The chip is connected to 3 control PINs ( RS, RW, CS) and to 4 data PINs.The chip mounted on the backpack could be both PCF8574T or PCF8574AT.We already said it is an I/O expander, hence it should be clear that it is somehow capable to read/write from its I/O PINs which are P0~ P7. The chip on the backpack is the root of the problem.In order to understand how we have edited the old code, it is very important to take a look to the backpack schematic. SCL, which should be connected to I2C serial clock.SDA, which should be connected to I2C serial data.GND, which should be connected to ground.It is a black PCB designed to be soldered on the backside of the LCD, indeed,as the display, it has a 1×16 PIN header connector.Īside the 1×16 header connector, the backpack has another 1×4 header which PINs are: The backpack used in this tutorial is almost standard. Backpack schematic I2C backpack PCF8574AT The backpack reduce the number of wires to 4: 2 for supply and 2 for the I2C communication. This solution is available as a backpack ready to be soldered on our display. The most used solution is based on PCF8574A, an I2C I/O expander from NXP. Today it is possible to buy our display with the SIPO already mounted with a small extra amount (about 3.90 USD instead of 2.80 USD considering a 16×2 LCD). Using a SIPO, it is possible to drive a HD44780 with less wires accepting a small extra charge due to additional hardware. Nowadays, serial communication are largely used and SIPO ( Serial-in Parallel-Out) converters are very cheap. With fastest MCU it is possible to use the 4-bit mode and with a couple of welds and a potentiometer we can reduce to 9 the required lines. At that time serial communications were not so widespread because their were costly and involved constraint about clock speed. Because of that, the HD44780 comes with a parallel bus and requires up to 16 lines to work (8 for data, 3 for handshake, 2 for supply, 1 for contrast and 2 for backlight). Of course we will explain how to edit old code in order to get it work with this new hardware configuration.Įven if it is still popular, this controller was made commercially available in the late eighties.

In this article we will step over introducing an I2C backpack for that display. We have already use it providing a source code to use a 16×2 LCD with a STM32. The HD44780 is a de-facto standard controller for display.

2 wire library for stm