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ID 421C: Designing with Segmented LCD Displays

ID 421C: Designing with Segmented LCD Displays. BNS Solutions. Travis Weisberger. Embedded Systems Engineer. 13 October 2010. Version: 1.3. BNS Solutions. Battery systems Medical White goods Service industries Avionics LED Lighting Entertainment Industrial controls

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ID 421C: Designing with Segmented LCD Displays

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  1. ID 421C: Designing with Segmented LCD Displays BNS Solutions Travis Weisberger Embedded Systems Engineer 13 October 2010 Version: 1.3

  2. BNS Solutions • Battery systems • Medical • White goods • Service industries • Avionics • LED Lighting • Entertainment • Industrial controls • Engineering Education • Embedded Tools • “Engineering and Technical Marketing Solutions as simple as possible, But Not Simpler™” A little something about BNS… Engineering consulting firm Located in Walpole, Massachusetts Renesas Platinum Alliance partner Work in numerous industries including

  3. Bio • Travis Weisberger • Northeastern University BS CE/EE • With BNS Solutions since 2009 • Started LCD with DPAC (Dynamically Controlled Alarm Clock) • Graphic LCD which uses same methods • Jerry Issa • University of Michigan BSE in CE • Boston University MS CSE • With BNS Solutions since 2007 • First implemented LCD control in home heating solutions

  4. Innovation

  5. Agenda • Hardware • What is a segmented LCD • Types of LCDs • Controlling the hardware • Software • Remapping backplane • Information blocks • The “screen” approach for controlling a system • Questions • Feedback

  6. The Hardware 6

  7. LCD (Liquid Crystal Display) • There is a viewing angle(usually 12 or 6 o’clock) • Not light emitting, so require a backlight in low light situations • Mostly concerned with twisted nematic (TN) displays • Light is polarized and then either blocked or passed by the liquid crystals, causing light and dark areas • Applying voltage twists the liquid crystal to create visual segments

  8. Types of TN LCDs Reflective - has reflector behind, used only in well lit environments, cannot be backlit Transmisive - requires light to pass through, must be backlit Transflective – hybrid that can work backlit or in bright environments

  9. Driving an LCD segment Without a LCD driver 2 digital I/Os Waveform

  10. Multiplexing If there are more segments than available pins, the segments will need to be multiplexed between multiple backplanes (or commons) The disadvantage is increasing drive complexity and reduced contrast

  11. Bias • When there are greater 1 backplane, the voltage must be biased to several levels • The bias level is defined by the LCD itself • Bias is using several drive voltages which are multiples of each other

  12. Designs for 2 Backplanes Without a LCD driver Several methods using: 4 digital I/Os 2 digital I/Os Backplane waveform

  13. 4 I/Os By using 4 I/O pins for the commons, the configuration is easy to implement 5V – output high 2.5V – one high/one low 0V - both low for 0V.

  14. 2 I/Os By using 2 I/O pins for the commons, the configuration is easy to implement but requires the tri-state to be used 5V – output high 2.5V – tri-state/ float 0V – output low

  15. Backplane Code Show code in HEW

  16. DC Drift • One problem with LCDs is with DC drift • Occurs when a constant non-zero voltage potential is held on a single segment • Causes the crystals in the segment to “drift” to a half on-half off position, which gives a grey look (never clear, never black) • Changing potentials will solve this • Persistent application will cause permanent degradation

  17. Hardware LCD Drivers Some processors, such as the R8C/Lx family, contain integrated LCD drivers Greatly simplifies management of large LCDs with many segment and backplane lines

  18. The Software 18

  19. Remapping the LCD • Logical remap of display segment to an array of com and segment pins • Map display segments to array for: • Ease of reuse and portability • Maintainability in the face of hardware changes • Self documenting and readable code • Can further map characters to sets of lines in 7 or 14 segments to display • Examples in HEW

  20. Segmented Displays

  21. Segmental Building Blocks Multiple segments can be formed to create dynamic icons Segments that make a character Characters that make an information area

  22. Designing Software for LCD • Screens • State machine • Control state of system • Controls what is on LCD • Controls what key/button presses do • Easy to update and maintain • Straight forward and easy to read

  23. Screen Table • Everything is defined here • Each screen controls and corresponds the state of the system • Each table entry has • Section for fixed icons • Section for pointers for data to be displayed in data area • Section for formatting • Section for blinking icons • Other sections for state depended items • Examples in HEW

  24. Controlling Keyboard / Buttons • Can be one of several methods • 1 large switch statement, each case being a screen with a switch statement for all the buttons • Function table where rows are screen and columns are buttons • Button case control what button press does (or does not do) in that screen • Examples in HEW

  25. Questions? 25

  26. Thank You! 26

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