STC8A8K64D4 Series Manual
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The simple principle of segment code LCD driving is shown in Figure 1.
LCD is a special kind of liquid crystal. The arrangement direction of the crystal will be reversed under the action of an
electric field, which changes its light transmittance, so that the display content can be seen. LCD has a torsional voltage
threshold. The content will be displayed if the voltage across the LCD is higher than this threshold, and the content will not
be displayed if the voltage is lower than this threshold. There are usually 3 parameters in LCD: working voltage, DUTY
(corresponding to COM number) and BIAS (ie bias, corresponding threshold). For example, ‘3.0V, 1/4 DUTY, 1/3 BIAS’
means that the LCD display voltage is 3.0V, 4 COM, the threshold is about 1.5V. The content will be displayed if the voltage
across a certain LCD segment is 3.0V, and the content will not be displayed if the voltage is 1.0V. However, the LCD's
response to the driving voltage is not very sensitive. For example, the display may be faint if the voltage is 2V. This is usually
called a ‘ghost image’. Therefore, it is necessary to ensure that the voltage is larger than the threshold value if you want to
display something, and the voltage is smaller than the threshold value if you do not display anything.
Note: The LCD should be driven by AC, and DC voltage cannot be applied to the two ends of the LCD. Otherwise it will
be damaged for a long time DC applying. The average voltage of the driving voltage applied to the LCD must be 0. Time-
sharing is used to LCD. At any time, if one COM scan is valid, the other COM is invalid.
The scheme circuit for driving ‘1/4Duty, 1/ 2BIAS, 3V’ is shown in Figure 1. The scanning principle of LCD is shown
in Figure 3. The MCU works at 3.0V or 3.3V. Each COM is connected with a 20K resistor in series to a capacitor C1 aiming
to obtain the midpoint voltage of 1/2VDD after RC filtering. When it is the turn of a COM scan, the connected IO is set to
push-pull output mode, and the remaining COMs are set to high impedance. If the SEG connected to this COM is not used to
display, the SEG output is in phase with the COM, and if it is not used to display, it is inverted. After scanning is finished, the
I/O corresponding to this COM is set to high impedance. Each COM is connected to the 1/2VDD voltage on the capacitor C1
through a 20K resistor. The SEG outputs high or low level according to whether it is used to display or not. The voltage
applied to the LCD segment is +-VDD when displayed, and +-1/2VDD when not displayed, which can ensure that the average
DC voltage across the LCD is 0.
The circuit for driving the ‘1/4Duty, 1/3BIAS, 3V’ is shown in Figure 4. The scanning principle of LCD is shown in
Figure 5. The MCU works at 5V. The IOs connected to SEG output 1.5V and 3.5V through the resistor divider. The IOs
connected to COM output 0.5V, 2.5V (at high impedance), 4.5V. The common point of the voltage-dividing resistor is
connected to a capacitor C1 to abtain a mid-point voltage of 1/2VDD after RC filtering. When it is the turn of a certain COM
scan, the IO is set to push-pull output mode. If the SEG connected to this COM is not used to display, the SEG output is in
phase with COM, and if it is used to display, it is inverted. After scanning is finished, the I/O corresponding to this COM is
set to high impedance. This COM is connected to a 2.5V voltage through a 47K resistor. The SEG outputs high or low level
according to whether it is used to display or not. The voltage applied to the LCD is +-3.0V when displayed, and +-1.0V when
not displaying, which can meet the LCD scanning requirements.
When sleep of power saving is required, all IOs used to dirve COMs and SEGs output low level, and the extra current
will not appear in LCD drive part.
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