When the product needs a segment LCD display, if an MCU without an LCD driver is used, an external LCD driver IC is required, which will increase the cost and PCB area. In fact, many small items, such as a large number of small household appliances, do not need to display many segment codes. The common ones are 4 8 colons with decimal points or clocks ":", so if you use the IO port to scan and display directly, it will be reduced PCB area reduces costs.

 

However, this scheme is not suitable for driving too many segments (occupying too much IO), and it is not suitable for very low power consumption occasions.

The simple principle of segment LCD driving: as shown in Figure 1.

LCD is a special liquid crystal. Under the action of an electric field, the arrangement direction of the crystal will be twisted, thus changing its light transmittance, so that the display content can be seen. The LCD has a torsion threshold. When the voltage across the LCD is higher than this threshold, the content will be displayed. When it is below this threshold, it will not be displayed. Usually LCD has 3 parameters: working voltage, DUTY (corresponding to the number of COM) and BIAS (ie bias voltage, corresponding to the threshold), such as 4.5V, 1/4 DUTY, 1/3 BIAS, indicating that the LCD display voltage is 4.5V, 4 COMs, the threshold is about 1.5V. When the voltage applied to a certain segment of the LCD is greater than 1.5V (generally 4.5V), it will be displayed, and 1.5V will not be displayed. However, the LCD's response to the driving voltage is not very obvious. For example, when 2V is applied, the display may be weak, which is commonly referred to as "ghosting". Therefore, it is necessary to ensure that when the display is driven, the voltage is more than the threshold value, and when it is not displayed, the voltage is more than the threshold value.

Note: DC voltage cannot be applied to both ends of the LCD, otherwise it will be damaged if the time is a little longer, so ensure that the average voltage of the driving voltage applied to both ends of the LCD is 0. When LCD uses the split scan method, one COM scan is valid at any time, and the other COM is in an invalid state.

The circuit for driving 1/4Duty 1/2BIAS 3V is shown in Figure 1, and the LCD scanning principle is shown in Figure 3. The MCU works at 3V, and the two-way port is used as COM. The PUSH-PULL or STANDARD output port is connected to SEG, and each COM is connected to one. 47K resistor to a capacitor, RC filter to get a midpoint voltage. When it is the turn of a COM scan, it is set to PUSH-PULL output. If the SEG connected to this COM is not displayed, the SEG output is in phase with the COM, and if it is displayed, it is reversed. After scanning, the IO of this COM is set to high impedance, so this COM is connected to the 1/2VDD voltage through a 47K resistor, and SEG continues to output a square wave, so that the voltage applied to the LCD is +-VDD when displayed. When it is not displayed, it is +-1/2VDD, which ensures that the average DC voltage across the LCD is 0.

The circuit for driving 1/4Duty 1/3BIAS 3V is shown in Figure 4, and the LCD scanning principle is shown in Figure 5. The MCU works at 5V, the SEG line outputs 1.5V, 3.5V through resistor divider, and the COM line outputs 0.5V through resistor divider. , 2.5V (in high resistance), 4.5V. When it is the turn of a COM scan, it is set to PUSH-PULL output. If the SEG connected to this COM is not displayed, the SEG output is in phase with the COM, and if it is displayed, it is reversed. After scanning, the IO of this COM is set to high impedance, so this COM is connected to a voltage of 2.5V through a 47K resistor, and SEG continues to output a square wave, so that the voltage applied to the LCD is +-3.0V when displayed. When it is not displayed, it is +-1.0V, which fully meets the scanning requirements of LCD.