TSL230R−LF, TSL230AR−LF, TSL230BR−LF
PROGRAMMABLE LIGHT-TO-FREQUENCY CONVERTERS
TAOS079A
−
OCTOBER 2006
APPLICATION INFORMATION
Output-frequency scaling
Output-frequency scaling is controlled by two logic inputs, S2 and S3. Scaling is accomplished on chip by
internally connecting the pulse-train output of the converter to a series of frequency dividers. Divided outputs
available are divide-by 2, 10, 100, and 1 (no division). Divided outputs are 50 percent-duty-cycle square waves
while the direct output (divide-by 1) is a fixed-pulse-width pulse train. Because division of the output frequency
is accomplished by counting pulses of the principal (divide-by 1) frequency, the final-output period represents
an average of n (where n is 2, 10, or 100) periods of the principal frequency. The output-scaling-counter registers
are cleared upon the next pulse of the principal frequency after any transition of the S0, S1, S2, S3, or OE lines.
The output goes high upon the next subsequent pulse of the principal frequency, beginning a new valid period.
This minimizes the time delay between a change on the input lines and the resulting new output period in the
divided output modes. In contrast with the sensitivity adjust, use of the divided outputs lowers both the full-scale
frequency and the dark frequency by the selected scale factor.
The frequency-scaling function allows the output range to be optimized for a variety of measurement
techniques. The divide-by-1 or straight-through output can be used with a frequency counter, pulse
accumulator, or high-speed timer (period measurement). The divided-down outputs may be used where only
a slower frequency counter is available, such as a low-cost microcontroller, or where period measurement
techniques are used. The divide-by-10 and divide-by-100 outputs provide lower frequency ranges for high
resolution-period measurement.
Measuring the frequency
The choice of interface and measurement technique depends on the desired resolution and data acquisition
rate. For maximum data-acquisition rate, period-measurement techniques are used.
Using the divide-by-2 output, data can be collected at a rate of twice the output frequency or one data point every
microsecond for full-scale output. Period measurement requires the use of a fast reference clock with available
resolution directly related to reference-clock rate. Output scaling can be used to increase the resolution for a
given clock rate or to maximize resolution as the light input changes. Period measurement is used to measure
rapidly varying light levels or to make a very fast measurement of a constant light source.
Maximum resolution and accuracy may be obtained using frequency-measurement, pulse-accumulation, or
integration techniques. Frequency measurements provide the added benefit of averaging out random or
high-frequency variations (jitter) resulting from noise in the light signal or from noise in the power supply.
Resolution is limited mainly by available counter registers and allowable measurement time. Frequency
measurement is well suited for slowly varying or constant light levels and for reading average light levels over
short periods of time. Integration (the accumulation of pulses over a very long period of time) can be used to
measure exposure, the amount of light present in an area over a given time period.
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2006, TAOS Inc.
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The
LUMENOLOGY
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Company
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TAOS [ TEXAS ADVANCED OPTOELECTRONIC SOLUTIONS ]
