Campbell Scientific CR3000 Micrologger - Manual (Page 325)

Section 8. Operation
325
Table 74. Frequency Resolution Comparison
0.5 s Scan 5.0 s Scan
PulseCount(), POption = 1 FR = 2 Hz FR = 0.2 Hz
TimerIO(), Function = 2 FR = 0.0011 Hz FR = 0.00011 Hz
Q — When more than one pulse is in a scan interval, what does TimerIO() return
when configured to return a frequency? Does it average the measured periods and
compute the frequency from that (f = 1/T)? For example:
Scan (50,mSec,10,0)
TimerIO (WindSpd(),11111111,00022000,60,Sec)
A — In the background, a 32-bit timer counter is saved each time the signal
transitions as programmed (rising or falling). This counter is running at a fixed
high frequency. A count is also incremented for each transition. When the
TimerIO() instruction executes, it uses the difference of time between the edge
prior to the last execution and the edge prior to this execution as the time
difference. The number of transitions that occur between these two times divided
by the time difference gives the calculated frequency. For multiple edges
occurring between execution intervals, this calculation does assume that the
frequency is not varying over the execution interval. The calculation returns the
average regardless of how the signal is changing.
8.1.5.4 Pulse Measurement Problems
8.1.5.4.1 Pay Attention to Specifications
The table Example of Differing Specifications for Pulse Input Channels
(p. 325)
compares specifications for pulse-input channels to emphasize the need for
matching the proper device to application. Take time to understand signals to be
measured and compatible channels.
Table 75. Example of Differing Specifications for Pulse-Input
Channels
Pulse Channels
P1, P2
Digital I/O Channels
C1, C2, C3, C4,
C5, C6, C7, C8
High Frequency Max 250 kHz 400 kHz
Max Input Voltage 20 Vdc 16 Vdc
State Transition
Thresholds
Count upon transition from
<0.9 to >2.2 Vdc
Count upon transition from
<1.2 to >3.8 Vdc
8.1.5.4.2 Input Filters and Signal Attenuation
Pulse-input channels are equipped with input filters to reduce spurious noise that
can cause false counts. The higher the time constant (τ) of the filter, the tighter
the filter. Table Time Constants
(p. 326)
lists τ values for pulse-input channels. So,
while TimerIO() frequency measurement may be superior for clean signals, a
pulse channel filter (much higher τ) may be required to get a measurement on a
dirty signal.