Jamie Unger-Fink John David Eriksen

Outline Intro to LCDs Power Issues Energy Model New Reduction Techniques Results Conclusion

LCD Intro STN vs TFT Large power consumer even in high-performance embedded systems

Why so much power? Hand-held embedded systems usually execute interactive programs Lots 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 Model Display System LCD Controller Frame Buffer LCD panel & bus LCD backlight Inverter

System-level Approach Must utilize detailed energy consumption characteristics System-level simulator Locate major energy consuming components Minor quality loss but no major degradation Compare CPU/memory power consumption to display consumption

Reference Platform 32 bit RISC 206 MHz 32 bit 64MB 66MHz 8KB 2-way-set-associative data and instruction caches

Reference Platform (4) Samsung SDRAM, 2” bus length, 2.7 pF capacitance Fairchild buffer, 4 pF capacitance Bus-hold circuit, 0.5 pF capacitance SDRAM data ports, 5.3 pF capacitance Buffer for memory address bus, 4.0 pF capacitance Address port input, 15 pF capacitance CPU and main memory

Reference Platform LCD controller and frame buffer memory 32 bit frame buffer Controller implemented in Xilinx Spartan II Xpower estimate: V core voltage, 3.3V I/O voltage, 66MHz, 10 pF load LCD panel and bus 640 x 480, 6.4”, 18-bit transmissive color TFT LCD (VGA) LCD backlight and inverter CCFT backlight tube, 12V supply inverter

LCD Power Consumption Power Consumption per color Power consumption at pixel clock freq 25MHz

Energy Consumption Example: MPEG4 player

New Energy Reduction Techniques Variable-duty-ratio refresh Dynamic-color-depth control Brightness compensation with backlight dimming Contrast enhancement with backlight dimming

Variable-duty-ratio refresh CRT compatible interface Can exploit CRT/LCD differences to save power Variable-duty-ratio implemented with DTMG Reduce to 50% duty with no flicker

LCD sub-pixel circuit Two capacitive components, C LC an C ST C ST needs to be refreshed For TFT LCD, if refresh rate is higher than C ST time constant, no flicker at all Don’t need a high rate like 120 Hz

Dynamic-color-depth control Modify pixel organization to reduce color depth when appropriate CPU independent

Dynamic-color-depth control During rendering, CPU draws image in full depth During sweeping, LCD controller adjusts the color depth to save energy Can shut down 8 LSB when we use 8-bit depth Application dependent energy gain MPEG4 player – mW MP3 player – 250 mW Image viewer – 253 mW Document viewer – mW Text editor – mW

Backlight dimming techniques Brightness compensation Contrast enhancement

Brightness compensation When you dim the backlight, you decrease the luminance Need to compensate by increasing brightness, as long as number of saturated pixels is small I = ρLY I – Perceived Intensity ρ – LCD transmittance L – Backlight Luminance Y – Image Luminance

Contrast enhancement If too many saturated pixels in image, contrast enhancement may be used Will not work if there is a continuous color spectrum Can dim the backlight more aggressively than with Brightness compensation

Results By 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, 1Hz Image viewer: 640 x 480, updates every 3 s Document viewer: 640 x 480, new page every 5 s Text editor: updates 3 new characters per second

Results Application Specific Parameters Aggregate Power Reduction

Conclusion New low power techniques Minimal quality loss As hand-held devices become smaller, low power displays become more important Battery life Heat dissipation Average power consumption savings: 25%