Osram ams TSL2521 User manual

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ams OSRAM Group

Application Note
AN001042
TSL2521 ALS/Flicker
Settings and Comparison to Legacy ALS Devices
v1-01 •2022-Jan-27

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TSL2521 ALS/Flicker
Content Guide
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Content Guide
1Introduction ................................... 3
2TSL2521 ALS and Flicker
Settings.......................................... 4
2.1 ALS Channels/Modulators............................4
2.2 Sequencer....................................................5
2.3 Residual Measurement ................................5
2.4 ALS and Flicker Timing................................9
2.5 ALS/Flicker Gain and AGC.........................11
2.6 Wait Time ...................................................12
3ALS Result Data Format ............. 13
3.1 Internal Data Format ..................................13
3.2 16-Bit Data Full Count Results in ALS_DATA
Registers ....................................................14
3.3 20-Bit Data Result in ALS_DATA Registers
Including 4-Bit Residuals............................14
3.4 24-Bit ALS Result Data Transferred Via
FIFO............................................................16
3.5 Saturation and Maximum Value of ALS
Result..........................................................16
4Flicker Result Data Format .........19
4.1 Internal Flicker Data Format.......................19
4.2 Flicker Data Reduction Options..................19
4.3 Flicker Data Transfer Via FIFO...................20
5Revision Information...................22
6Legal Information.........................23

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TSL2521 ALS/Flicker
Introduction
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1 Introduction
TSL2521 offers new ALS and Flicker measurement features on a very small space of 2.0 mm x
1.0 mm x 0.5 mm compared to legacy devices. ALS and flicker are running on the same time base,
the FIFO is larger, it has some data compression features, and measurements can be planned in
sequences with individual settings. Additionally, a new residual measurement feature offers higher
resolution at lower gain.
These new features –especially regarding timing and sequences –require registers different than on
legacy ALS devices like TCS3707 –consequently the registers have now different names.
This document explains the settings for ALS measurements compared to legacy ALS devices, e.g. the
TCS3707 and shows the new relation between ALS and Flicker measurements.

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TSL2521 ALS and Flicker Settings
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2 TSL2521 ALS and Flicker Settings
2.1 ALS Channels/Modulators
The datasheets of ALS legacy devices used to name the measurement channels after their filters, e.g.
the Clear channel, Red channel, Blue Channel and so on. It was possible on devices like TCS3707 to
change the Diode-to-channel connection via a multiplexer, but this was not easy during measurements
and therefore not often used.
The TSL2521 has only two ALS channels but it is very easy to change the connection between the
individual diodes –no matter which filter is on this diode –and the ALS engine. Consequently, channel
related registers are not named after the filter characteristic of the diodes –like Clear (C) and Infrared
(IR) –but after the two available ALS modulators: modulator 0 and modulator 1.
Figure 1 shows the number and filter type of the six available photodiodes on TSL2521. The
photodiode numbers are used to connect the photodiodes to the modulators in the SMUX registers
MEAS_SEQR_STEP0_MOD_PHDX_SMUX_L up to MEAS_SEQR_STEP3_MOD_PHDX_SMUX_H.
The default setting has to be changed since it refers to a different filter layout: modulator 0 should be
connected to the four Clear diodes 1, 2, 3, and 4; modulator 1 should be connected to the two IR
diodes 0 and 5. There is no separate channel/modulator for the flicker measurements, every
modulator can be used for both ALS and Flicker measurements at the same time. On TSL2521 flicker
measurements should be done preferred on modulator 0 with the Clear diodes.
The GUI and the drivers provided by ams OSRAM have both mentioned changes implemented.
Figure 1:
Location, Filter Type, and Number of Photodiodes of TSL2521 (top view)

