Campbell Scientific CR3000 Micrologger - Manual (Page 322)

Section 8. Operation
322
Edge Counting (C1 - C8)
Rising edges (transitions from <1.5 Vdc to >3.5 Vdc) or falling edges (transitions
from >3.5 Vdc to <1.5 Vdc) of a square-wave signal can be counted.
Switch Closure (C1 - C8)
Two schemes are available for connecting switch-closure sensors to the CR3000.
If a switch is to close directly to ground, an external pull-up resistor is should be
used as shown in figure Using a Pull-up Resistor on Digital I/O Channels C1 - C8
(p. 323).
Alternatively, if the switch is to close ground through a digital I/O port,
connect the sensor to the CR3000 as diagrammed in figure Connecting Switch
Closures to Digital I/O
(p. 323).
Mechanical switch closures have a tendency to bounce before solidly closing.
Bouncing can cause multiple counts. The CR3000 incorporates software switch
debounce in switch-closure mode for channels C1 – C8.
Note Maximum switch-closure measurement frequency of C1 – C8 is 150 kHz.
8.1.5.2.2 Low-Frequency Mode
Low-frequency mode enables edge timing and measurement of period (not period
averaging) and frequency. For information on period averaging, see Period
Averaging
(p. 327).
Edge Timing (C1 - C8)
Time between pulse edges can be measured. Results can be expressed in terms of
microseconds or Hertz. To read more concerning edge timing, refer to CRBasic
Editor Help for the TimerIO() instruction. Edge-timing resolution is
approximately 540 ns136 ns.
Edge Timing (C1 - C8)
Open collector (bipolar transistors) or open drain (MOSFET) sensors are typically
measured as frequency sensors. Channels C1 – C8 can be conditioned for open
collector or open drain with an external pull-up resistor as shown in figure Using
a Pull-up Resistor on Digital I/O Channels C1 - C8
(p. 323).
The pull-up resistor
counteracts an internal 100-kΩ pull-down resistor, allowing inputs to be pulled to
> 3.8 V for reliable measurements.
8.1.5.3 Pulse Measurement Tips
• The PulseCount() instruction, whether measuring pulse inputs on pulse
channels (P1 through P4) or on digital I/O channels (C1 – C8), uses
dedicated 24-bit counters to accumulate all counts over the user-specified
scan interval. The resolution of pulse counters is one count or 1 Hz.
Counters are read at the beginning of each scan and then cleared. Counters
will overflow if accumulated counts exceed 16,777,216, resulting in
erroneous measurements.
• Counts are the preferred PulseCount() output option when measuring the
number of tips from a tipping bucket rain gage or the number of times a door
opens. Many pulse output sensors, such as anemometers and flow meters, are