Counter Input Applications

On-demand refers to the fact that software can read the counter contents at any time without disturbing the counting process. The following figure shows an example of single point edge counting.

You can also use a pause trigger to pause (or gate) the counter. When the pause trigger is active, the counter ignores edges on its Source input. When the pause trigger is inactive, the counter counts edges normally.

You can route the pause trigger to the Gate input of the counter. You can configure the counter to pause counting when the pause trigger is high or when it is low. The following figure shows an example of on-demand edge counting with a pause trigger.

The counter begins counting when it is armed. The arm usually occurs between edges on the Gate input, but the counting does not start until the desired edge. You can select whether to read the high pulse or low pulse first using the StartingEdge property in NI-DAQmx.

The following figure shows an example of an implicit buffered pulse measurement.

The counter performs a pulse measurement on the Gate. On each sample clock edge, the counter stores the high and low ticks in the FIFO of the last pulse to complete. A DMA controller transfers the stored values to host memory.

The following figure shows an example of a sample clocked buffered pulse measurement.

In this method, you route a pulse of known duration (T) to the Gate of a counter. You can generate the pulse using a second counter. You can also generate the pulse externally and connect it to a DIO terminal. You only need to use one counter if you generate the pulse externally.

Route the signal to measure (fx) to the Source of the counter. Configure the counter for a single pulse-width measurement. If you measure the width of pulse T to be N periods of fx, the frequency of fx is N/T. The following figure illustrates this method.

Another option is to measure the width of a known period instead of a known pulse.

When measuring a large range of frequencies with two counters, you generate a long pulse using the signal to measure. You then measure the long pulse with a known timebase. The USB-6421 can measure this long pulse more accurately than the faster input signal.

You can route the signal to measure to the Source input of Counter 0, as shown in the following figure. Assume this signal to measure has frequency fx. NI-DAQmx automatically configures Counter 0 to generate a single pulse that is the width of N periods of the source input signal.

NI-DAQmx then routes the Counter 0 Internal Output signal to the gate of Counter 1. You can then route a signal of known frequency (fk) as a counter timebase to the Counter 1 Source input. NI-DAQmx configures Counter 1 to perform a single pulse-width measurement. Suppose the result is that the pulse width is J periods of the fk clock.

From Counter 0, the length of the pulse is N/fx. From Counter 1, the length of the same pulse is J/fk. Therefore, the frequency of fx is given by fk = fk * (N/J).

Which Method is Best?

This depends on the frequency to be measured, the rate at which you want to monitor the frequency and the accuracy you desire.

• Low frequency measurements with one counter is a good method for many applications. However, the accuracy of the measurement decreases as the frequency increases.
• High frequency measurements with two counters is accurate for high frequency signals. However, the accuracy decreases as the frequency of the signal to measure decreases. At very low frequencies, this method may be too inaccurate for your application. Another disadvantage of this method is that it requires two counters (if you cannot provide an external signal of known width). An advantage of high frequency measurements with 
two counters is that the measurement completes in a known amount of time.
• Measuring a large range of frequencies with two counters measures high and low frequency signals accurately. However, it requires two counters, and it has a variable sample time and variable error % dependent on the input signal.

The following table summarizes some of the differences in methods of measuring frequency.

Considerations for Choosing Frequency Measurement Method

For all frequency measurement methods, assume the following:

Here is how these variables apply to each method, with a summary in the following table:

One counter—With one counter measurements, a known timebase is used for the source frequency (fk). The measurement time is the period of the frequency to be measured, or 1/fk.

Two counter high frequency—With the two counter high frequency method, the second counter provides a known measurement time. The gate frequency equals 1/measurement time.

Two counter large range—The two counter larger range measurement is the same as a one counter measurement, but now the user has an integer divide down of the signal. An internal timebase is still used for the source frequency (fk), but the divide down means that the measurement time is the period of the divided down signal, or N/fx where N is the divide down.

For a practical example, consider measuring a 50 kHz signal. Assuming that the measurement times for the sample clocked (with averaging) and two counter frequency measurements are configured the same, the following table summarizes the results.

From these results, you can see that while the measurement time for one counter is shorter, the accuracy is best in the sample clocked and two counter large range measurements. For another example, the following table shows the results for 5 MHz.

The following table summarizes some of the differences in methods of measuring frequency.

X1 Encoding

When channel A leads channel B in a quadrature cycle, the counter increments. When channel B leads channel A in a quadrature cycle, the counter decrements. The amount of increments and decrements per cycle depends on the type of encoding—X1, X2, or X4.

The following figure shows a quadrature cycle and the resulting increments and decrements for X1 encoding. When channel A leads channel B, the increment occurs on the rising edge of channel A. When channel B leads channel A, the decrement occurs on the falling edge of channel A.

X2 Encoding

The same behavior holds for X2 encoding except the counter increments or decrements on each edge of channel A, depending on which channel leads the other. Each cycle results in two increments or decrements, as shown in the following figure.

X4 Encoding

Similarly, the counter increments or decrements on each edge of channels A and B for X4 encoding. Whether the counter increments or decrements depends on which channel leads the other. Each cycle results in four increments or decrements, as shown in the following figure.

Channel Z Behavior

Some quadrature encoders have a third channel, channel Z, which is also referred to as the index channel. A high level on channel Z causes the counter to be reloaded with a specified value in a specified phase of the quadrature cycle. You can program the counter reload to occur in any one of the four phases in a quadrature cycle.

Channel Z behavior—when it goes high and how long it stays high—differs with quadrature encoder designs. You must refer to the documentation for your quadrature encoder to obtain timing of channel Z with respect to channels A and B. You must then ensure that channel Z is high during at least a portion of the phase you specify for reload. For instance, in Figure 7-21, channel Z is never high when channel A is high and channel B is low. Thus, the reload must occur in some other phase.

In the following figure, the reload phase is when both channel A and channel B are low. The reload occurs when the phase is true and channel Z is high. Incrementing and decrementing takes priority over reloading. Thus, when the channel B goes low to enter the reload phase, the increment occurs first. The reload occurs within one maximum timebase period after the reload phase becomes true. After the reload occurs, the counter continues to count as before. The following figure illustrates channel Z reload with X4 decoding.