7.3.1 Human Eye Matching

The OPT3007 spectral response closely matches that of the human eye. If the ambient light sensor measurement is used to help create a good human experience, or create optical conditions that are optimal for a human, the sensor must measure the same spectrum of light that a human sees.

The device also has excellent infrared light (IR) rejection. This IR rejection is especially important because many real-world lighting sources have significant infrared content that humans do not see. If the sensor measures infrared light that the human eye does not see, then a true human experience is not accurately represented.

Furthermore, if the ambient light sensor is hidden underneath a dark window (such that the end-product user cannot see the sensor) the infrared rejection of the OPT3007 becomes significantly more important because many dark windows attenuate visible light but transmit infrared light. This attenuation of visible light and lack of attenuation of IR light amplifies the ratio of the infrared light to visible light that illuminates the sensor. Results can still be well matched to the human eye under this condition because of the high infrared rejection of the OPT3007.

7.3.2 Automatic Full-Scale Range Setting

The OPT3007 has an automatic full-scale range setting feature that eliminates the need to predict and set the optimal range for the device. In this mode, the OPT3007 automatically selects the optimal full-scale range for the given lighting condition. The OPT3007 has a high degree of result matching between the full-scale range settings. This matching eliminates the problem of varying results or the need for range-specific, user-calibrated gain factors when different full-scale ranges are chosen. For further details, see the Automatic Full-Scale Setting Mode section.

7.3.3 I²C Bus Overview

The OPT3007 offers compatibility with both I²C and SMBus interfaces. The I²C and SMBus protocols are essentially compatible with one another. The I²C interface is used throughout this document as the primary example with the SMBus protocol specified only when a difference between the two protocols is discussed.

The OPT3007 is connected to the bus with two pins: an SCL clock input pin and an SDA open-drain bidirectional data pin. The bus must be controlled by a master device that generates the serial clock (SCL), controls the bus access, and generates start and stop conditions. To address a specific device, the master initiates a start condition by pulling the data signal line (SDA) from a high logic level to a low logic level while SCL is high. All slaves on the bus shift in the slave address byte on the SCL rising edge, with the last bit indicating whether a read or write operation is intended. During the ninth clock pulse, the slave being addressed responds to the master by generating an acknowledge bit by pulling SDA low.

Data transfer is then initiated and eight bits of data are sent, followed by an acknowledge bit. During data transfer, SDA must remain stable while SCL is high. Any change in SDA while SCL is high is interpreted as a start or stop condition. When all data are transferred, the master generates a stop condition, indicated by pulling SDA from low to high while SCL is high. The OPT3007 includes a 28-ms timeout on the I²C interface to prevent locking up the bus. If the SCL line is held low for this duration of time, the bus state machine is reset.

7.4.1 Automatic Full-Scale Setting Mode

The OPT3007 has an automatic full-scale-range setting mode that eliminates the need for a user to predict and set the optimal range for the device. This mode is entered when the configuration register range number field (RN[3:0]) is set to 1100b.

The first measurement that the device takes in auto-range mode is a 10-ms range assessment measurement. The device then determines the appropriate full-scale range to take its first full measurement.

For subsequent measurements, the full-scale range is set by the result of the previous measurement. If a measurement is towards the low side of full-scale, the full-scale range is decreased by one or two settings for the next measurement. If a measurement is towards the upper side of full-scale, the full-scale range is increased by one setting for the next measurement.

If the measurement exceeds the full-scale range, resulting from a fast increasing optical transient event, the current measurement is aborted. This invalid measurement is not reported. If the scale is not at its maximum, the device increases the scale by one step and a new measurement is retaken with that scale. Therefore, during a fast increasing optical transient in this mode, a measurement can possibly take longer to complete and report than indicated by the configuration register conversion time field (CT).

7.5.1 Writing and Reading

Accessing a specific register on the OPT3007 is accomplished by writing the appropriate register address during the I²C transaction sequence. Refer to Table 1 for a complete list of registers and their corresponding register addresses. The value for the register address (as shown in Figure 19) is the first byte transferred after the slave address byte with the R/W bit low.

Figure 19. Setting the I²C Register Address

Writing to a register begins with the first byte transmitted by the master. This byte is the slave address with the R/W bit low. The OPT3007 then acknowledges receipt of a valid address. The next byte transmitted by the master is the address of the register that data are to be written to. The next two bytes are written to the register addressed by the register address. The OPT3007 acknowledges receipt of each data byte. The master may terminate the data transfer by generating a start or stop condition.

When reading from the OPT3007, the last value stored in the register address by a write operation determines which register is read during a read operation. To change the register address for a read operation, a new partial I²C write transaction must be initiated. This partial write is accomplished by issuing a slave address byte with the R/W bit low, followed by the register address byte and a stop command. The master then generates a start condition and sends the slave address byte with the R/W bit high to initiate the read command. The next byte is transmitted by the slave and is the most significant byte of the register indicated by the register address. This byte is followed by an acknowledge from the master; then the slave transmits the least significant byte. The master acknowledges receipt of the data byte. The master may terminate the data transfer by generating a not-acknowledge after receiving any data byte, or by generating a start or stop condition. If repeated reads from the same register are desired, continually sending the register address bytes is not necessary; the OPT3007 retains the register address until that number is changed by the next write operation.

Figure 20 and Figure 21 show the write and read operation timing diagrams, respectively. Note that register bytes are sent most significant byte first, followed by the least significant byte.

Figure 20. I²C Write Example

1. An ACK by the master can also be sent.

Figure 21. I²C Read Example

7.6.1 Internal Registers

The device is operated over the I²C bus with registers that contain configuration, status, and result information. All registers are 16 bits long.

There are four main registers: result, configuration, low-limit, and high-limit. There are also two ID registers: manufacturer ID and device ID. Table 1 lists these registers.