Display Power

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

Introduction

OLPC has consistently strived to provide a laptop with the lowest power consumption possible. Early work on the XO-3 A1 prototype provided an excellent opportunity to directly compare differences in power consumption.

The results show power savings (130 mW) in every use case, and significant power savings (> 1W) in several important use cases (use with indoor/outdoor lighting and reading/browsing).

In the XO Laptop display architecture (labelled XO-1.75 in the above diagram), the system display controller outputs video data as 18 bit +3.3V LVTTL to a Display CONtroller (DCON). This DCON performs data manipulations and filtering if necessary, then performs the function of an LCD module Timing Controller (TCON) before outputting a +1.8V LVTTL signal which is routed directly the LCD gate and source drivers mounted on the display glass. The display is a transflective display custom to OLPC.

The XO-3 A1 prototype used a different approach, due to time and money constraints. Instead of a DCON chip, it uses a conventional LVTTL/LVDS converter chip (the TI SN75LVDS83B) and a recent technology (2011) display. This display uses an LVDS interface, and integrates the LVDS receiver and TCON into the source driver mounted on the glass.

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	930 mW	1900 mW
Total	1351 mW	2465 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.

System Suspended and Display On

The presence of the DCON in the display path allows further power savings, by allowing the system to go into a suspended state while maintaining an image on the display. This use mode is not supported by the XO-3 A1 prototype, which has no DCON. In this mode, the XO-1.75 laptop is capable of sustaining a displayed image for only 850 mW (no backlight, linux_sus_noback.html) vs. 1850 mW (linux_idle.html) if automatic suspend is disabled.

XO-1.75 Suspend Power w. No Backlight

	Suspended	Idle
Display Subsystem	130 mW	135 mW
LCD Module	220 mW	265 mW
Other system power	500 mW	1450 mW
Total	850 mW	1850 mW

In the suspended case, much of the remaining system power draw is the 802.11b/g network unit, which remains powered and functional.