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PCF8574 i2c Driver Module for 1602 LCD Displays

The module showing the various connection

These modules use a PCF8574 i2c io expander chip to allow a standard 1602 LCD module via an i2c bus.

A 10k pot allows for adjustment of the LCD contrast, the ‘LED’ pins control the LCD backlight, and there are pads on the pcb for links to set the i2c address.

The module mounted on the rear of the LCD

They can be easily mounted on the rear of the LCD by means of 16 pin headers.

1. Connections

The pcf8574 io pins are connected to the LCD module as follows:

pcf8574 lcd
pin io signal pin
4 P0 RS 4
5 P1 RW 5
6 P2 E 6
7 P3 BL 15
9 P4 D4 11
10 P5 D5 12
11 P6 D6 13
12 P7 D7 14

LCD module pinouts

Note that some other modules have different connections between the pcf8574 and the LCD pins. Using an Ohm meter is an easy way to determine which goes where.

2. Pullup Resistors

Pullup resistors are fitted to the SDA and SDL line and will need to be removed if the i2c bus is pulled up elsewhere. The annoying power indicater LED can also be removed to save a few milliwatts.

Module showing pullup resistor locations and backlight control.

The pullup resistors are valued at around 4.3k to 4.7k ohms depending on the particular module.

3. Backlight Control

The LCD’s backlight is controlled via the pin header at left. The top pin in VCC and a jumper can be used to turn the backlight on.

The lower pin is the input to the base of a transistor, which when high, turns on the backlight. The current draw is around 4ma when driven from a 3.3v logic output.

With the jumper in place, the P3 bit of the PCF8574 can be used to turn the backlight on and off.

4. I2C Addresses

The A2,A1,A0 links set the I2C address. As shown, with no links fitted, the (7 bit) address is 0×27. The links control the least significant 3 bits, fitting the link sets the bit low.

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