BH1721FVC Datasheet, PDF (11/16 Pages) Rohm – Digital 16bit Serial Output Type Ambient Light Sensor IC

BH1721FVC
Technical Note
●Adjust measurement result for influence of optical window. (sensor sensitivity adjusting )
BH1721FVC is possible to change sensor sensitivity. And it is possible to cancel the optical window influence (difference with
/ without optical window) by using this function. Adjust is done by changing measurement time. For example, when
transmission rate of optical window is 50% (measurement result becomes 0.5 times if optical window is set), influence of
optical window is ignored by changing sensor sensitivity from default to 2 times
Sensor sensitivity is shift by changing the value of MTreg (measurement time register). MTreg value has to set 2 times if
target of sensor sensitivity is 2 times. Measurement time is also set 2 times when MTreg value is changed from default to 2
times. Low 4bit value is fixed”1100”. Please change high 6bit value of this register via to I²C Bus interface.
ex) Procedure for changing target sensor sensitivity to 2 times.
Please change Mtreg from”01_0010_1100” (default) to”10_0101_1100” (default * 2).
1) Changing High bit of Mtreg
ST
Slave Address
R/W
0
Ack
010_10010
Ack SP
2) Changing Low bit of Mtreg
ST
Slave Address
R/W
0
Ack
011_1XXXX
Ack SP
* X value is ignore.
3) Input Measurement Command.
ST
Slave Address
R/W
0
Ack
0001_0000
Ack SP
* This example is High Resolution mode, but it accepts the other measurement.
4) After about 240ms, measurement result is registered to Data Register. (High Resolution mode is typically 120ms, but
measurement time is set twice. )
The below table is seeing the changeable range of MTreg.
Min.
changeable
range of MTreg
binary
decimal
00_1000_1100
( sensitivity : default * 0.47 )
140
( sensitivity : default * 0.47 )
Typ.
01_0010_1100
default
300
default
Max.
11_1111_1100
( sensitivity : default * 3.40 )
1020
( sensitivity : default * 3.40 )
It is possible to detect 0.25lx by using this function at H-resolution mode.
The below formula is to calculate illuminance per 1 count.
Illuminance per 1 count (lx / count) = 1 / 1.2 *(300 / X)
1.2 : Measurement accuracy
300 : Default value of MTreg ( dec )
X : MTreg value
The below table is seeing the detail of resolution.
MTreg value
00_1000_1100
01_0010_1100
11_1111_1100
lx / count
1.79
0.83
0.25
Please input the opecode at PowerDown state to change Mtreg.There is a possibility of malfunctioning when the opecode to
change Mtreg is input while the illuminance measurement is going .
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11/15
2010.02 - Rev.A