TSL2580, TSL2581

LIGHT-TO-DIGITAL CONVERTER

TAOS098 − MARCH 2010

21

The LUMENOLOGY

r Company

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Copyright

E 2010, TAOS Inc.

www.taosinc.com

APPLICATION INFORMATION: SOFTWARE

Interrupts

The interrupt feature of the TSL258x device simplifies and improves system efficiency by eliminating the need

to poll the sensor for a light intensity value. Interrupt mode is determined by the INTR field in the INTERRUPT

CONTROL Register. The interrupt feature may be disabled by writing a field value of 00h to the Interrupt Control

Register (02h) so that polling can be performed.

The versatility of the interrupt feature provides many options for interrupt configuration and usage. The primary

purpose of the interrupt function is to signal a meaningful change in light intensity. However, it can also be used

as an end-of-conversion signal. The concept of a meaningful change can be defined by the user both in terms

of light intensity and time, or persistence, of that change in intensity. The TSL258x device implements two

16-bit-wide interrupt threshold registers that allow the user to define thresholds above and below a desired light

level. An interrupt will then be generated when the value of a conversion exceeds either of these limits. For

simplicity of programming, the threshold comparison is accomplished only with Channel 0. This simplifies

calculation of thresholds that are based, for example, on a percent of the current light level. It is adequate to

use only one channel when calculating light intensity differences because, for a given light source, the channel 0

and channel 1 values are linearly proportional to each other and thus both values scale linearly with light

intensity.

To further control when an interrupt occurs, the TSL258x device provides an interrupt persistence feature. This

feature allows the user to specify a number of conversion cycles for which a light intensity exceeding either

interrupt threshold must persist before actually generating an interrupt. This can be used to prevent transient

changes in light intensity from generating an unwanted interrupt. With a value of 1, an interrupt occurs

immediately whenever either threshold is exceeded. With values of N, where N can range from 2 to 15, N

consecutive conversions must result in values outside the interrupt window for an interrupt to be generated. For

example, if N is equal to 10 and the integration time is 402 ms, then an interrupt will not be generated unless

the light level persists for more than 4 seconds outside the threshold.

Two different interrupt styles are available: Level and SMBus Alert. The difference between these two interrupt

styles is how they are cleared. Both result in the interrupt line going active low and remaining low until the

interrupt is cleared. A level style interrupt is cleared by selecting the Special Function in the COMMAND register

and writing a 0 to the Interrupt Clear field value. The SMBus Alert style interrupt is cleared by an Alert Response

as described in the Interrupt Control Register section and SMBus specification.

To configure the interrupt as an end-of-conversion signal so that every ADC integration cycle generates an

interrupt, the interrupt PERSIST field in the Interrupt Control Register (02h) is set to 0000b. Either Level or

SMBus Alert style can be used. An interrupt will be generated upon completion of each conversion. The interrupt

threshold registers are ignored.