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.

Defining a Custom Character#

Each character is defined by 8 bytes, one per row, where the lower 5 bits of each byte set the pixels (MSB is leftmost, but only bits 4–0 matter). For example, a simple smiley face might be 0x00, 0x0A, 0x00, 0x11, 0x0E, 0x00, 0x00, 0x00 — sketching it out on a 5x8 grid and converting each row to a binary value produces the character data. There are plenty of online generators that allow clicking pixels and spit out the byte array, which saves the tedious manual bit-twiddling.

To load a character, a “set CGRAM address” command points to the character slot (slot number times 8), followed by 8 data bytes. After that, writing character code 0x00 to the display shows the custom character in slot 0, 0x01 shows slot 1, and so on. Most libraries wrap this in a createChar(slot, data) function.

Practical Uses#

The 8-slot limit forces some creativity. Common patterns:

Progress bars: Define characters for empty, 1/5 filled, 2/5 filled, etc., then string them across a row for a smooth progress indicator wider than a single character
Battery icons: A set of battery outlines at different charge levels — full, three-quarter, half, quarter, empty
Arrows and navigation symbols: Custom arrows that look better than the > and < characters from the built-in font
Unit symbols: Degree symbol (°) is actually in the HD44780 ROM already at 0xDF, but other symbols like “µ” might need custom characters
Simple animations: Redefining a CGRAM slot while the character is displayed, and the display updates immediately — making it possible to animate a spinning icon by cycling through definitions in a timer loop

Tips#

Use an online CGRAM character generator to design glyphs visually rather than computing byte arrays by hand
Define all custom characters during initialization, right after the HD44780 wake-up sequence
For progress bars, define a set of partially-filled characters (1/5, 2/5, 3/5, etc.) and string them across a row for smooth-looking fractional progress

Caveats#

All 8 slots are global — Character code 0x00 displays the same glyph everywhere it appears on screen. Slot 0 can’t mean one thing on row 1 and another on row 2
CGRAM is volatile — Custom characters are lost on power cycle. Reload them as part of the initialization routine
Redefining a character updates every instance immediately — This is useful for animation (cycle a slot’s definition in a timer), but it means partially updating character definitions isn’t possible without visible glitches

In Practice#

A custom character that looks partially garbled or shows the wrong pattern often means the CGRAM address calculation is off by one slot — each slot is 8 bytes apart
Characters that work after a reset but not on cold boot may indicate the CGRAM write is happening before the initialization sequence completes — add a delay before defining characters
On some HD44780 clones, redefining a character while it’s on screen may briefly show an intermediate state — adding a short delay after the CGRAM write before returning to DDRAM eliminates the flicker