Experiment 7: Character LCD Display Interface

Complete Lab Manual

For the complete experiment including learning objectives, theoretical background, and detailed explanations, download the PDF manual: Download Experiment 7 PDF

Learning Objectives

After completing this experiment, you will be able to:

• Understand the architecture and operation of the HD44780-based 16x2 LCD module.
• Differentiate between command and data registers and their respective control signals.
• Configure the LCD in 4-bit mode to minimize GPIO pin usage.
• Implement the initialization sequence for 4-bit LCD operation.
• Send commands and data to the LCD using proper timing and enable pulse sequences.
• Display text strings at specific cursor positions on the LCD.
• Implement scrolling and dynamic display effects using LCD commands.
• Integrate LCD interfacing with GPIO and interrupt-driven input from push buttons.

Experiment Overview

This experiment introduces LCD interfacing using the HD44780-based 16x2 character display module. You will learn the controller's architecture, implement 4-bit communication mode to minimize GPIO usage, and create a reusable LCD driver library with functions for initialization, command transmission, and data display. By the end of this lab, you will be able to interface an LCD with the TM4C123 microcontroller and create interactive display applications combining LCD output with GPIO input and interrupts.

Theoretical Background

HD44780 LCD Controller Architecture

The HD44780 is an LCD controller/driver developed by Hitachi and widely adopted as an industry standard for character-based LCD modules. It provides a simple microprocessor interface for controlling alphanumeric displays with minimal external components.

LCD Module Overview

The 16x2 LCD module consists of:

• HD44780 Controller: Manages display operations, character generation, and timing
• LCD Panel: 2 rows x 16 columns of character positions
• Character Generator ROM (CGROM): Contains 208 predefined character patterns (alphanumeric, symbols, Japanese kana)
• Character Generator RAM (CGRAM): 64 bytes for up to 8 user-defined custom characters (5x8 pixels each)
• Display Data RAM (DDRAM): 80 bytes storing characters currently displayed (40 bytes per line)
• Backlight: Optional LED backlight for improved visibility

The controller handles all low-level display refresh, character rendering, and cursor management automatically. The microcontroller simply writes characters to DDRAM and issues commands for cursor positioning, display control, and special effects.

LCD Registers

The HD44780 has two main registers accessible by the microcontroller:

Instruction Register (IR)

The Instruction Register receives commands that control display operations:

• Clear display and return cursor to home
• Set cursor position in DDRAM
• Control display on/off, cursor visibility, and blinking
• Set entry mode (cursor direction, display shift)
• Shift cursor or entire display left/right
• Configure interface width (4-bit or 8-bit), number of lines, and font size

Data Register (DR)

The Data Register receives character codes (ASCII) to be displayed:

• Writing to DR displays a character at the current cursor position
• The cursor automatically advances after each write (direction set by entry mode)
• Reading from DR retrieves the character at the current cursor position (rarely used)

LCD Pin Configuration

The HD44780 interface consists of 16 pins (some modules have 18 pins with additional backlight control):

Pin	Name	Description
1	VSS	Ground (0V)
2	VDD	Power supply (+5V or +3.3V)
3	V0	Contrast adjustment (connect to potentiometer)
4	RS	Register Select: 0 = Instruction (command), 1 = Data (character)
5	RW	Read/Write: 0 = Write to LCD, 1 = Read from LCD (usually grounded for write-only)
6	E	Enable: Falling edge latches data/command into LCD
7-14	D0-D7	8-bit data bus (D0-D3 unused in 4-bit mode)
15	A	Backlight anode (+5V, typically with series resistor)
16	K	Backlight cathode (Ground)

4-Bit vs. 8-Bit Communication Mode

The HD44780 supports two communication modes:

8-Bit Mode

• Uses all 8 data pins (D0-D7)
• Each command or character requires one write cycle
• Faster communication (single byte transfer)
• Requires 11 GPIO pins total (3 control + 8 data)

