Introduction: Beginning Microcontrollers Part 13: Intro to LCD Interfacing
The microcontroller is a wonderful piece of engineering and it can do many things (with the help of some great programming), but it is still an opaque black box. If you want it to share information, or show you what it is trying to do, you need to connect (interface) an output device. An output device provides you a way to show information from the microcontroller. That is to say, the output device allows the microcontroller to "output" information to the "device". We have already worked with another output device, called the LED (Light Emitting Diode), which gives off light when you program it to do such. Now we will take an in-depth look at interfacing and programming the LCD (Liquid Crystal Display).
The LCD is a much more informative output device than a single LED. The LCD is a display that can effortlessly show characters on its screen. LCDs range in size, price and configuration, from having a couple of lines to large displays. Some are even specifically designed for single applications, having only that ability to display set graphics. We will be using an LCD that has the ability to display four (4) lines of characters that has a 20 character line length. This will be sufficient to show quite a bit of information. Another popular LCD has 2 lines and 16 characters per line.
In the video tutorial (posted above), we will look at how the LCD receives information and control, and the requirements to make sure the information is sent to the LCD in the way that it can appropriately accept information. So, what does all that mean?
First, we have a speed discrepancy between the LCD and the microcontroller. The microcontroller is considerably faster than the LCD, so the microcontroller's program must be fully aware of this and compensate for the time that the LCD is busy working on things you told it just prior. Fortunately, the LCD can inform us of this busy status. We want to create a function to wait until the LCD is not busy. For the LCD to accept information from the microcontroller, or let it give you information, we must turn its enable pin on and off while the information is present for the LCD to accept.
Now we will discuss the pins on the LCD. The most basic of the pins are the power pins for the display to be able to function firstly. There is a VDD pin for 5 volts and a VSS pin for ground. There is a V0 pin for the adjustment of the LCD contrast. Some LCDs even have an LED backlight which are generally the last two pins.
Just like the microcontroller, the LCD has a row of 8 pins to serve as its port. The pins that serve as its port is D0, D1, D2, D3, D4, D5, D6 and D7. These pins are generally used to pass information into the LCD, but it can also be set to pass information back to the microcontroller. You're probably thinking "You told me this is an output device!". Well, sure, it is, but from time to time, it will need to inform you of its state (busy or not). Two types of information can be passed through these pins: data to display on the LCD screen, or control information that is used to do things such as clearing the screen, controlling the cursor position, turning the display on or off and so forth. These data pins are connected to the pins of the desired port on the microcontroller. For example, if you wanted the LCD to be connected to PORTB, the D0 would be connected to Pin0 of Port B, and: D1-PortB Pin1, D2-PortB Pin2, D3-PortB Pin3, D4-PortB Pin4, D5-PortB Pin5, D6-PortB Pin6 and D7-PortB Pin7. This way, there is pin to pin consistency, and if you pass a character in the form of a hexadecimal number, the LCD will receive it in the proper way. If not, there will unexpected results, unless you are using a unique form of byte structure, but don't let that get in your way.
There are other pins on the LCD that help the LCD accept information, and tell the LCD to receive a character or control, or control the read or write (input or output) function of the pins. For clarification, the read/write is microcontroller-centric: the LCD "read" mode is the process of passing information from the LCD to the microcontroller (microcontroller port set as input or reading or listening).; the LCD "write" mode is exchanging information from the microcontroller to the LCD (microcontroller set to output or writing). Without taking into consideration
The pin on the LCD that is responsible for the read and write state is labeled R/W. The pin on the LCD that is responsible for whether the information sent is character or control, is the RS pin (Register Select). And the pin that helps the LCD accept information is called the EN pin (Enable).
There are three basic things you will want to do with an LCD for the proper functioning (more advanced functions can be performed with these three fundamental routines): (1) to make sure the LCD is not busy; (2) Control the LCD's cursor, or display function; and (3) Write a character to the LCD for it to display. Each of these will require its own process:
(1) Checking if the LCD is busy (If you try to display a character to the LCD while the LCD is busy, then the LCD will just ignor the character and it will not be displayed).
- We set the port to receive data on the microcontroller (Data direction as input).
- We put the LCD in read mode (RW on).
- We put the LCD in command mode (RS off).
- And the port now magically contains the data from the LCD (D7 pin will be ON if the LCD is busy and OFF if the LCD is not busy).
(2) Send a command to the LCD
- We check to see if the LCD is busy (Perform the steps in #1 above).
- We set the port direction as output so we can send information to the LCD.
- We turn RW off so we can write.
- We turn RS off for command mode.
- We fire up the data lines with the command we want (simply making the port equal to a number that associates to a specific command).
- We turn on the enable and then turn it off.
- The LCD will magically perform the command.
(3) Send a character to the LCD: This is the same as sending a command except the RS is on and the port will equal the character corresponding to the ASCII code.
So, we are really just turning pins on and off, just like we did with the LEDs from the past tutorials. It's as simple as that. The only catch is that they must be done in the correct sequence.
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