Here's little project of electronic module for ubiquitous Hitachi HD4470 based alphanumeric displays. Main reason for making the project was to drive the display using I2C bus. There are many existing I2C extension boards, with most popular built upon NXP PCF8574 8 bit port expander. This module however, use NXP PCA9535 16 bit expander and provide few additional functions.

PCB of the module was made in KiCAD 4.0. Generated gerber files were sent to the PCD manufacturer and after few minor modifications and few weeks later, PCB of a prototype were sent back. Here are the results.

• Full 8 bit communication with the display. It uses PCA9535 16 bit expander, so we have spare IO ports to communicate using full 8 bit width words. In popular I2C expansion modules 4 bits are used to drive control lines of the display and 4 other bits are used as data lines. This module project, was made to provide full 8 bit communication. No need to send data bytes as two 4 bit chunks although sent as 8 bits of which 4 bits are meaningful and 4 other are just ignored.

• You can read data from the display. Not all I2C modules provide ability to read from the display. Some of implementations have hard-wired R/W line assuming user will always write to the display. Using this module you are able to read data from the module. Attached example libraries use this feature to read the busy flag of the display. No longer arbitrary delays in your code, your code waits as long as it is required for the display to finish.

• Contrast and backlight intensity are handled programmatically. Another eight IO lines are used for driving control lines of a display and changing backlight intensity, setting contrast and other functions. Usually I2C expansion modules use a potentiometer to set contrast of a display, depending on the module, contrast setting may differ, so you take a screwdriver and change the pot until the display shows its content with desired contrast. The module has two electronic potentiometers to set the contrast and for changing backlight intensity. Where on popular I2C modules you're able to switch backlight on or off. Potentiometers use simple Up/Down communication protocol.

• You can programmatically switch on or off the display power supply. Additional line is used to switch the display power on or off. If for some reason the display should be switched off (to save battery usage for example), you can do it from your code. However there are some issues regarding this version of module. Read this Wiki article about known issues and how to get rid of them.

If you like to read more about the module please follow this link: https://github.com/lucidm/i2lcd/wiki