Low-Power Color TFT LCD Display for Hand-Held Embedded Systems

Jamie Unger-FinkJohn David Eriksen

OutlineIntro to LCDsPower IssuesEnergy ModelNew Reduction TechniquesResultsConclusion

LCD IntroSTN vs TFTLarge power consumer even in high-

performance embedded systems

Why so much power?Hand-held embedded systems usually

execute interactive programsLots of slack time, dynamic power

management can save CPU and memory access power

Shutting down LCD or turning off backlight results in unacceptable quality degradation

Display cannot ‘sleep’, so how do we reduce power here?Need new energy reduction techniques

System Energy ModelDisplay System

LCD ControllerFrame BufferLCD panel & busLCD backlightInverter

System-level ApproachMust utilize detailed energy consumption

characteristicsSystem-level simulatorLocate major energy consuming componentsMinor quality loss but no major degradationCompare CPU/memory power consumption to

display consumption

Reference Platform32 bit RISC CPU @ 206 MHz32 bit 64MB SDRAM @ 66MHz8KB 2-way-set-associative data and

instruction caches

Reference Platform(4) Samsung SDRAM, 2” bus

length, 2.7 pF capacitanceFairchild buffer, 4 pF

capacitanceBus-hold circuit, 0.5 pF

capacitanceSDRAM data ports, 5.3 pF

capacitanceBuffer for memory address

bus, 4.0 pF capacitanceAddress port input, 15 pF

capacitance

CPU and main memory

Reference PlatformLCD controller and frame buffer memory

32 bit frame bufferController implemented in Xilinx Spartan IIXpower estimate: 136.7 mW @ 2.5V core

voltage, 3.3V I/O voltage, 66MHz, 10 pF loadLCD panel and bus

640 x 480, 6.4”, 18-bit transmissive color TFT LCD (VGA)

LCD backlight and inverterCCFT backlight tube, 12V supply inverter

LCD Power ConsumptionPower Consumption per color

Power consumption at pixel clock freq 25MHz

Energy ConsumptionExample: MPEG4 player

New Energy Reduction TechniquesVariable-duty-ratio refreshDynamic-color-depth controlBrightness compensation with backlight

dimmingContrast enhancement with backlight

dimming

Variable-duty-ratio refreshCRT compatible interfaceCan exploit CRT/LCD differences to save

powerVariable-duty-ratio implemented with DTMGReduce to 50% duty with no flicker

LCD sub-pixel circuitTwo capacitive components, CLC an CST

CST needs to be refreshedFor TFT LCD, if refresh rate is higher than

CST time constant, no flicker at allDon’t need a high rate like 120 Hz

Dynamic-color-depth controlModify pixel organization to reduce color

depth when appropriateCPU independent

Dynamic-color-depth controlDuring rendering, CPU draws image in full

depthDuring sweeping, LCD controller adjusts the

color depth to save energyCan shut down 8 LSB when we use 8-bit depthApplication dependent energy gain

MPEG4 player – 315.7 mWMP3 player – 250 mWImage viewer – 253 mWDocument viewer – 251.8 mWText editor – 250.1 mW

Backlight dimming techniquesBrightness

compensationContrast

enhancement

Brightness compensationWhen you dim the backlight, you decrease

the luminanceNeed to compensate by increasing

brightness, as long as number of saturated pixels is small

I = ρLYI – Perceived Intensity

ρ – LCD transmittanceL – Backlight LuminanceY – Image Luminance

Contrast enhancementIf too many saturated pixels in image,

contrast enhancement may be usedWill not work if there is a continuous color

spectrumCan dim the backlight more aggressively

than with Brightness compensation

ResultsBy using the new techniques outlined in the

paper, energy consumption can be reduced by 15% to 27%

MPEG4 player: 320x240 pixels, 30Hz fram rate

MP3 player: 100x50 pixel user interface, 1HzImage viewer: 640 x 480, updates every 3 sDocument viewer: 640 x 480, new page every

5 sText editor: updates 3 new characters per

second

ResultsApplication Specific Parameters

Aggregate Power Reduction

ConclusionNew low power techniquesMinimal quality lossAs hand-held devices become smaller, low

power displays become more importantBattery lifeHeat dissipation

Average power consumption savings: 25%

Questions?