AN3114 datasheet(14/43 Pages) STMICROELECTRONICS

14 / 43 page

LCD controller drive signals

AN3114

14/43

Doc ID 16829 Rev 3

Discrimination ratio calculation for quadruplex mode:

COM3 - SEG0 [ON] = (2VDD/3 - VDD/3) + (2VDD/3 - VDD/3) + (2VDD/3 - VDD/3) + (0 -

VDD) + (VDD/3 - 2VDD/3) + (VDD/3 - 2VDD/3) + (VDD/3 - 2VDD/3) + (VDD - 0) => VDC = 0

COM3 - SEG0 [OFF] = (2VDD/3 - VDD/3) + (2VDD/3 - VDD/3) + (2VDD/3 - VDD/3) + (0 - 0)

+ (VDD/3 - 2VDD/3) + (VDD/3 - 2VDD/3) + (VDD/3 - 2VDD/3) + (0 - 0) => VDC = 0

2.4

Octaplex LCD drive

In an octaplex LCD drive, eight backplanes are used. Each segment line is connected to

eight LCD segments, which other side is connected to one, two, four or eight of the eight

backplanes. Consequently, only (S/8)+8 MCU pins are necessary to drive an LCD with S

LCD segments. This mode is available only in medium+ and high density

STM8L152xx/STM8L162xx devices. For example, to drive an LCD with 320 LCD segments

(40 ×8), only 48 I/O ports are required (40 I/O ports to drive the segment lines and 8 I/O

ports to drive the backplanes).

Five different voltage levels have to be generated on the common lines: 0, VDD/4, VDD/2,

3VDD/4 and VDD. The Segment line voltage levels are also 0, VDD/4, VDD/2, 3VDD/4 and

VDD. The LCD segment is inactive if the RMS voltage is below the LCD threshold voltage

Vth, and is active if the LCD RMS voltage applied is above the threshold. Figure 9 shows

typical backplane, Segment lines and LCD waveforms. The intermediate voltages VDD/4,

and 3VDD/4 are required for backplane voltages. When a backplane or COM is active (0 V

during an odd frame and VDD during an even frame), the others are made inactive by

applying to them 3VDD/4 during an odd frame and VDD/4 during an even frame.

ON RMS

()

VSi

()

∑

------------------------

VDD

-------------

⎝⎠

⎛⎞

VDD

()

VDD

–

----------------

⎝⎠

⎛⎞

VDD

–

()

----------------------------------------------------------------------------------------------------------------------

0,577VDD

OFF RMS

()

VSi

()

∑

------------------------

VDD

-------------

⎝⎠

⎛⎞

VDD

–

----------------

⎝⎠

⎛⎞

------------------------------------------------------------

0,333VDD

ON RMS

()

OFF RMS

()

-----------------------------

0,395

0,176

---------------

1,732

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