When you send the ASCII code for a character like “A” to an LCD module, the LCD controller looks up the appropriate 5x8-pixel pattern in ROM (read-only memory) and displays that pattern on the LCD. That character-generator ROM contains 192 bit-mapped characters corresponding to the alphabet, numbers, punctuation, and a limited set of Japanese Kanji characters, and Greek symbols.

The ROM is part of the main LCD controller (e.g., HD44780, KS0066, etc.), is mask-programmed, and cannot be changed by the user. The manufacturers do offer alternative symbol sets in ROM for European and Asian languages, but most U.S. distributors stock only the standard character set shown in the LCD Serial Backpack manual.

Alphanumeric LCD controllers do not allow you to turn individual pixels on or off—they just let you pick a particular pattern (corresponding to an ASCII code) and display it on the screen. If you can't change the ROM and you can't control pixels, how do you create graphics on these LCDs? It's easy!

There is a 64-byte block of RAM (random-access memory) that the LCD controller uses in the same way as it does the ROM-based character-generator. When the controller receives an ASCII code in the range that's mapped to the CGRAM, it uses the bit patterns stored there to display a pattern on the LCD. The main difference is that you can write to CGRAM, thereby defining your own graphic symbols.

A character LCD that uses the HD44780 or the KS0068 controller allows for eight programmable characters. The character patterns must initially be programmed into the CGRAM of the LCD. First, the CGRAM must be select by setting the CGRAM address for the character that is to be programmed. Each eight consecutive addresses starting at CGRAM address zero are assigned to one programmable graphical character. Each bit of the data at each address sets the pixel on (1) or off (0). The pixels on a particular row at the right of the character display have the lowest binary value. The rows of pixels start at the top and move down the character as the addresses increase in value as shown in Fig. 1.

A WEB-based character calculator can be used to help determine the list of bytes that need to be written to the CGRAM.^{1 2} Be advised that the calculator in the second reference only generates seven bytes of data and does not specify data for the bottom row of pixels. Writing to the CGRAM is a lot like moving the cursor to a particular position on the display and displaying characters at that new location. The steps are shown in Listing 1.

Listing 1. Steps to program graphical characters.

• Reset RS and R/W pins of the LCD to prepare the LCD to accept instructions.
• Set the CG RAM address by sending an instruction byte from 64 to 127 (locations 0-63 in CG RAM)
• Switch to DATA MODE by changing setting the RS pin.
• Send bytes with the bit patterns for your symbol(s). The LCD controller automatically increments CG RAM addresses, just as it does cursor positions on the display.
• To leave CGRAM, switch to COMMAND MODE to set the address counter to a valid display address (e.g., 128, 1st character of 1st line); the clear-screen instruction (byte 1); or the home instruction (byte 2). Now bytes are once again being written to the visible portion of the display.
• To see the custom character(s) you have defined, print ASCII codes 0 through 7.