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AN2687 Datasheet(PDF) 17 Page - STMicroelectronics |
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AN2687 Datasheet(HTML) 17 Page - STMicroelectronics |
17 / 22 page 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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