mcf51jf128 Freescale Semiconductor, Inc, mcf51jf128 Datasheet - Page 701

• Manufacturer Part Number: mcf51jf128
• Description: Mcf51jf128 Reference Manual
• Manufacturer: Freescale Semiconductor, Inc

If CR1[SE]=1, the filter takes samples of COUTA on each positive transition of the
sample input. The output state of the filter changes when CR0[FILTER_CNT]
consecutive samples all agree that the output value has changed.
30.8.4.2 Latency Issues
The FPR[FILT_PER] value (or SAMPLE period) should be set such that the sampling
period is just larger than the period of the expected noise. This way a noise spike will
only corrupt one sample. The CR0[FILTER_CNT] value should 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 CR0[FILTER_CNT] power.
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.
The values of FPR[FILT_PER] (or SAMPLE period) and CR0[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 CR0[FILTER_CNT]
power.
Freescale Semiconductor, Inc.
Mode #
2A
2B
3A
3B
1
CR1[
EN]
0
1
1
1
1
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.
CR1[
WE]
Table 30-16. Comparator Sample/Filter Maximum Latencies
X
0
0
0
0
CR1[
SE]
X
0
0
1
0
MCF51JF128 Reference Manual, Rev. 2, 03/2011
CR0[FILTER
_CNT]
0x00
0x01
0x01
X
X
Table continues on the next page...
FPR[FILT_P
> 0x00
0x00
ER]
X
X
X
Preliminary
Note
Sampled, Non-Filtered mode
Continuous Mode
Operation
Disabled
Chapter 30 Comparator (CMP)
T
PD
T
Maximum Latency
PD
+ (FPR[FILT_PER] x
+ T
T
per
SAMPLE
N/A
T
) + T
PD
per
+ T
per
1
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