tsl2550 ETC-unknow, tsl2550 Datasheet - Page 9

• Manufacturer Part Number: tsl2550
• Description: Ambient Light Sensor With Smbus Interface
• Manufacturer: ETC-unknow

ADC Register
The LUMENOLOGY r Company
The TSL2550 contains two ADC registers (channel 0 and channel 1). Each ADC register contains two
components to determine the logarithmic ADC count value: CHORD bits and STEP bits. The CHORD bits
correspond to the most significant portion of the ADC value and specifies a segment of the piece-wise linear
approximation. The STEP bits correspond to the least significant portion of the ADC count value and specifies
a linear value within a segment. CHORD and STEP bits all equal to 0 corresponds to a condition in which the
light level is below the detection limit of the sensor. CHORD and STEP bits all equal to 1 corresponds to an
overflow condition.
Each of the two ADC value registers contain seven bits as described in Table 3. The specific ADC value register
read depends on the last written RSEL field to the command register, as described above and in the Operation
section, below.
The MSB of the ADC register (VALID bit B7) is used to indicate that data has been written to the ADC register
after the ADC and internal oscillator are activated as described in Command Register section.
Bits 6 through 0 contain the 7-bit code representing the ADC count value, which is proportional to a
photodetector current. In this code, the ADC count value is represented by a piece-wise linear approximation
to a log function. The transfer function is broken into 8 chords of 16 steps each. (This code is very similar to -law
code used in audio compression — it differs in that it does not have a sign bit and it is not inverted.) Table 4 shows
the relationship between the CHORD and STEP bits and the CHORD and STEP numbers and values. These
are used to calculate the ADC count value.
CHORD
VALID
VALID
FIELD
VALID
STEP
B7
6 to 4
3 to 0
BITS
7
B6
C2
ADC channel data is valid. One indicates that the ADC has written data into the
channel data register, since ADCEN was asserted in the COMMAND register.
CHORD number.
STEP number.
CHORD BITS
Table 3. ADC Register Data Format
B5
C1
t
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B4
C0
DESCRIPTION
B3
S3
B2
S2
STEP BITS
t
B1
S1
TAOS029 – SEPTEMBER 2002
Copyright E 2002, TAOS Inc.
B0
S0
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