TSL2521 ALS/Flicker
ALS Result Data Format
Application Note
• PUBLIC
AN001042
• v1-01 • 2022-Jan-27
23
│ 17
3.5.1
Analog Saturation
Analog saturation means that the signal from the photodiodes amplified by the gain stage is that high
that it cannot be checked fast enough by the modulator circuit. It is signaled by the bits
als_datax_analog_saturation_status dedicated for each modulator in register
ALS_STATUS
, the
result set of all modulators needs to be ignored in this case. The ALS_DATAx register of the affected
modulator is forced to the value 0xFFFF additionally, in case of 24-bit FIFO format to 0xFFFFFF, in
case of 32-bit FIFO format to 0xFFFFFF. Analog saturation sets the modulator interrupt flag mint in
register
STATUS
as well.
In case of manual gain setting the gain needs to be reduced, in case of enabled saturation AGC this is
done automatically. If a gain of only 0.5x does not remove the saturation condition the number of used
photodiodes for this filter needs to be reduced with the SMUX registers in the sequencer section.
3.5.2
Digital Saturation
The als_digital_saturation flag in register
STATUS2
is set if an internal ALS results cannot be
expressed in the data format chosen by als_msb_position in register
MEAS_MODE1
. Additionally, the
ALS_DATA
x register of the affected modulator is forced to the value 0xFFFE, in case of 24-bit FIFO
format to 0xFFFFFE. Digital saturation sets the modulator interrupt flag mint in register
STATUS
as
well.
Like for analog saturation this can be solved by reducing the gain - if not done by the AGC
automatically
– or by reducing the number of photodiodes connected to the affected modulator. In
contrast to analog saturation a reduction of the ALS integration time has the same effect.
3.5.3
Maximum Value of ALS Result
The maximum value of the ALS result can be important to check because it can be reached,
depending on the als_nr_samples and sample_time settings, without getting analog or digital
saturation. Nevertheless, measurements beyond this maximum value will always show the same value
and are therefore not linear anymore.
This maximum value needs to be checked on host side and in case of occurrence marked as
saturation due to maximum value reached, the sensor does not recognize this nonlinearity. Equation 7
shows the calculation for measurements when Residuals are disabled, the factor 16 at the end takes
care for the 4 bits reserved for Residuals, it has to be omitted if the 16-bit data results transfer
described above is used.
Equation 7:
𝐴𝐿𝑆_𝑚𝑎𝑥_𝑣𝑎𝑙𝑢𝑒 = ((𝑎𝑙𝑠_𝑛𝑟_𝑠𝑎𝑚𝑝𝑙𝑒𝑠 + 1) ∗ (𝑠𝑎𝑚𝑝𝑙𝑒_𝑡𝑖𝑚𝑒 + 1) − 2) ∗ 16
In case of enabled Residual measurement the calculation is not that easy anymore since the
maximum value can change a bit with the Residual measurement time and due to other reasons.
In order to be on the save side the result of the following calculation and every bigger value should be
considered as saturation due to maximum value reached.
