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EX1200-1538: Counter/Timer Operation
29
Jitter
Jitter
T= ?
F
IGURE
3-15:
E
FFECT OF
J
ITTER ON
F
REQUENCY
M
EASUREMENTS
In Figure 3-15, the logic pattern has a long rise time and sharp fall time. When the signal is read
by the analog/digital counter input channels, the jitter creates uncertainty when trying to identify
the point at which the logical high begins. To reduce the effect of jitter, the signal can be inverted,
as shown in Figure 3-16, to utilize the sharp fall time of the signal as the rise time on which the
measurement is based.
Jitter
Jitter
T (Stable)
F
IGURE
3-16:
S
IGNAL
I
NVERSION TO
D
ECREASE THE
E
FFECT OF
J
ITTER
RPM Measurement
The
RPM
(revolutions per minute) function measures the rotational velocity of a toothed-wheel
sensor. This function measures the tooth-to-tooth period (similar to
Frequency
mode) and
converts it into units of revolutions per minute (RPM). In-line with frequency measurements,
RPM measurements use averaging methods to determine the returned value.
The RPM measurement algorithm compensates for toothed-wheels that have a missing or extra
tooth to mark their index position. Without this feature, RPM measurement would have bumps or
sags. When configuring the
Tooth Count
parameter, the nominal number of teeth should be used.
For example, a wheel nominally has twelve teeth, but has a missing tooth (A) or an extra tooth (B)
to provide an index position as shown in Figure 3-17. In this case, the
Tooth Count
parameter
should be set to“12” and
RpmToothState
should be set to
CounterRpmToothStateExtra
or
CounterRpmToothStateMissing
and the measurement will be compensated accordingly.