Light Sensor

A light sensor is built into the XO-1.75. It measures the amount of light reaching the laptop from outside.

XO-1.75 C1 has the sensor is next to the storage indicator. Any use of the wireless activity indicator or the panel backlight affects the readings. The storage indicator is turned off briefly during each sample capture.

XO-1.75 B1 has a restricted form of the feature, in that the wireless activity indicator is affected by the sensor, and any use of the storage indicator will affect the reading.

XO-1.75 A3 has a restricted form of the feature, in that the sensor is affected by the battery and power LED.

The sensor is managed by the embedded controller, and is averaged and updated at a rate that depends on the brightness of the light. Reading the averaged value can be done up to 213 times per second, but there is no great advantage to reading that quickly.

Testing

In OpenFirmware at the Ok prompt, type

ok als@ .d (cr  d# 200 ms  many

and press enter. The sensor readings will be displayed continuously, until you press a key.

(If you always get the same reading, please report the defect, and also type

ok test /leds

and observe the flashing pattern. Normal pattern on a B1 is wireless indicator bright, storage indicator bright, wireless indicator dim.)

Programming

• your program must read /sys/devices/platform/olpc-ols.0/power_state and parse the result as an integer,
• your program should avoid reading power_state at a rate more than the sample rate the sensor is configured for, because this consumes extra power by making the CPU and the EC more busy,

Internals

The sensor is a reverse biased LED. The EC charges the LED as if it is a capacitor, and then times how long it takes to discharge. The discharge time is mostly related to the ambient light, and slightly related to operating temperature of the embedded controller.

XO-1.75 C1 Restriction

The sensor is adjacent to the storage indicator. The EC disables the storage indicator while taking a sample. The wireless indicator, if it is used, and the screen backlight, will affect the sample.

XO-1.75 B1 Restriction

The sensor is adjacent to the wireless indicator. The EC disables the wireless indicator while taking a sample. The storage indicator, if it is used, and the screen backlight, may affect the sample.

XO-1.75 A3 Restriction

The sensor is adjacent to the battery indicator, and as a result generates distorted samples when the laptop is connected to a DC power cable.

References

• runin-light-sensor script for testing data stream from sensor,
• kernel implementation first included in os40,
• embedded controller implementation,
• OpenFirmware implementation.