Display Power

This page discusses the power savings provided by the OLPC XO Laptop display architecture.

OLPC has consistently strived to provide a laptop with the lowest power consumption possible. The work on early XO-3 prototypes (which did not incorporate the OLPC display architecture) provides an excellent opportunity to measure the differences in power consumption.

System Idle and Display On

The first comparison is the power saved when the system is running with the display turned on. The XO-1.75 display subsystem consumes 424 mW (not including backlight or memory) compared to 565 mW on the XO-3 A1 prototype.

Display Power

	XO-1.75	XO-3
MMP2 Display Ctrl.	90 mW	70 mW
DCON	125 mW	-
LVDS Converter	-	150 mW
LCD Module	206 mW	345 mW
Total	435 mW	565 mW

The real power savings comes from the transflective aspect of the OLPC display, which allows operation without a backlight in bright light, and operation with less backlight power indoors. The backlight in the OLPC display uses between 0 (in bright light) and 0.9W. The backlight in the display in the XO-3 A1 prototype uses between 0.5 and 1.9W ! Starting with the XO-1.75, the XO laptop even automatically turns off the backlight if the ambient light levels make it superfluous (such as outdoors).

Display Power w. Backlight

	XO-1.75	XO-3
Display Subsystem	215 mW	220 mW
LCD Module	206 mW	345 mW
Backlight	500 mW	1000 mW
Total	935 mW	1565 mW

Memory Power

The amount of power required by the host frame buffer memory is not included in the above numbers. This was done as the main system memory on the XO-3 only uses two DDR3 chips (128Mx16), versus the four used in the XO-1.75 (128Mx8). Another problem with a direct comparison is the different size of the frame buffer --- 1200 x 900 on XO-1.75 and 1024 x 800 on the XO-3. For comparison, the measured increase in power draw when outputting video is 146 mW on XO-1.75 and 70 mW on XO-3.

Measurement Error

The slight decrease in display controller power consumption between XO-1.75 and XO-3 is not explained (although it is proportional to the change in display resolution, 75%).

Likewise, there is some uncertainty in the power draw measurements on the +3.3V bus on XO-3. We would expect the SoC's +3.3V power consumption to decrease slightly when the display controller is disabled, as the LVTTL video output becomes quiescent. On XO-1.75, we see a 15 mA decrease (ofw_nodisp.html vs ofw_idle.html). On XO-3, we see an increase of roughly 40 mA (ofw_nodisp2.html vs ofw_idle2.html), possibly due to leakage in the display subsystem.