The 8-bit mode of LCD interfacing with PIC has been explained earlier. In the 4-bit mode the (8-bit) data/command is sent in nibble (four bits) format to LCD. The higher nibble is sent first followed by the lower nibble. In 4-bit mode only four data pins (D4-D7) of LCD are connected to the controller. The control pins (RS, RW and EN) are connected the same way as in 8-bit mode. The connections of LCD with PIC18F4550 are shown in the adjoining circuit diagram. Please note that here only PortB is used to connect data lines as well as control lines unlike in 8-bit mode. Refer LCD interfacing with PIC in 8-bit mode.
LCD is configured for 4-bit mode by sending appropriate instruction known as Function Set. The Function Set is hexadecimal instruction for LCD MPU unit which selects the working modes of LCD. The Function Set is given below along with its description.
DL - Data Length
N - No. of Lines
F - Font
According to the table, the value of Function Set for 4–bit mode will be [ 0010 0000 ] 0x20. The value of Function Set for the LCD configuration : 2 line (N=1), 5x7 dots (F=0) and 4-bit (DL=0) mode will be [ 0010 1000 ] 0x28.
It is important to note that when the power supply is given to LCD, it remains in 8-bit mode. In this state if 0x20 is sent, lower nibble will not be received by LCD because only four data lines (D4-D7) are connected, so 0x02 is sent instead of 0x20.