HD44780 character LCD: pinout, 4-bit wiring and initialization

Driving a 16x2, 16x4 or 20x4 character LCD: pin functions, contrast, 4-bit vs 8-bit mode, the initialization sequence, DDRAM row addresses and timing.

The Hitachi HD44780 (and its many clones: KS0066, SPLC780, ST7066) drives almost every character LCD module — 16x1, 16x2, 20x2, 16x4, 20x4 and 40x2. Once you know one, you know them all.

Pinout

Pin	Name	Function
1	VSS	Ground
2	VDD	Supply, usually +5 V (3.3 V versions exist)
3	V0 / VEE	Contrast — wiper of a 10 kΩ pot between VDD and GND
4	RS	Register select: 0 = command, 1 = data
5	R/W	0 = write, 1 = read. Tie to GND if you never read
6	E	Enable — data is latched on the falling edge
7–14	D0–D7	Data bus; in 4-bit mode only D4–D7 are used
15, 16	A, K	Backlight anode and cathode (check whether a series resistor is fitted)

4-bit or 8-bit?

4-bit mode saves four pins: every byte is sent as two nibbles, high nibble first, each with its own pulse on E. It is what most projects use. 8-bit mode is a little faster and simpler to code but costs eight data lines. If R/W is tied to ground you can't read the busy flag, so use fixed delays instead — generous ones.

Initialization sequence (4-bit)

Wait at least 40 ms after power-up (the datasheet asks for 15 ms after VDD reaches 4.5 V, 40 ms after 2.7 V — waiting longer does no harm).
With RS = 0, send the nibble 0x3; wait at least 4.1 ms.
Send 0x3 again; wait at least 100 µs.
Send 0x3 a third time, then 0x2 — the controller is now in 4-bit mode.
Send full commands (two nibbles each): 0x28 function set (4-bit, 2 lines, 5×8 font), 0x08 display off, 0x01 clear (wait 2 ms), 0x06 entry mode (increment, no shift), 0x0C display on, cursor off.

The triple 0x3 exists because the controller may be in an unknown state — mid-byte in 4-bit mode, for example — after a reset without a power cycle. Skipping it is the classic reason a display works after power-up but shows garbage after pressing reset.

Useful commands

Command	Code	Execution time
Clear display	`0x01`	1.52 ms
Return home	`0x02`	1.52 ms
Entry mode: increment	`0x06`	37 µs
Display on, cursor off / on / blinking	`0x0C` / `0x0E` / `0x0F`	37 µs
Function set 8-bit / 4-bit, 2 lines	`0x38` / `0x28`	37 µs
Set CGRAM address (custom characters)	`0x40` + address	37 µs
Set DDRAM address (cursor position)	`0x80` + address	37 µs

Times are for the standard 270 kHz oscillator; clones can be slower, so add margin. The enable pulse must be at least 450 ns wide — 1 µs is a safe choice.

Row addresses

Display	Row 1	Row 2	Row 3	Row 4
16x2, 20x2, 40x2	`0x00`	`0x40`	—	—
16x4	`0x00`	`0x40`	`0x10`	`0x50`
20x4	`0x00`	`0x40`	`0x14`	`0x54`

So to put the cursor on row 2, column 5 of a 20x4, send 0x80 + 0x40 + 4 = 0xC4. On a 20x4, rows 3 and 4 are simply the continuation of rows 1 and 2 in memory — which is why text “jumps” from row 1 to row 3 if you just keep writing. Many 16x1 modules are wired internally as 8x2: characters 9–16 live at 0x40.

Custom characters

Eight user characters (codes 0–7) live in CGRAM. Each is 5×8 pixels, stored as eight bytes where the low five bits are the pixels. Set the CGRAM address to 0x40 + 8 × n, write eight bytes, then set a DDRAM address again before printing text — otherwise your text goes into CGRAM.

Troubleshooting

Only a row of solid blocks: the module has power but hasn't been initialized — check E and RS wiring, and the init delays.
Nothing visible at all: contrast. Turn the pot through its whole range; many modules need V0 at only a few hundred millivolts.
Random characters: wrong nibble order, E pulse too short, or no delay after clear/home.
I2C “backpack” modules (PCF8574) usually answer at address 0x27 or 0x3F, but the mapping of expander bits to RS/E/backlight varies between boards — use the library settings that match yours.