Counter/Timer Overview
Signal Compatibility
Units used for stability are typically parts per million (ppm) and
Signal compatibility indicates the signal levels a counter/timer can parts per billion (ppb). For example, the frequency of a 10 MHz
measure or output, such as TTL/CMOS.
oscillator with 10 ppm stability can be 10 MHz 100 Hz; with
100 ppb stability it can be 10 MHz 1 Hz.
Buffered Operations and DMA
The best technique for improving oscillator stability is to precisely
The National Instruments counter/timers can capture numerous control its temperature as is done in an oven-controlled crystal
data points without dead times. These types of measurements, called oscillator (OCXO). The PXI-6608 features such an oscillator.
buffered operations, are valuable in applications that range from
statistical analysis on production lines to experiments in molecular Debouncing and Glitch Removal
chemistry. For instance, when you configure a counter for buffered Noisy signals containing glitches and/or bouncing effects pose
period measurement, data is moved from the counter into a buffer. special challenges for some counter/timer measurements. Noise may
Each edge that initiates a measurement also causes a transfer of the be introduced in the source of the signal, such as with
count into the buffer, as shown in Figure 1. With buffered operations, electromechanical relays, or in the connection if there are strong
data is transferred to the computer memory using DMA or sources of interference in the vicinity of the system. NI 660x devices
interrupts. DMA offers a considerable performance advantage; if contain programmable digital filters that eliminate measurement
your application requires this performance simultaneously on errors caused by spurious spikes and bouncing. Figure 2 shows an
multiple counter/timers, you must know how many DMA channels example of digital filtering.
are available on a particular counter/timer device. For example, if a
device contains three DMA channels and eight counter/timers, Calibration
you can simultaneously perform three high-speed and five lower- Calibration is a key component of any measurement solution. In the
speed, interrupt-based, buffered operations. On NI 660x devices, case of counter/timers, timebase calibration ensures that the
National Instruments implements DMA with the NI MITE chip, frequency and time measurements are accurate. Calibration
which is optimized for measurement applications.
certificates enclosed with the National Instruments counter/timers
and periodic calibration satisfy your ISO-9000 requirements,
certifying that your instrument has been properly calibrated. See
Timebase Stability
Timebase stability can be important when you need to make high- page 21 for more information.
quality measurements. Crystal oscillators typically form the basis of
electrical circuits used to drive timing of a measurement application.
In an ideal case, the oscillation frequency would be constant, but in
reality, many factors influence the the behavior of an oscillator.
A commonly used measure of quality for an oscillator is stability.
Input
Start of Measurement
Filtered Output
Figure 2. Debouncing and Glitch Removal
T1
T2
T3
T4
Input Signal
T
T
T
T
T
T
T
T
T
1
1
2
1
2
3
1
2
3
4
T
Measurement Data
Buffer
Figure 1. Buffered Period Measurement with Counter/Timers
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