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AN2687 Datasheet(PDF) 17 Page - STMicroelectronics

No. de pieza AN2687
Descripción Electrónicos  STM8S20xxx LCD software driver
Download  22 Pages
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Fabricante Electrónico  STMICROELECTRONICS [STMicroelectronics]
Página de inicio  http://www.st.com
Logo STMICROELECTRONICS - STMicroelectronics

AN2687 Datasheet(HTML) 17 Page - STMicroelectronics

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AN2687
Software contrast control with the first method
Doc ID 14296 Rev 1
17/22
5
Software contrast control with the first method
Software contrast control is under patent from STMicroelectronics. The use of this technique
with a non-STMicroelectronics microcontroller has to be agreed by STMicroelectronics.
LCD contrast is entirely controlled by software with no need for any external component.
LCD contrast can be adjusted to the optimal value depending on the operating voltage of the
used LCD. LCD contrast is controlled by varying the dead phase timing as shown in the LCD
timing diagrams. This corresponds to the variation of the compare event value.
Dead time can be used to either decrease or increase the LCD Vrms. Dead time is the
voltage compensation time to regulate the rms voltage up and down. Dead time can be
implemented either after each control period or at the end of the frame. To avoid flickering,
the dead time duration must be adjusted depending on the quality of LCD and the frequency
of the frame.
In the example shown in Figure 6, the Vrms value of the LCD decreases when the dead
time duration increases and the Vrms value increases when the dead time duration
decreases. The opposite process is shown in Figure 7.
Two push-buttons connected to PA4 and PA5 are used on the demo board to decrease and
increase contrast, respectively, using this software method.
Relatively high software contrast control steps were chosen with the object of reaching
maximum or minimum contrast by pressing the push-buttons around 10 times. This can be
further fine-tuned if needed.
5.1
Contrast calculations
Let the frame period be T + xT, where:
T is the Active time
xT is the Dead time
In the formulas below, x corresponds to the proportion of dead time and Vx corresponds to
the voltage during the Dead time.
Vrms(on) =
Vrms(on) =
Vrms(on) =
Vrms(on) =
1
TxT
+
-----------------
ft

2
dt
0
TxT
+

1
TxT
+
-----------------
V
DD

2
dt
V
DD

2
dt
V
DD
2
-----------


2
dt
Vx

2
dt 8
0
xT
8
-------
+
2T
8
-------
T
+
T
8
---
2T
8
-------
+
0
T
8
---





1
TxT
+
-----------------V
DD
2 T
8
---
V
DD
2 T
8
---
V
DD
4
-----------
2
6T
8
-------
Vx

2 xT
8
-------


8
+
+
+



1
1x
+
------------
14V
DD
32
------------------
2
Vx

2
x
+




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