LCD Glass segment code screen driver

1 segment code screen driving principle

Segmented LCDs consist of several segments (either a single pixel or a complete symbol), each containing a layer of liquid crystal molecules aligned between two electrodes. Under the influence of the electric field, the ordering direction of the crystal will be distorted, thus changing its light transmittance, so that the displayed content can be seen. To distort the crystal, the voltage difference across the electrodes must be greater than a certain threshold to display the content.

Generally, the segment code screen has two poles, one is the segment electrode and the other is the common pole, as shown in the figure below. SEG1 ~ SEGn are segment electrodes, COM is common pole. The voltage of the above-mentioned electric field is applied to both ends of the segment electrode and the common electrode.

However, the segment code LCD is not like the digital tube, which can be displayed by applying a certain amount of DC forward voltage. If a DC voltage is applied to both ends of the segment code screen SEG and COM, it will cause irreversible damage to the liquid crystal.

If you need a certain segment of liquid crystal display, then apply an alternating current with a certain voltage difference between the SEG and COM of a certain segment of liquid crystal. The pressure difference is small or zero.

2-segment screen driver implementation

Three main parameters of LCD segment code screen
Operating Voltage
The operating voltage of the segment code LCD screen.

Duty cycle (Duty)
This parameter is also known as the number of COMs and is defined as 1 / (the number of common terminals on the given LCD). Generally, the driving method of LCD adopts the method of time-division dynamic scanning. Therefore, the ratio of the effective gate time of each COM to the entire scan period, that is, the duty cycle (Duty) is fixed, equal to 1/COM.

Bias (BIAS)
The voltage level used when driving the LCD. Defined as 1 / (number of voltage levels used to drive the LCD display - 1)

Basic drive method
For example, the working voltage of the LCD screen is about 3.3V, just apply a voltage difference of about 3.3V between the two motors of the LCD (for example, the COM terminal is 3.3V, and the SEG terminal is 0V), and at an appropriate time interval, the two electrodes Voltage reversal (for example, COM terminal is 0V, SEG terminal is 3.3V) can make LCD display.

When the liquid crystal is not displayed, ensure that the voltage difference between the two electrodes is 0V (such as COM is 3.3V, SEG is 3.3V), and the voltage of the two poles is reversed at an appropriate time interval (such as COM is 0V, SEG is 0V).

Driver scheme
MCU does not have LCD driver peripherals
If the IO port is directly driven, the bias ratio can only be 1/2. This method needs to add a pull-up resistor and a pull-down resistor to the COM port, and the resistance value is about 100KΩ - 200KΩ.
MCU + dedicated LCD display driver chip
Commonly used driver chips are HT1621 and HT1622, and the corresponding bias ratios are 1/3 and 1/4.
MCU with LCD peripheral
Such as STM32L073 series chips.

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