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INTRODUCTION TO 16X2 LCD
HD44780 microcontroller based LCD DISPLAY
This component is specifically manufactured to be used with microcontrollers, which means that it cannot be activated by standard IC circuits. It is used for displaying different messages on a miniature liquid crystal display. The model described here is for its low price and great capabilities most frequently used in practice. It is based on the HD44780 microcontroller ( Hitachi ) and can display messages in two lines with 16 characters each. It can display all the letters of alphabet, Greek letters, punctuation marks, mathematical symbols etc. It is also possible to display symbols made up by the user. Other useful features include automatic message shift (left and right), cursor appearance, LED backlight etc. LCD Display Pins Along one side of the small printed board of the LCD display there are pins that enable it to be connected to the microcontroller. There are in total of 14 pins marked with numbers (16 if there is a backlight). Their function is described in the table bellow:
LCD Screen
An LCD screen can display two lines with 16 characters each. Every character consists of 5x8 or 5x11 dot matrix. This book covers a 5x8 character display which is most commonly used. Display contrast depends on the power supply voltage and whether messages are displayed in one or two lines. For this reason, varying voltage 0-Vdd is applied to the pin marked as Vee. A trimmer potentiometer is usually used for this purpose. Some of the LCD displays have built-in backlight (blue or green LEDs). When used during operation, a current limiting resistor should be serially connected to one of the pins for backlight power supply (similar to LED diodes).
CGROM Memory
CGROM memory contains a standard character map with all characters that can be displayed on the screen. Each character is assigned to one memory location: The addresses of CGROM memory locations match the characters of ASCII. If the program being currently executed encounters a command ‘send character P to port’ then the binary value 0101 0000 appears on the port. This value is the ASCII equivalent to the character P. It is then written to an LCD, which results in displaying the symbol from the 0101 0000 location of CGROM. In other words, the character ‘P’ is displayed. This applies to all letters of alphabet (capitals and small), but not to numbers. As seen on the previous map, addresses of all digits are pushed forward by 48 relative to their values (digit 0 address is 48, digit 1 address is 49, digit 2 address is 50 etc.).
Accordingly, in order to display digits correctly it is necessary to add the decimal number 48 to each of them prior to being sent to an LCD. What is ASCII? From their inception till today, computers can recognize only numbers, but not letters. It means that all data a computer swaps with a peripheral device has a binary format even though the same is recognized by the man as letters (the keyboard is an excellent example). In other words, every character matches a unique combination of zeroes and ones. ASCII is character encoding based on the English alphabet. ASCII code specifies a correspondence between standard character symbols and their numerical equivalents. CGRAM Memory Apart from standard characters, the LCD display can also display symbols defined by the user itself. It can be any symbol in the size of 5x8 pixels. RAM memory called CGRAM in the size of 64 bytes enables it. Memory registers are 8 bits wide, but only 5 lower bits are used. Logic one (1) in every register represents a dimmed dot, while 8 locations grouped together represent one character. It is best illustrated in figure below:
LCD Connecting Depending on how many lines are used for connecting an LCD to the microcontroller, there are 8- bit and 4-bit LCD modes. The appropriate mode is selected at the beginning of the operation in the process called 'initialization'. The 8-bit LCD mode uses outputs D0- D7 to transfer data as explained on the previous page. The main purpose of the 4-bit LCD mode is to save valuable I/O pins of the microcontroller. Only 4 higher bits (D4-D7) are used for communication, while others may be left unconnected. Each piece of data is sent to the LCD in two steps- four higher bits are sent first (normally through the lines D4-D7), then four lower bits. Initialization enables the LCD to link and interpret received bits correctly. Data is rarely read from the LCD (it is mainly transferred from the microcontroller to the LCD) so it is often possible to save an extra I/O pin by simple connecting the R/W pin to the Ground. Such a saving has its price. Messages will be normally displayed, but it will not be possible to read the busy flag since it is not possible to read the display either. Fortunately, there is a simple solution. After sending a character or a command it is important to give the LCD enough time to do its job. Owing to the fact that the execution of a command may last for approximately 1.64mS, it will be sufficient to wait about 2mS for the LCD. LCD Initialization The LCD is automatically cleared when powered up. It lasts for approximately 15mS. After this, it is ready for operation. The mode of operation is set by default, which means that: 1.Display is cleared. 2.Mode DL= 1 - Communication through 8-bit interface N= 0 - Messages are displayed in one line F= 0 - Character font 5 x 8 dots
3.Display/Cursor on/off D= 0 - Display off U= 0 - Cursor off B= 0 - Cursor blink off 4.Character entry ID= 1 Displayed addresses are automatically incremented by 1 S= 0 Display shift off Let's do it in mikroC.. void main() { ADCON1 = 0x0f; Lcd_Init(&PORTD); // Initialize LCD on PORTC Lcd_Cmd(LCD_CLEAR); // Clear display Lcd_Cmd(LCD_CURSOR_OFF); // Cursor off Lcd_Out(1, 1, "www.waayoo.com"); Delay_ms(500) ; }
