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

2 BNS Solutions A little something about BNS… Engineering consulting firm Located in Walpole, Massachusetts Renesas Platinum Alliance partner Work in numerous industries including 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™”

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

Innovation 4

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 5

66 The Hardware

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

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 Segments that make a character Characters that make an information area Multiple segments can be formed to create dynamic icons

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

Questions? 25

26 Thank You!