To write an LCD driver, first identify the display’s controller and interface: there is no universal LCD command set. For a common HD44780U-compatible character display, the core driver sends commands and character data over a 4-bit or 8-bit bus, performs the controller’s initialization sequence, and observes its timing. A graphics display such as one using the ST7735 needs a different driver.
Identify the controller before writing code
“LCD” describes a display technology, not the protocol your microcontroller must implement. Check the module documentation for its controller, pinout, supported interface, display geometry, and electrical requirements. An HD44780U-compatible character module and an ST7735 graphics module differ in what they display and how a host communicates with them.
| Display type | What the driver sends | Typical interface described here | What the software must handle |
|---|---|---|---|
| HD44780U-compatible character LCD | Commands and character codes for a character display | Parallel 4-bit or 8-bit bus | Initialization, command/data selection, timing, and character-cell addressing |
| ST7735 graphics LCD | Commands, parameters, and display data | Selectable serial modes, including a 4-line mode | Its own initialization, serial framing, and graphics-memory writes |
For the HD44780U, the controller datasheet documents the interface and instruction behavior. The actual module documentation remains essential for its wiring and electrical specifications: Hitachi HD44780U datasheet.
Build the HD44780 driver in layers
Keep microcontroller-specific GPIO operations separate from the controller protocol. This makes the driver easier to inspect and adapt when the board or pin mapping changes.
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- 1602 LCD screen can display 2 lines x 16 characters, with i2c serial interface, blue display.
- Built-in independent potentiometer, backlight can be adjusted through the back potentiometer.
- Power supply: 5v; I2C address: 0x27; wiring method: GND—GND, VCC—VCC, SDA—A4, SCL—A5.
- Compatible with most development boards, such as Arduino, Raspberry pi, Tinkerboard, Nano pi, Banana pi, stm32, etc.
- Widely used in: Internet of Things, school electronics projects, smart buildings, maker DIY projects, etc., can display letters, characters, numbers, real-time clock or temperature.
- Board-support layer: name and control the RS, R/W, E, and data pins. Configure pin directions and known idle levels.
- Bus primitive: implement an enable-pulse operation and a function that places a nibble or byte on the bus. Match the pulse and data timing to the controller datasheet.
- Byte operations: provide
write_command(byte)andwrite_data(byte). Set RS low for an instruction and high for character data; set R/W for the intended direction. - Display operations: build routines such as
clear,home,set_cursor,write_char, andwrite_stringon top of the byte operations.
In 8-bit mode, a bus operation transmits a byte. In 4-bit mode, each byte takes two transfers: send the high nibble first, then the low nibble. Keep RS and R/W correct for both transfers. When reading in 4-bit mode, the byte likewise arrives in two nibble cycles.
Initialize the controller before ordinary writes
Initialization is a protocol sequence, not just a matter of setting GPIO outputs. The controller’s starting state can depend on power-supply conditions, so establish known pin directions and output levels, wait after power-up, and follow the documented interface-recovery sequence before sending normal function-set and display-control instructions.
Rank #2
- 2004 LCD screen can display 4 lines x 20 characters, with i2c serial interface, blue display.
- Compatible with most development boards, such as Arduino, Raspberry pi, Tinkerboard, Nano pi, Banana pi, stm32, etc.
- Power supply: 5v; I2C address: 0x27; wiring method: GND—GND, VCC—VCC, SDA—A4, SCL—A5.
- Built-in independent potentiometer, backlight can be adjusted through the back potentiometer.
- Widely used in: Internet of Things, school electronics projects, smart buildings, maker DIY projects, etc., can display letters, characters, numbers, real-time clock or temperature.
During a 4-bit startup handshake, send the initial recovery values as single nibbles. Once the controller has recognized 4-bit mode, send each subsequent byte as two nibbles. Use the sequence and timing requirements in the controller documentation for the specific controller and module rather than assuming that a microcontroller reset also reset the LCD.
The Microchip AVR-libc reference illustrates an implementation with waits of 15 ms before the first nibble, then 4.1 ms and 0.1 ms during initialization. Its comments note a 40 ms power-up wait at Vcc 2.7 V. These are implementation-specific examples, not universal constants; verify timing against the controller datasheet, supply conditions, and module documentation. See Microchip’s AVR-libc hd44780.c reference and hd44780.h command definitions.
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Rank #3
- 2inch LCD Display Module, IPS Screen, 240×320 Resolution, SPI Interface,requires minimum GPIO for controlling.
- Operating voltage: 3.3V/5V (Please ensure that the power supply voltage and logic voltage are consistent, otherwise it will not work properly).
