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Using LCD Displays

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The principle is straightforward - apply a voltage to the electrode segments that you want to display. However there are two problems!

1. If you apply a dc voltage to the electrode, it will eventually produce a permanent change in the pattern of the molecules, making the LCD display unusable.

The answer: Apply an ac voltage, so that the molecules are moved first one way and then the other, cancelling out any permanent change. With a high enough frequency, greater than about 50Hz, the flickering produced on the display will not be noticeable.

2. Actual LCD displays can have hundreds of electrodes, to allow them to generate a wide range of characters. That makes it a difficult device to drive on an electrode-by-electrode basis!

The answer: Use an LCD module, which contains not only the liquid crystal display, but also an integrated circuit, such as the Hitachi HD 44780, to take care of much of the hard work.

The LCD module is controlled by inputting instructions and data via the data inputs, and control signals, via the control lines. Instructions include clearing the display, moving the cursor back to the home position, and defining factors such as the number of display lines and character font. The control lines tell the module whether the signal on the data inputs is an instruction, or data, and enable the LCD module, allowing it to carry out the instructions.

On the PICmicro development board, the four data inputs lines and two control lines are connected to Port B of the PICmicro chip.

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