Low-Power Color TFT LCD Display for Hand
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Transcript Low-Power Color TFT LCD Display for Hand
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 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,
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%