NOA1312 Datasheet, PDF (8/16 Pages) ON Semiconductor – High-Precision Ambient Light Sensor

NOA1312
DESCRIPTION OF OPERATION
Ambient Light Sensor Architecture
The NOA1312 employs a sensitive photo diode fabricated
in ON Semiconductor’s standard CMOS process
technology. The major components of this sensor are as
shown Figure 2. The photons which are to be detected pass
through the photopic filter limiting extraneous photons and
thus performing as a band pass filter on the incident wave
front. The filter only transmits photons in the visible
spectrum which are primarily detected by the human eye and
exhibits excellent IR rejection. The photo response of this
sensor is as shown in Figure 4.
The ambient light signal detected by the photo diode is
converted to digital signal using a variable slope integrating
ADC with a resolution of 16−bits, unsigned. The ADC value
is stored in the ALS_DATA register where it can be read by
the I²C interface.
Sensor Accuracy Trim
Highly accurate ambient light intensity reading can be
obtained from the NOA1312 by following a simple
trimming procedure which stores the trim value in the
EEPROM memory. This not only enables more accurate
readings, but also provides a way to match readings between
devices. Matching to better than 2% is achievable when
devices share a common power supply and thermal
environment.
The NOA1312 uses a type of binary weighted trim code
approach which allows the output count to be calibrated to
a known light intensity. The factory default code stored in
the EEPROM is shown in Table 8. Figure 7 shows the
trimming of the output counts when Ev = 100 lux. The
trimming operation is performed by writing an 8−bit code in
the range of 0 to 255 (0x00 to 0xFF) to the EEPROM
OUTPUT_TRIM register 0x16. Changes to the trim register
are reflected in real time on the output value of the device.
As indicated in Table 5, the trimming range is from 315
(when trim code is 0xFF) to 825 (when trim code is 0x00).
One possible trimming algorithm is to perform a binary
search starting with trim code 0x00, refining the search to
find a trim code providing 520 counts at Ev=100 lux within
the desired accuracy.
Modes of Operation
The NOA1312 can be placed in any of the following
modes of operation by programming registers over the I²C
bus:
1. Interrupt driven mode
2. Polling mode
3. Power−down mode
In the interrupt driven mode, once the NOA1312 is
configured, no I²C activity is necessary until the ambient
light intensity goes above the value programmed in the
interrupt threshold register (see INT_SELECT register 0x03
for details). When this occurs, the device signals an interrupt
on the INT pin. Then it is up to the I²C master host to read
the ALS_DATA count from the device.
In polling mode, interrupts are typically disabled, but the
NOA1312 continuously takes measurements and the I²C
master host reads out the most recent count whenever it
desires to do so, typically in a timed repeat loop.
In power−down mode, the NOA1312 stops taking
ambient light measurements and powers down most of the
internal circuitry and the INT pin is deactivated. Power is
maintained to preserve the register values (static memory)
and a portion of the I²C remains active to monitor for a
power−on command to the NOA1312.
I²C Interface
The NOA1312 acts as an I²C slave device and supports
single register read and write operations, in addition to block
read and block write operations. All data transactions on the
bus are 8−bits long. Each data byte transmitted is followed
by an acknowledge bit. Data is transmitted with the MSB
first.
The I²C bus address of this device can be 0x29, 0x39 or
0x49, depending on the state of AD pin. When AD is
connected to VDD, the address is 0x29. When AD is not
connected (floating), the address is 0x39. When AD is
connected to VSS, the address is 0x49. The AD connection
must not be changed after power is applied to the device.
Figure 19 shows an I²C write operation. Write
transactions begin with the master sending an I²C start
sequence followed by the seven bit slave address (e.g. 0x29)
and the write(0) command bit. The NOA1312 will
acknowledge this byte transfer with an appropriate ACK.
Next the master will send the 8−bit register address to be
written to. Again the NOA1312 will acknowledge reception
with an ACK. Finally, the master will begin sending 8−bit
data segment(s) to be written to the NOA1312 register bank.
The NOA1312 will send an ACK after each byte and
increment the address pointer by one in preparation for the
next transfer. Write transactions are terminated with either
an I²C STOP or with another I²C START (repeated START).
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