4. Functions
CNT32-4MT(LPCI)
61
Counter error
Counter errors are classified into two types: digital filter error and abnormal input error.
Digital filter error
A digital filter error occurs when a signal faster than the digital filter setting is input to phase A or
phase B. Notification of the error is via a status (latch/clear), interrupt, or external output
(one-shot pulse).
The card monitors the input signal level for digital filter errors using the filter source clock which has a
period of 1/2 the setting value. The digital filter error is detected when the same level cannot be detected
for two or more consecutive times.
However, a filter error does not occur if the frequency of the input clock is equal to or close to an integer
multiple of the filer source clock.
The following are possible causes for a digital filter error.
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When a signal faster than the digital filter setting is input
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Noise is generated.
0
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100nsec
Even when the input signal is less than 200 nsec,
no filter error occurs as long as the input signal frequency is
an integer multiple of the source clock frequency.
When the signal is less than 200 nsec, a filter error is reported.
When the signal is not less than 200 nsec,
counting is performed normally.
Filter source clock
Phase-A/Phase-B
input signal
50nsec
Phase-A/Phase-B
input signal
Phase-A/Phase-B
input signal
Figure 4.16. Filter error (Set to 0.2
μ
sec)
