#398 LCD I²C Adapter

Using an I²C adapter to reduce the Arduino pins needed to drive an LCD.

Build

Notes

LCDs based on the Hitachi HD44780 or a compatible chipset are a popular display option for Arduino project. Common form factors are 1602 (16 column, 2 row) and 2004 (20 column, 4 row).

The basic LCDs require at least 6 Arduino pins to drive, and the standard LiquidCrystal library is available for this.

To save pins and simplify wiring, there are I²C adapters available - either separately or packaged with an LCD. These board basically use some form of Remote 8-Bit I/O Expander chip, most commonly the PCF8574 or PCF8574A, for example from NXP.

I am using a product “5pcs 1602 2004 LCD Adapter Plate IIC I2C / Interface lcd1602 I2C LCD Adapter for new” (aliexpress seller listing) that uses a PCF8574AT.

module

I²C Addressing

The address for the module is different for the PCF8574 and PCF8574A. The 7-bit hex address (without R/W bit) per the datasheet:

A2	A1	A0	PCF8574	PCF8574A
0	0	0	0x20	0x38
0	0	1	0x21	0x39
0	1	0	0x22	0x3A
0	1	1	0x23	0x3B
1	0	0	0x24	0x3C
1	0	1	0x25	0x3D
1	1	0	0x26	0x3E
1	1	1	0x27	0x3F

There are 3 unsoldered address pads on the module (A0, A1, A2) that are used to adjust the unit address. On the board I have, each of these are pulled high with a 1kΩ resistor by default. Closing the address pad will pull it low. Thus the default address for the PCF8574 is 0x27, and PCF8574A is 0x3F.

Code and Libraries

I²C is not supported by the standard LiquidCrystal library. When I searched for I²C LCD libraries:

it seems there is no clear leader
there are many independent forks of common source code
it’s a bit of a mess!

Never-the-less, I picked a reletively recent one: LiquidCrystal_I2C and it is working just fine.

The I2CAdapter.ino sketch is a simple test of the library.

Construction

Breadboard

Schematic