TSL2550

AMBIENT LIGHT SENSOR

WITH SMBus INTERFACE

TAOS029L − OCTOBER 2007

9

The LUMENOLOGY r Company

r

r

Copyright E 2007, TAOS Inc.

www.taosinc.com

The specific ADC value register read depends on the last read command written 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 device is powered up 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 μ-law

code used in audio compression — it differs in that it does not have a sign bit and it is not inverted.) Table 3 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.

Table 3. CHORD and STEP Numbers and Values vs Register Bits

CHORD

BITS

B6, B5, B4

C, CHORD

NUMBER

CHORD

VALUE

(Note A)

STEP

VALUE

(Note B)

STEP

BITS

B3, B2, B1, B0

S, STEP

NUMBER

000

0

0

1

0000

0

001

1

16

2

0001

1

010

2

49

4

0010

2

011

3

115

8

0011

3

100

4

247

16

0100

4

101

5

511

32

0101

5

110

6

1039

64

0110

6

111

7

2095

128

0111

7

1000

8

1001

9

1010

10

1011

11

1100

12

1101

13

1110

14

1111

15

NOTES: A. CHORD VALUE = INT (16.5

The ADC count value is obtained by adding the CHORD VALUE and the product of the STEP NUMBER and

STEP VALUE (which depends on CHORD NUMBER).

ADC Count Value + ((Chord Value) ) (Step Value)

(Step Number))

The ADC count value can also be expressed as a formula:

ADC Count Value + (INT (16.5

where:

C

is the CHORD NUMBER (0 to 7)

S

is the STEP NUMBER (0 to 15)

as defined in Table 3.