- Display color: RGB, 262K color.Backlight: LED
- Interface: SPI. LCD type: IPS. Driver: ST7789V. Resolution: 240(V) x 320 (H) RGB.
- Display size: 30.60（H）x 40.80（V）mm. Pixel size: 0.0975（H）x 0.0975（V）mm. Dimension: 58 x 35 (mm).
Handle instruction completion safely
You can determine when an instruction has completed by polling the busy flag or by waiting for a conservative execution interval. Busy-flag polling requires R/W to be connected and the data pins to switch safely to input. The flag is read from DB7; in 4-bit mode, the read operation uses two nibble cycles. If R/W is tied low, busy-flag reads are unavailable, so use waits instead.
In its no-polling implementation branch, the Microchip AVR-libc example uses 37 microseconds for ordinary operations and 1.52 milliseconds for long operations. Treat these as values from that implementation to compare with the timing requirements of your exact controller; clear and home are examples of long operations that need the longer completion time.
Rank #4
- This is a general LCD display Module, IPS screen, 2inch diagonal, 240×320 resolution, with embedded controller, communicating via SPI interface.
- SPI interface, requires minimum GPIO for controlling
- Comes with development resources and manual (examples for Raspberry Pi/Jetson Nano/ /STM32)
- Driver: ST7789 Interface: SPI Display color: RGB, 262K color
- Resolution: 240×320 Backlight: LED Operating voltage: 3.3V/5V
Implement useful character-display operations
Once command and data transfers work, add higher-level operations using the controller’s instruction set:
- Function set: configure bus width, line mode, and font as supported by the controller and module.
- Display control: select display, cursor, and blink behavior.
- Entry mode: configure address increment and display shift behavior.
- Clear and home: issue the relevant instruction and wait for its longer completion time.
- Cursor positioning: set the DDRAM address appropriate to the requested row and column.
- Character output: send character codes as data after positioning the cursor.
Do not assume every module geometry uses the same row-to-DDRAM-address mapping. Track the actual display’s row offsets from its documentation. Add custom glyph support only after ordinary output is reliable: custom characters use CGRAM addressing, which is distinct from DDRAM display addressing.
Best Value
- Three Displays For More Projects: Build a sensor dashboard, robot status panel and classroom demo at the same time, or keep spare modules ready for testing; each compact screen delivers 128x64 graphics with self-luminous pixels and no backlight
- Fixed Yellow-Blue Zones Make Status Information Easy To Scan: Use the yellow upper band for headings, alerts or icons and the blue lower area for readings and menus; the display colors are fixed by the OLED panel rather than programmable RGB, and the screen does not support touch input
- Four-Wire I2C Connection Saves Controller Pins: Connect GND, VCC, SCL and SDA according to the module labels, scan the I2C bus and use the default 7-bit address 0x3C; the 0x78 PCB marking represents the corresponding 8-bit write-address format used by some documentation
- Works With Common 3.3 V & 5 V Project Platforms: Add compact visual feedback to compatible microcontroller and single-board computer projects, but verify the module pin order, supply voltage, I2C logic levels, pull-up voltage and SSD1306 software configuration before powering
- Three Modules Plus Ten Dupont Wires: Includes 3 OLED display modules, 5 female-to-female and 5 male-to-female jumper wires; controller boards, breadboards and enclosures are not included, and multiple displays on one I2C bus require unique addresses where supported or an I2C multiplexer
Do not reuse this driver for an ST7735 display
An ST7735 is a graphics controller rather than a character-cell controller. Its datasheet specifies selectable 3-line and 4-line serial protocols. In 4-line mode, CSX selects the transaction, SCL clocks the bits, SDA carries serial data, and D/CX distinguishes commands from parameters or display data. The driver must also handle the controller’s graphics-memory operations and its own initialization; HD44780 nibble transfers and character addressing do not apply. See the Sitronix ST7735 datasheet, Version 1.9 (2010-01-19).
Quick Recap
Debug from the wiring upward
- Check the module: verify supply, ground, contrast, backlight, pinout, and electrical requirements against that module’s documentation.
- Check GPIO and pulses: confirm pin direction and idle levels. If available, use a scope or logic analyzer to inspect the enable pulse and data signals.
- Check the transfer: confirm RS selection, bus wiring, and high-then-low nibble order in 4-bit mode before testing more complex operations.
- Cold-start the display: run the controller-specific initialization after removing and restoring LCD power; resetting only the microcontroller may leave the LCD controller’s state unchanged.
- Add features gradually: first test visible text, then cursor movement and row mapping, then clear and home, and finally custom glyphs.
- Investigate intermittent behavior: revisit initialization waits, power-up assumptions, enable timing, wiring, and whether the module actually uses the controller-compatible interface you selected.
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