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

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

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

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  1. Low-Power Color TFT LCD Display for Hand-Held Embedded Systems Jamie Unger-Fink John David Eriksen

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

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

  4. 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

  5. System Energy Model • Display System • LCD Controller • Frame Buffer • LCD panel & bus • LCD backlight • Inverter

  6. 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

  7. Reference Platform • 32 bit RISC CPU @ 206 MHz • 32 bit 64MB SDRAM @ 66MHz • 8KB 2-way-set-associative data and instruction caches

  8. Reference Platform CPU and main memory • (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

  9. Reference Platform • LCD controller and frame buffer memory • 32 bit frame buffer • Controller implemented in Xilinx Spartan II • Xpower estimate: 136.7 mW @ 2.5V 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

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

  11. Energy Consumption • Example: MPEG4 player

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

  13. 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

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

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

  16. 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 – 315.7 mW • MP3 player – 250 mW • Image viewer – 253 mW • Document viewer – 251.8 mW • Text editor – 250.1 mW

  17. Backlight dimming techniques • Brightness compensation • Contrast enhancement

  18. 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

  19. 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

  20. 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

  21. Results • Application Specific Parameters • Aggregate Power Reduction

  22. 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%

  23. Questions?

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