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Technology Center

The IO port of MCU directly drives the segment code LCD.


Release time:

2018-12-07

When the product requires a segment code LCD display, ifLCDThe drive'sMCU, you need an externalLCDDriveICwhich increases the cost andPCBArea. In fact, many small projects, such as a large number of small household appliances, need to display the segment code is not much, the common is4One8Colon ":" with a decimal point or clock, so that if you useIOThe direct scanning of the port will reduce the PCB area and reduce the cost.
However, this solution is not suitable for driving too many segments (occupying too many IO), nor is it suitable for very low power consumption occasions.
Segment code LCD drive simple principle: as shown in Figure 1.
LCD is a special kind of liquid crystal. Under the action of electric field, the arrangement direction of the crystal will be reversed, thus changing its light transmittance, so that the display content can be seen. The LCD has a twist threshold whenLCDWhen the voltage at both ends is higher than this threshold, the content is displayed, and when it is lower than this threshold, it is not displayed. UsuallyLCDThere are3Parameters: working voltage,DUTY(correspondingCOMnumber) andBIAS(I. e., bias, corresponding to the threshold), such4.5V,1/4 DUTY,1/3 BIAS, indicatesLCDThe display voltage is4.5V,4OneCOM, the threshold is approximately1.5V, when added to a paragraphLCDVoltage at both ends is greater1.5VWhen (generally plus4.5V) shows, while plus1.5Vis not displayed. But...LCDThe reaction to the driving voltage is not very obvious, such as adding 2V, may be weak display, this is commonly referred to as "ghost". Therefore, it is necessary to ensure that when driving the display, the voltage should be greater than the threshold value, while when not displaying, the voltage should be much smaller than the threshold value.
Note: the two ends of the LCD can not be added DC voltage, otherwise a little longer time will be damaged, so to ensure that plusLCDThe average voltage across the drive voltage is0.LCDUse the time split scan method, any time aCOMScan is valid, additional COM is in an invalid state.
The scheme circuit for driving 1/4Duty 1/2BIAS 3V is shown in the figure.1,LCDScanning principle is shown in the figure3,MCUFor3VWork, do with two-way mouthCOM,PUSH-PULLorSTANDARDOutput port connectionSEG, and eachCOMAll take one47Kresistance to a capacitor,RCAfter filtering, a midpoint voltage is obtained. In the turn of a certainCOMWhen scanning, setPUSH-PULLoutput, if with thisCOMConnectedSEGdoes not show, thenSEGOutput.COMIn phase, if displayed, reverse phase. After scanning, thisCOMofIOSet it to high resistance, so thisCOMJust pass47KResistance connected1/2VDDvoltage, whileSEGContinue to output square wave, so addLCDThe voltage on the display is -VDD, it is when not displayed -1/2VDD, guaranteed.LCDThe average DC voltage at both ends is 0.
The scheme circuit for driving 1/4Duty 1/3BIAS 3V is shown in the figure.4,LCDScanning principle is shown in the figure5,,MCUFor5Vwork,SEGLine through the resistor voltage output1.5V,3.5V,COMLine through the resistor voltage output0.5V,2.5V(at high resistance),4.5V. In the turn of a certainCOMWhen scanning, setPUSH-PULLoutput, if with thisCOMConnectedSEGdoes not show, thenSEGOutput.COMIn phase, if displayed, reverse phase. After scanning, thisCOMofIOSet it to high resistance, so thisCOMJust pass47KResistance connected2.5Vvoltage, whileSEGContinue to output square wave, so addLCDThe voltage on the display is -3.0V, it is when not displayed -1.0V, fully satisfiedLCD scanning requirements.

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