HD44780 LCD- Instruction Mode

The HD44780 LCD can be in two modes: Character Mode or Instruction Mode.

The HD44780 has 8 data pins, which is how a user submits data to the LCD. When the LCD is in character mode, the data on the data pins represents the character(s) that the LCD will show. When the LCD is in instruction mode, the data on the data pins represents the instruction(s) that the LCD should execute.

There are many instructions that can be sent to the LCD. An example is an instruction to clear the screen of all that may be on it.

Below is the chart of many various instructions that the HD44780 LCD can execute:

Using this chart, we can send and have the LCD execute a number of instructions which can do very useful things.

How to Set the LCD to Instruction Mode

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

How to Wire

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

So in order to set the RS pin to Instruction mode, we wire the RS pin to low or ground.

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

How to Switch Between Both Modes

If you just want to set the LCD permanently to Instruction Mode, you can simply wire the RS pin to ground. However, usually, we connect the RS pin (pin 4) so that we can switch between the 2 modes whenever we want.

In this schematic above, the RS pin is wired so that we can switch between Character Mode and Instruction Mode.

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.