Display Power: Difference between revisions

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==System Suspended and Display On==
==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|XO-3 A1 prototype]], which has no [[DCON]]. In this mode, the [[XO_1.75_C2|XO-1.75 laptop]] is capable of sustaining a displayed image for only 850 mW (no backlight, [http://dev.laptop.org/~wad/cl2_c2/linux_sus_noback.html linux_sus_noback.html]) vs. 1850 mW ([http://dev.laptop.org/~wad/cl2_c2/linux_idle.html linux_idle.html]).
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|XO-3 A1 prototype]], which has no [[DCON]]. In this mode, the [[XO_1.75_C2|XO-1.75 laptop]] is capable of sustaining a displayed image for only 850 mW (no backlight, [http://dev.laptop.org/~wad/cl2_c2/linux_sus_noback.html linux_sus_noback.html]) vs. 1850 mW ([http://dev.laptop.org/~wad/cl2_c2/linux_idle.html linux_idle.html]) if automatic suspend is disabled.


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Revision as of 01:49, 10 April 2012

This page discusses the power savings provided by the OLPC 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 (130mw to in several important use cases (use with indoor/outdoor lighting and reading/browsing).


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
XO-1.75 Sus. XO-1.75 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.