TSL2550

AMBIENT LIGHT SENSOR

WITH SMBus INTERFACE

TAOS029L − OCTOBER 2007

7

The LUMENOLOGY r Company

r

r

Copyright E 2007, TAOS Inc.

www.taosinc.com

PRINCIPLES OF OPERATION

Analog-to-Digital Converter

The TSL2550 contains an integrating analog-to-digital converter (ADC) that integrates a photodiode current.

First it integrates channel 0 photodiode current and then it integrates channel 1 photodiode current. At the end

of the conversion cycle for each channel, the conversion result is transferred to the appropriate channel 0 or

channel 1 ADC register. The transfer is double-buffered to ensure that invalid data is not read during the transfer.

After the data is transferred, the TSL2550 automatically begins the next conversion cycle. A VALID bit is used

to indicate that data has been written to the ADC register after ADC is enabled.

Interface to the ADC and control of other device functions is accomplished using the standard 2-wire System

Management Bus (SMBus) interface. Both versions 1.1 and 2.0 of the SMBus are supported.

The ADC has two operating modes: standard and extended. In standard mode, the integration time is 400 mS

for each channel or 800 mS for both channel 0 and channel 1. Extended mode shortens the integration time

by a factor of five with a corresponding decrease in responsivity of 5

to operate at higher light levels, extending the overall dynamic range by a factor of five.

Digital Interface

The TSL2550 contains an 8-bit command register that can be written and read via the SMBus. The command

register controls the overall operation of the device. There are two read-only registers that contain the latest

converted value of each of the two ADC channels. The SMBus slave address is hardwired internally as 0111001

(MSB to LSB, A6 to A0).

Both the send byte protocol and the receive byte protocol are implemented in the TSL2550. The send byte

protocol allows single bytes of data to be written to the device (see Figure 6). The written byte is called the

COMMAND byte. The receive byte protocol allows single bytes of data to be read from the device (see Figure

7). The receive data can be either the previously written COMMAND byte or the data from one of the ADC

channels. In Figure 6 and Figure 7, the clear area represents data sent by the host and the shaded area

represents data returned by the ambient light sensor or slave device.

WR

7

Data Byte

Slave Address

S

1

AP

A

8

11

1

1

S = Start Condition

P = Stop Condition

Shaded = Slave Transmission

Figure 6. Send Byte Protocol

RD

7

Data Byte

Slave Address

S

1

AP

A

8

11

1

1

S = Start Condition

P = Stop Condition

Shaded = Slave Transmission

Figure 7. Receive Byte Protocol