Campbell Scientific CR5000 Measurement and Control Module - Manual (Page 145)

Section 7. Measurement Instructions
7-19
7.8 Digital I/O
PeriodAvg (Dest, Reps, Range, SEChan, Threshold, PAOption, Cycles,
Timeout, Mult, Offset)
This instruction measures the period of a signal on any single-ended input
channel. The specified number of Cycles are timed with a resolution of 70 ns,
making the resolution of the period measurement 70 ns divided by the number
of Cycles chosen.
Parameter
& Data Type Enter
Dest
Variable or
Array
The Variable in which to store the results of the instruction. When Reps are used the
results are stored in an array with the variable name. An array must be dimensioned to
have elements for all the Reps.
Reps
Constant
The number of repetitions for the instruction on adjacent channels.
Range
Constant
The voltage range for the measurement, which determines the gain applied to the signal
prior to a zero-crossing detector. Maximum frequency decreases with increasing gain.
Range Gain Signal (pk-pk) 1 Minimum Maximum
Code Min Max Pulse
Width
Frequency 2
mV5000 0.8 600 mV 10.0 V 2.5 µs 200 kHz
mV1000 4.0 100 mV 2.0 V 5.0 µs 100 kHz
mV200 20 4 mV 2.0 V 25 µs 20 kHz
1 Signals must cross threshold to trigger the voltage comparator.
2 Maximum frequency equals 1/(Twice Minimum Pulse Width) for 50% duty cycle
signals.
SEChan
Constant
The single-ended channel number on which to make the first measurement. If the channel
is entered as a negative number, all reps occur on that channel.
Threshold
Constant
The voltage in millivolts that the input must cross for a count to occur. For a signal
centered around CR5000 ground (Figure 7.8-1) the threshold should be 0. If the input
signal is a 0 to 5 V CMOS signal then a threshold of 2500 mV would result in the voltage
comparator switching at 2.5 V.
PAoption Specifies whether to output the period in µs or the frequency in Hz.
Numeric Code Voltage Range
0 Period of the signal is returned
1 Frequency of the signal is returned
Cycles
Constant
The number of cycles to be measured for the average calculation.
Timeout
Constant
The maximum time duration (in msec) that the logger will wait for the number of Cycles
to be measured for the average calculation.
Mult, Offset
Constant,
Variable,
Array, or
Expression A multiplier and offset by which to scale the raw results of the measurement.
Low-level signals are amplified prior to a voltage comparator for the period
averaging measurement. The internal voltage comparator is referenced to the
user entered threshold. The threshold parameter allows a user to reference the
internal voltage comparator to voltages other than 0 V. For example, a
threshold of 2500 mV allows a 0 to 5 V digital signal to be sensed by the
internal comparator without the need of any additional input conditioning
circuitry. The threshold allows direct connection of standard digital signals, but