The generic 16-character × 2-line character LCD. Almost every module sold under this description is driven by a Hitachi HD44780 or a pin- and command-compatible clone (Samsung KS0066, Sitronix ST7066), which is why the same wiring and the same library work across parts from different vendors.

Pinout

Source: SparkFun — Basic Character LCD Hookup Guide (original image). SparkFun tutorials are CC BY-SA 4.0.

Pin assignments

The SparkFun diagram above marks pins 7–10 "not used" because that guide wires the display in 4-bit mode. They are D0–D3 and are only idle in that mode; in 8-bit mode they carry the low nibble.

RGB-backlight variants have 18 pins — pins 16, 17, and 18 become the separate red, green, and blue cathodes.

Minimum wiring

Six microcontroller pins, the usual arrangement:

Tying R/W to ground makes the display write-only, which costs nothing in practice — the busy flag is the only reason to read from it, and library code just waits out the worst-case timing instead. That saves a pin.

In the Arduino LiquidCrystal library this is:

#include <LiquidCrystal.h>

// LiquidCrystal(rs, enable, d4, d5, d6, d7)
LiquidCrystal lcd(12, 11, 5, 4, 3, 2);

void setup() {
  lcd.begin(16, 2);       // columns, rows
  lcd.print("hello");
}

Notes and gotchas

• The backlight needs a series resistor. Pin 15 goes to a bare LED anode on many bare modules — no onboard current limiting. Connect it straight to 5 V and you cook the backlight. Something in the 100–220 Ω range is typical; check the specific module's datasheet, because some do include a resistor and adding another just dims the display.
• Blank display with the backlight on is usually contrast, not code. Pin 3 needs to sit near the bottom of its range. If a trimpot sweep produces no visible characters at any setting, then suspect wiring.
• Line 2 is not contiguous with line 1. DDRAM for a 16x2 starts line 1 at address 0x00 and line 2 at 0x40, so writing past column 16 does not wrap to the second line — it runs into unseen address space. Use lcd.setCursor(0, 1) rather than relying on wrapping.
• Contrast bias may want a negative rail. Standard TN modules bias fine against ground, but some extended-temperature parts need V0 below ground, which a plain trimpot between VDD and GND cannot supply.
• 3.3 V logic is not the same as a 3.3 V module. HD44780 modules are specified for 5 V logic and 5 V LCD bias. A 3.3 V microcontroller can usually drive the control and data lines directly since they are inputs and the thresholds happen to work, but the module itself generally still wants 5 V on VDD to get usable contrast. Modules explicitly sold as 3.3 V exist; do not assume a given one is.
• An I²C backpack trades pins for a chip. PCF8574-based adapter boards solder onto the 16-pin header and reduce the interface to SDA, SCL, and power. Cheap and popular, and it costs six pins' worth of wiring, at the price of a slower interface and a second address to keep track of.

References

• Basic Character LCD Hookup Guide — SparkFun, source of the pinout diagram above
• HD44780U datasheet — Hitachi, the authority on command set, timing, and DDRAM/CGRAM addressing
• Arduino LiquidCrystal reference — library API

Used in

• Altoids Tin CW Keyer