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TSL2521 ALS and Flicker Settings
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Information
In application cases with high transmissivity glass direct sunlight could cause saturation even at
gain setting of 0.5x especially on modulator 0 connected to the Clear diodes. This can be avoided
by connecting less than 4 of the Clear diodes to modulator 0 in the SMUX registers of the
sequencer.
2.2 Sequencer
TSL2521 has a programmable sequencer for ALS and Flicker measurements built in. There are 4
sequencer steps –step 0 to step 3 –available, but not all need to be used.
In ALS legacy devices like TCS3707 there is no comparable sequencer available, so that would be
like using only step 0 of the TSL2521. For a simple two channel ALS/Flicker measurement one step
might be enough, therefore the default settings of the TSL2521 only use step 0.
For more flexibility, some settings can be chosen for each sequencer step individually, most of them
organized in a 4-bit pattern - each bit of the pattern represents a single step. ALS needs to be enabled
in such a pattern in measurement_sequencer_als_pattern in register MEAS_SEQR_ALS_FD_1 for
both modulators, by default ALS with both modulators is enabled for step 0 only.
Flicker measurement can be enabled for each modulator individually, so there are two pattern fields,
measurement_sequencer_mod0_fd_pattern and measurement_sequencer_mod1_fd_pattern, both in
register MEAS_SEQR_FD_0.
The time a sequencer step lasts is defined by the ALS integration time, the Flicker measurement time,
or the Wait time –whatever is activated and lasts longer. The same is valid for the whole sequencer
cycle if only step 0 is performed.
If in a step there is neither ALS nor Flicker nor Wait time enabled by using the respective patterns then
this step will be omitted.
2.3 Residual Measurement
Residual measurement is a new feature on TSL2521 that enables to measure Sub-LSBs on ALS and
flicker data at the end of the integration time. Residuals represent fractions of full counts: the
integrating ADC –the ALS modulator –like on legacy devices normally counts only a 1 when the
capacitor charged by the photocurrent reached a certain voltage. When the integration time is over,
normally the remaining charge that did not reach this voltage is discharged. The idea of the Residual
measurement is that this remaining charge is measured to get fractions of full counts.

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Since the Residual measurement is done with a resolution of max. 4 bits on this device that results in
4 extra resolution bits almost for free. That way it is possible to use lower gains that have a better
linearity than higher gains, because the Residual bits provide additional resolution on LSB side.
Nevertheless, that has a limit too, so the Residual measurement resolution is reduced at higher gains
–that is why residual measurement is mainly suited to increase the resolution at medium and lower
gains.
There are two different gain tables, the default one reduces the evaluated number of residual bits
starting at gain 128x upwards by 1 for each gain step. The second gain table is valid up to gain 256x
and always uses 4 bits for residuals. It is recommended to use the second gain table and restrict the
maximum gain to 256x. Adding the 4 bits Residual resolution results in an effective gain of 4096x in
this case –but at the gain non-linearity of 256x, which is rather low.
In order to use the second gain table mod_gain_select has to be set to 3 before starting the
measurement, this field can be found in register MOD_GAIN_H as shown in Figure 2. The field
measurement_sequencer_max_mod_gain has to be set to 9 in register CFG8 to restrict the modulator
gain to 256x.
Figure 2:
MOD_GAIN_H
Addr: 0xED
MOD_GAIN_H
Bit
Bit Name
Default
Access
Bit Description
7:6
Reserved
0
5:4
MOD_GAIN_SELECT
0
RW
One bit for each channel to
select second gain table.
This register has to be
written before enabling the
sequencer!
3:0
Reserved
0
Figure 3 shows the Number of Residual bits for the gain steps of the two gain tables. If less than 4
Residual bits are used the non-used bits starting from LSB side are set to 0. This leads to higher
minimum step sizes of the ALS or Flicker sample results as well.
Figure 3:
Number of Residual Bits Depending on Gain Table Chosen by mod_gain_select
Mod_gainx [gain]
Number of Residual Bits for
mod_gain_select = 0
Number of Residual Bits for
mod_gain_select = 1
0 [0.5x] –8 [128x]
4
4
9 [256x]
3
4
10 [512x]
2
Reserved
11 [1024x]
1
Reserved

