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An Evaluation System for the TAOS Light-to-Frequency Converters
time. This integration time is controlled independently for each daughterboard. The frequency is then
determined by dividing the count value by the integration time to get units of Hz (cycles per second).
Since the number of counts obtained during the integration time interval determines the measurement
resolution, resolution can be improved by using longer integration times. The maximum count value
that can be obtained is 65535 (16-bit), so integration time must be chosen so that this maximum
count value is not exceeded.
Period measurement uses the technique of measuring the time between pulses (i.e. low-to-high
transitions) on the sensor output pin. The EVM uses an internal hardware-based timer to measure this
time interval. The measurement resolution is 2µs with a maximum count value of 65535, therefore the
maximum time interval (period) that can be measured is 0.131s.
Long period measurement uses a software loop to measure the time interval rather than a
hardware-based timer. After establishing an initial condition (such as a high-to-low or low-to-high
transition) this technique samples the output state from the sensor and increments a value used as a
counter. This process repeats until the state changes from the initial condition. The period is then
proportional to the number of counts obtained. Actual time is calculated by multiplying the count
value by the time it takes to execute a single pass through the loop. On the BS2pe used on the EVM,
this loop time is 0.00119s, or 1.19ms, and the maximum count value is 65535. Therefore the
maximum time interval (period) that can be measured is about 78s.
Each of the above three measurement techniques can give different resolution for a given frequency.
In general, for high frequencies it is best to use the Frequency measurement mode since this gives a
relatively high count value in a reasonable amount of time (integration time). For lower frequencies, a
longer integration time is required to get a high count value (high resolution). In this case Period
measurement mode can be used, which will result in a higher count value in a shorter amount of time,
since only the measurement of one period, or cycle, is required. For very low frequencies (less than
about 8Hz, or 1/0.131s), the maximum count of the Period measurement mode can be exceeded. In
this case the Long Period measurement mode can be used, allowing periods up to 78s (about 0.01Hz)
to be measured, although measurement times of this length are not practical in most cases.
Measurement mode can be selected independently for the daughterboards in sockets A and B.
Update Readings
The host software application periodically polls the EVM automatically and requests a measurement.
This polling, or update, interval can be adjusted in several steps between 0.1s and 10s using the Rate
slider. Note that this update rate is independent of the integration time used to measure frequency.
Therefore if the selected integration time exceeds the selected update rate, the update rate will be
decreased to allow the frequency measurement to be made. Additionally, if two daughterboards are
installed and measuring frequency, both integration times must be accommodated. Therefore the
actual update rate may be much slower than the rate selected. The actual update interval is shown
below the Rate slider. Automatic measurement update can be disabled and enabled using the Auto
Update check box.
Strip Chart Data Source
A strip chart may be invoked which provides a visual representation of the measurement data (for
more information on the Strip Chart see the section entitled Strip Chart). Option buttons select the
source of the data for the strip chart from either Socket A or Socket B.
Daughterboard Windows
Daughterboard windows provide control and display measurement results for each daughterboard
installed in the EVM motherboard. Although there are six different daughterboards, there are only two
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