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



+