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Mod_gainx [gain]
Number of Residual Bits for
mod_gain_select = 0
Number of Residual Bits for
mod_gain_select = 1
12 [2048x]
0
Reserved
13 [4096x]
0
Reserved
The additional Residual resolution can be expressed either as decimal places after the full integration
counts or –as internally done - with additional bits on LSB side to avoid a floating-point format. Since
there are 4 bits added on the LSB side for Residuals –no matter if they are measured or not –the way
to get back to the same number space comparable to legacy devices is to divide the results by 16, in
this case the decimal places show the residual amount.
The GUI provided by ams OSRAM can show both methods, Figure 4 with the “Integer view box”
checked shows the internal integer view with 4-bit Residuals on LSB side like in Figure 5.
Figure 4:
TSL2521 GUI Configuration Tab With
Checked “Integer View” Box
Figure 5:
Internal Integer View With 4-Bit Residuals On
LSB Side
In contrast, “Integer View” box unchecked like in Figure 6 does the mentioned division by 16 to show
full counts and Residuals as fractions separated by the decimal point like in Figure 7, that way it can
be compared to the value of legacy devices e.g. for dark count measurements.

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TSL2521 ALS and Flicker Settings
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Figure 6:
TSL2521 GUI Configuration Tab With
Unchecked “Integer View” Box
Figure 7:
Residuals Expressed as Fractions of Full
Counts
In order to activate the Residual measurement it needs to be enabled per step in
measurement_sequencer_mod0_residual_enable_pattern for modulator 0 and in
measurement_sequencer_mod1_residual_enable_pattern for modulator 1, both fields are in register
MEAS_SEQR_RESIDUAL_0.
One difference is that in case of enabled Residual measurement the last sampling time is reduced by
the Residual measurement time. This reduces the ALS integration time accordingly but only by a very
small amount and in case of no flicker measurement during the same step most times it can be
neglected as shown in Equation 1.
Equation 1:
     󰇛  󰇛󰇜󰇜
In case of flicker measurements within the same step after each flicker sample measurement one
residual measurement is done. This reduces the whole ALS integration time more significantly as
shown in Equation 2. This needs to be considered when preparing or using a lux equation.
Equation 2:

 󰇛  󰇜   󰇛
 󰇛󰇜󰇜
Information
It is recommended to place ALS measurement and Flicker measurement into separate steps of the
sequencer in order to avoid noticeable influence of Residual measurement time onto the ALS
integration time. This is what the GUI and the drivers provided by ams OSRAM do as well.

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2.4 ALS and Flicker Timing
For ALS measurements one of the most important parameters is the ALS integration time. On legacy
devices like the TCS3707 that was the product of the modulator clock period at e.g. 2.78 µs, the value
in the ASTEP register (+1) and the value in the ATIME register (+1) as shown in Equation 3.
Equation 3:
󰇟󰇠    󰇛  󰇜 󰇛  󰇜
On TSL2521, it is almost the same but the modulator clock changed to 1.3889 µs and the register/bit
field names are different as Equation 4 shows.
Equation 4:
󰇟󰇠    󰇛  󰇜 󰇛  󰇜
On TSL2521, the first term –the modulator clock multiplied by sample_time –is the base sampling
time both for flicker measurements and for ALS measurements. If Flicker measurement is activated for
a modulator within the active sequencer step, every value at the end of the sampling time is pushed to
the FIFO –as often as set in fd_nr_samples in registers FD_NR_SAMPLES0 and
FD_NR_SAMPLES1 - and increases at the same time the ALS result –as often as set in
als_nr_samples in registers ALS_NR_SAMPLES0 and ALS_NR_SAMPLES1.
Therefore, the Flicker sampling frequency is the reciprocal of the sampling time set with register
SAMPLE_TIME, as shown in Equation 5.
Equation 5:
  
  󰇛  󰇜
The whole flicker measurement time is defined by the mentioned sampling time and the register
FD_NR_SAMPLES, see Equation 6.
Equation 6:
   󰇛  󰇜 󰇛  󰇜
Note that there is only one new ALS result after the ALS integration time for each modulator 0 and
modulator 1 since all single sample values are summed up. In contrast, there are fd_nr_samples
flicker results per chosen modulator in the FIFO after the Flicker measurement time of one step. If that
number of flicker results is more than the FIFO can handle –256 values with 16-bit width are normally
possible –the FIFO needs to be read out during the flicker measurement time to prevent a FIFO
overflow. Figure 8 and Figure 9 show the relation between ALS and flicker measurement within the
same step 0 of the sequencer with the chosen settings.