Power consumption report ST72F324 MCU board

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LCD Software Driver Using ST7 MCU
February, 2004
MCU / Shanghai
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Agenda

• LCD basics
• LCD software driver implementation

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LCD panel

• Segment LCD(we talk about this kind)
• Dot matrix LCD

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LCD display principal

• Equivalent circuit basically a capacitor(tens
• of pF) in series with a resistor
• Driving waveform requirement
• LCD driver must apply an AC voltage on
  COM-Segment. When the Root Mean Square (RMS)
  value is above a certain threshold, that segment
  is made opaque, or say dark
• Absolute DC (mean) value on COM-Segment must be
  very low (under 100 mV typically). Otherwise, the
  life time of the LCD can be shortened
• Its frequency must be in the range 30 2000 Hz
  typically. If too low, the display flickers. If
  too high, driving consumes more power

COM
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agenda

• LCD basics
• LCD software driver implementation

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LCD Software Driver block diagram
General purpose port 2
Vcc
LCD panel
R4
ST72(F) 324J2
Vcc 4.25.5V or 2.83.6V
COM1
COM2


GND
General purpose port 1n
Seg 1
Seg n
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½ bias ½ duty driving waveforms

• COM output average voltage
• Vdd/2
• SEG output average voltage
• Vdd/2
• This guarantees COM-SEG
• theoretically to be 0V

Off COM1 Off COM2
On COM1 Off COM2
Off COM1 On COM2
On COM1 On COM2
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½ bias ? duty driving waveforms

• Different COMs output active
• voltage(Vdd or Vss) in turn

Off COM1 Off COM2 Off COM3
On COM1 Off COM2 Off COM3
Off COM1 On COM2 Off COM3
Off COM1 Off COM2 On COM3
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½ bias ¼ duty driving waveforms

• At specific active phase,
• segment outputs reverse voltage
• to turn on that COM-SEG,
• and outputs same voltage level
• not to turn on that COM-SEG

Off COM1 Off COM2 Off COM3 Off COM4
On COM1 Off COM2 Off COM3 Off COM4
Off COM1 On COM2 Off COM3 Off COM4
Off COM1 Off COM2 On COM3 Off COM4
Off COM1 Off COM2 Off COM3 On COM4
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Software flow chart
Timer Int Entry
Reset Entry
Initialization Port, Variable and Timer, Enable
interrupt
Get values from COM table Output to COMs
Interrupt Program is very short
LCD phase need update?
Get values from Segment buffer Output to SEGs
main routine performs segment buffer renewing
N
Y
Update LCD phase Renew segment buffer
Timer Int Exit
Key processing to switch driving waveform
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Average DC voltage on COM-Segment

• DC multimeter UT70D Professional Digital
  Multimeter DC mV scale, Agilent 34401A 6 ½
  Digit Multimeter
• Note Result from DC multimeter is more reliable
  than that from oscilloscope

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Display effects comparison
Note Effect for ½ bias ? duty LCD is not so good
as ½ bias ½ dutys, the reason may be it has no
reflection film at the back plane
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Conclusion

• Software driver Display effect Software driver
  can provide good result when driving ½ bias
  LCD(with whatever duty like ½, ? or ¼ duty)
• Costs very low for hardware(just several
  resistors). Software overhead is low too
• Tips Use more accurate resistors(1 may be a
  good choice) to minimize DC voltage on
  COM-Segment for longer LCD panel life time
• Conclusion Software LCD driver performs very
  well in many specific applications

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Brightness(contrast) adjustable solution

• Two types of solution(patented)
• One solution about 10 step, but requirement
  lower
• Another solution as accurate as the timer can
  provide, but requirement higher(accurate timer
  based)

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LCD software driver library

• Target to ease users work and time
• Support up to 324 segments, easy to port to
  other matrix (only several minutes needed) under
  COSMIC C compiler
• Embedded COM waveform generating, segment
  waveform generating
• Verified to be done

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References

• Application note from ST AN 1447(Software driver
  for 4-multiplexed LCD with a standard ST62) and
  Application note-ST7 software LCD driver