When the ASCII code for any character, say ‘A’, is sent to be displayed on LCD module, the module's controller looks up the appropriate 5x8-pixel pattern in ROM (read-only memory) and displays that pattern on the LCD. There are 8 symbol locations where a custom character can be stored as shown in the following right table. These locations will have a particular bitmap layout corresponding to the custom character. To display an arrow sign, the bitmap values are mapped to a base address location, say 64 (ASCII code 0).
The symbol locations with their base addresses are given below:
This is achieved by first sending the address location (64) to LCD command register. Next, the bitmap values (0, 4, 2, 31, 2, 4, 0, 0) are sent to the LCD data register. Now the arrow sign gets stored at the first address. Now whenever this symbol is to be displayed, its location needs to be sent to the command register of LCD.
There's a 64-byte hunk of RAM (random-access memory) that the LCD controller uses in the same way as character-generator (CG) ROM. When the controller receives an ASCII code in the range that's mapped to the CG RAM, it uses the bit patterns stored there to display a pattern on the LCD. The concept here lies in the fact one can write to the CG RAM, thereby defining one’s own graphic symbols. Each byte of CG RAM is mapped to a five-bit horizontal row of pixels, and LCD characters are typically eight rows high, so 64 bytes of CG RAM is enough to define eight custom characters. These characters correspond to ASCII codes 0 through 7. When an LCD is first powered up, CG RAM contains random garbage bits. If necessary, CG RAM may be cleared by writing 00 into each CG RAM cell.
Writing to CG RAM
Writing to CG RAM is a lot like moving the cursor to a particular position on the display and displaying characters at that new location. The steps involved are:
- Set RS (Register Select) and R/W (Read/Write) pins of the LCD to initialize 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 the setting of RS pin
- Send bytes with the bit patterns for your symbol(s). The LCD controller automatically increments CG RAM addresses, in the same way as it increments cursor positions on the display.
- To leave CG RAM, switch to Command Mode to set 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 display the defined custom character print ASCII codes 0 through 7.
In the circuit, output of microcontroller AT89C51 (from port P2) goes to data pins of LCD numbered 7-14. The control pins RS (pin4), R/W (pin5) and EN (pin6) are connected to the pins 0, 1 and 6 of port P3 of the controller (P3^0, P3^1 & P3^6, respectively).