ISL29001 Datasheet, PDF (7/11 Pages) Intersil Corporation – Light-to-Digital Sensor

ISL29001
Data Registers
The ISL29001 contains four 8-bit data registers. These
registers cannot be specifically addressed, as is conventional
with other I²C peripherals; instead, performing a read operation
on the device always returns all available registers in ascending
order. See Table 2 for a description of each register.
TABLE 2. DATA REGISTERS
ADDRESS
CONTENTS
00H Least-significant byte of most recent sensor reading.
01H Most-significant byte of most recent sensor reading.
02H Least-significant byte of integration counter value
corresponding to most recent sensor reading.
03H Most-significant byte of integration counter value
corresponding to most recent sensor reading.
The first two 8-bit data registers contain the most recent
sensor reading. The meaning of the specific value stored in
these data registers depends on the command written via
the I²C interface; see Table 1 for information on the various
commands. The first byte read over the I²C interface is the
least-significant byte; the second is the most significant. This
byte ordering is often called “little-endian” ordering.
The third and fourth 8-bit data registers contain the
integration counter value corresponding to the most recent
sensor reading. The ISL29001 includes a free-running
oscillator, each cycle of which increments a 16-bit counter. At
the end of each integration period, the value of this counter
is made available in these two 8-bit registers. Like the
sensor reading, the integration counter value is read across
the I²C bus in little-endian order.
Note that the integration counter value is only available
when using one of the three externally-timed operating
modes; when using internally-timed modes, the device will
NAK after the two-byte sensor reading has been read.
Internal Timing
When using one of the three internal timing modes, each
integration period of the ISL29001 is timed by 32,768 clock
cycles of an internal oscillator. The nominal frequency of the
internal oscillator is 327.6kHz, which provides 100ms
internally-timed integration periods. The oscillator frequency
is dependent upon an external resistor, R_EXT, and can be
adjusted by selecting a different resistor value. The
resolution and maximum range of the device are also
affected by changes in R_EXT.
The oscillator frequency can be calculated using Equation 1:
f_osc
=
327.6kHz ⋅ ¹----⁰---⁰----^k---^Ω---
R_EXT
(EQ. 1)
Accordingly, the integration time, t_int, is also a function of R_EXT,
as shown in Equation 2.
The full scale range in lux, FSR, is also scaled by R_EXT.
t_int
=
100
m
s
⋅
--^R-----^E---^X----^T---
100 k Ω
(EQ. 2)
FSR
=
10000
l
u
x
⋅
-¹---⁰---⁰----^k----^Ω--
R_EXT
(EQ. 3)
R_EXT is nominally 100kΩ, and provides 100ms internal
timing and a 1 to 10,000lux range for Diode 1. Doubling this
resistor value to 200kΩ halves the internal oscillator
frequency, providing 200ms internal timing. In addition, the
maximum lux range of Diode 1 is also halved, from
10,000 lux to 5,000 lux, and the resolution is doubled, from
3.3 counts per lux to 6.6 counts per lux.
The acceptable range of this resistor is 50kΩ (providing
50ms internal timing, 20,000 lux maximum reading, ~1.6
counts per lux) to 500kΩ (500ms internal timing, 2,000lux
maximum reading, ~16 counts per lux).
TABLE 3. R_EXT RESISTOR SELECTION GUIDE
R_EXT
(kΩ)
INTEGRATION
TIME
LUX RANGE RESOLUTION,
(ms)
(lux)
COUNTS/LUX
50 (Min)
50
20,000
1.6
100
100
10,000
3
Recommended
200
200
5,000
6
500 (Max)
500
2,000
16
When using one of the three internal timing modes, the
ISL29001’s resolution is determined by the ratio of the max
lux range to 32,768, the number of clock cycles per
integration.
Equation 4 describes the light intensity as a function of the
sensor reading, and as a function of the external resistor.
E(Lux)
=
--^F----^S-----^R-----
32768
⋅
Data1
E(Lux)
=
-------¹---------
32768
⋅
--------¹---⁰---^,---⁰----⁰---⁰----^l--^u---^x---------
(R_EXT ⁄ 100kΩ)
⋅
Data1
(EQ. 4)
where E is the measured light intensity, Data1 is the sensor
reading, and R_EXT is external resistor value.
External Timing
When using one of the three external timing modes, each
integration period of the ISL29001 is determined by the time
which passes between consecutive external timing
commands received over the I²C bus. The user starts the
integration by sending an external command and stops the
integration by sending another external command. The
integration time, t_int, therefore is determined by Equation 5:
t_int
=
-ⁱ-^I--²---^C---
f_I2C
(EQ. 5)
where:
i_I2C is the number of I²C clock cycles to obtain the t_int.
f_I2C is the I²C operating frequency.
7
FN6166.6
December 10, 2008