TSL2571
LIGHT-TO-DIGITAL CONVERTER
TAOS117A − FEBRUARY 2011
ALS Operation
The ALS engine contains ALS gain control (AGAIN) and two integrating analog-to-digital converters (ADC) for
the Channel 0 and Channel 1 photodiodes. The ALS integration time (ATIME) impacts both the resolution and
the sensitivity of the ALS reading. Integration of both channels occurs simultaneously and upon completion of
the conversion cycle, the results are transferred to the data registers (C0DATA and C1DATA). This data is also
referred to as channel
count.
The transfers are double-buffered to ensure data integrity.
ATIME(r 1)
2.72 ms to 696 ms
CH0
ALS
CH0
CH1
ADC
CH1
CH0
Data
C0DATAH(r 0x15), C0DATA(r 0x14)
ALS Control
CH1
Data
C1DATAH(r 0x17), C1DATA(r 0x16)
AGAIN(r 0x0F, b1:0)
1y, 8y, 16y, 120y Gain
Figure 6. ALS Operation
The registers for programming the integration and wait times are a 2’s compliment values. The actual time can
be calculated as follows:
ATIME = 256 − Integration Time / 2.72 ms
Inversely, the time can be calculated from the register value as follows:
Integration Time = 2.72 ms
×
(256 − ATIME)
In order to reject 50/60-Hz ripple strongly present in fluorescent lighting, the integration time needs to
be programmed in multiples of 10 / 8.3 ms or the half cycle time. Both frequencies can be rejected with a
programmed value of 50 ms (ATIME = 0xED) or multiples of 50 ms (i.e. 100, 150, 200, 400, 600).
The registers for programming the AGAIN hold a two-bit value representing a gain of 1
×
, 8
×
, 16
×,
or 120
×
. The
gain, in terms of amount of gain, will be represented by the value AGAINx, i.e. AGAINx = 1, 8, 16, or 120.
Lux Equation
The lux calculation is a function of CH0 channel count (C0DATA), CH1 channel count (C1DATA), ALS gain
(AGAINx), and ALS integration time in milliseconds (ATIME_ms). If an aperture, glass/plastic, or a light pipe
attenuates the light equally across the spectrum (300 nm to 1100 nm), then a scaling factor referred to as glass
attenuation (GA) can be used to compensate for attenuation. For a device in open air with no aperture or
glass/plastic above the device, GA = 1. If it is not spectrally flat, then a custom lux equation with new coefficients
should be generated. (See TAOS application note).
Counts per Lux (CPL) needs to be calculated only when ATIME or AGAIN is changed, otherwise it remains a
constant. The first segment of the equation (Lux1) covers fluorescent and incandescent light. The second
segment (Lux2) covers dimmed incandescent light. The final lux is the maximum of Lux1, Lux2, or 0.
CPL = (ATIME_ms
×
AGAINx) / (GA
×
53)
Lux1 = (C0DATA − 2
×
C1DATA) / CPL
Lux2 = (0.6
×
C0DATA − C1DATA) / CPL
Lux = MAX(Lux1, Lux2, 0)
Copyright
E
2011, TAOS Inc.
r
r
The
LUMENOLOGY
r
Company
8
www.taosinc.com
TAOS [ TEXAS ADVANCED OPTOELECTRONIC SOLUTIONS ]
