Character LCDs on Embedded Systems Development

HD44780 & Compatible Controllers

Mon, 01 Jan 0001 00:00:00 +0000

HD44780 & Compatible Controllers#

The HD44780 is the controller behind nearly every cheap character LCD in the embedded world — the 16x2 and 20x4 modules that have been around since the 1980s. It’s genuinely the “hello world” of embedded displays. The blue or green LCDs with white or black text found at hackerspaces are almost certainly HD44780-based. The controller is so ubiquitous that dozens of manufacturers clone it, and the interface has become a de facto standard.

I2C Backpack Modules

Mon, 01 Jan 0001 00:00:00 +0000

I²C Backpack Modules#

Most character LCD projects in the last decade use an I²C backpack rather than wiring up the parallel interface directly. These tiny PCBs solder onto the back of a standard HD44780 module and reduce the connection down to four wires: VCC, GND, SDA, and SCL. The most common chip on these boards is the PCF8574 (or PCF8574A), an 8-bit I/O expander that bit-bangs the HD44780’s parallel interface over I²C.

Custom Characters & CGRAM

Mon, 01 Jan 0001 00:00:00 +0000

Custom Characters & CGRAM#

The HD44780 has a lesser-known feature that’s surprisingly useful: it supports up to 8 custom characters displayed alongside the built-in font. These live in CGRAM (Character Generator RAM), a small writable area separate from the main character ROM. Each custom character is a 5x8 pixel bitmap, referenced using character codes 0x00 through 0x07. It’s not much, but it’s enough for battery icons, signal bars, custom arrows, or mini progress bars.