximum latency values for the various modes of
operation in the absence of noise. Filtering latency is restarted each time an actual output
transition is masked by noise.
CMP functional description
Kinetis KE1xZ256 Sub-Family Reference Manual, Rev. 3, 07/2018
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NXP Semiconductors
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