HD44780 LCD- Register Select (RS) Pin

The Register Select (RS) pin (pin 4) of a HD44780 LCD is the pin which enables a user to select the instruction mode or the character mode of a LCD.

Depending on which mode is selected, the data on the 8 data pins (D0-D7) is treated either as instruction or character data.

Instruction Mode

When the instruction mode is selected, the data that the LCD receives on the 8 data pins (D0 to D7) will be interpreted as instruction data. These can be instructions such as "clear the display", "move the cursor", etc. Depending on which pins are high and low on the 8 data pins determines the instruction that the LCD will be sent and, thus, execute. For example, when in instruction mode, the LCD will clear the entire screen display when fed the voltages 00000001 on its 8 data pins.

Character Mode

When the character mode is selected, the data that the LCD receives on the 8 data pins (D0 to D7) will be interpreted as character data. Depending on which pins are high and low on the 8 data pins determines the character(s) which will be shown on the LCD. For example, when in character mode, the LCD will display the "$" (dollar sign) when the data pins are fed 01000010.

How to Wire

Instruction Mode- RS pin set to low (or ground)
Character Mode- RS pin set to high (or Vcc)

This is the full schematic of how to wire up a HD44780 LCD:

In order to set the RS pin to instruction mode, we simply connect a jumper wire from pin 4 of the HD44780 LCD to ground. To set the RS pin to character mode, we connect a 1KΩ from pin 4 of the LCD to Vcc to give it positive voltage.

Switch Between Both Modes

Being that both instruction and character modes are used when using a HD44780 LCD, many times we want to go back and forth between these 2 modes. The way to go back and forth between these 2 modes in an easy way is to connect a SPST switch between Vcc and ground. When the switch is open, the LCD is in character mode. When the switch is closed, the LCD is in instruction mode.

This allows us flexibility to go between Instruction and Character Mode of the RS pin.