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
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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 CPU @ 206 MHz
 32 bit 64MB SDRAM @ 66MHz
 8KB 2-way-set-associative data and instruction caches
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
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
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, 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
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 – 315.7 mW
 MP3 player – 250 mW
 Image viewer – 253 mW
 Document viewer – 251.8 mW
 Text editor – 250.1 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,
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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%