4-Bit Mode

• Uses only upper 4 data pins (D4-D7), D0-D3 are left unconnected
• Each command or character requires two write cycles (upper nibble, then lower nibble)
• Slightly slower due to two transfers per byte
• Requires only 7 GPIO pins total (3 control + 4 data)
• Preferred mode for GPIO-constrained systems

4-Bit Communication Protocol:

1. Set RS and RW to appropriate values
2. Place upper 4 bits (bits 7-4) on D7-D4
3. Generate enable pulse: E high → delay → E low
4. Place lower 4 bits (bits 3-0) on D7-D4
5. Generate another enable pulse: E high → delay → E low

In this experiment, we use 4-bit mode with the following connections:

• PB0 → RS (Register Select)
• PB2 → E (Enable)
• PB4-PB7 → D4-D7 (Data pins)
• RW → Ground (write-only operation)

LCD Initialization Sequence

Proper initialization is critical for reliable LCD operation. The HD44780 requires a specific sequence of commands with precise timing delays, especially during the transition to 4-bit mode.

Initialization Steps (4-Bit Mode)

The initialization sequence must account for the fact that the LCD powers up in 8-bit mode and must be switched to 4-bit mode:

1. Wait for power-on stabilization: Delay at least 40 ms after VDD rises to 4.5V
2. Initial function set (8-bit interface): Send 0x30 (upper nibble only) three times:
   • First time: Wait > 4.1 ms
   • Second time: Wait > 100 μs
   • Third time: Wait > 100 μs
3. Switch to 4-bit mode: Send 0x20 (upper nibble only), wait > 100 μs
4. Configure display parameters: Now send full 8-bit commands in two nibbles:
   • 0x28: Function set — 4-bit mode, 2 lines, 5x8 font
   • 0x0C: Display control — Display ON, cursor OFF, blink OFF
   • 0x06: Entry mode set — Increment cursor, no display shift
   • 0x01: Clear display
   • 0x02: Return home

Critical Note: Steps 2-3 send only the upper nibble because the LCD is still in 8-bit mode. After step 3 completes, the LCD switches to 4-bit mode, and all subsequent commands must be sent as two nibbles (upper first, then lower).

LCD Command Set

The HD44780 supports a comprehensive set of commands for display control, cursor manipulation, and special effects.

Command	Hex Code	Description
Clear Display	0x01	Clears entire display, sets DDRAM address 0, cursor to home
Return Home	0x02	Sets DDRAM address 0, cursor to home (display content unchanged)
Entry Mode Set	0x04-0x07	Controls cursor direction and display shift
	0x04	Cursor moves left, no shift
	0x06	Cursor moves right, no shift (default)
	0x05	Cursor moves left, display shifts right
	0x07	Cursor moves right, display shifts left
Display Control	0x08-0x0F	Controls display, cursor, and blink on/off
	0x08	Display OFF
	0x0C	Display ON, cursor OFF, blink OFF
	0x0E	Display ON, cursor ON, blink OFF
	0x0F	Display ON, cursor ON, blink ON
Cursor/Display Shift	0x10-0x1F	Shifts cursor or display left/right
	0x10	Shift cursor left
	0x14	Shift cursor right
	0x18	Shift display left
	0x1C	Shift display right
Function Set	0x20-0x3F	Sets interface length, line number, font
	0x28	4-bit mode, 2 lines, 5x8 font
	0x38	8-bit mode, 2 lines, 5x8 font
Set DDRAM Address	0x80 + addr	Positions cursor at DDRAM address
	0x80	Beginning of line 1 (address 0x00)
	0xC0	Beginning of line 2 (address 0x40)

DDRAM Address Mapping

The Display Data RAM (DDRAM) is 80 bytes, but only 32 bytes (16 per line) are visible at any time. Understanding the address mapping is essential for cursor positioning:

Display Position	DDRAM Address	Command
Line 1, Column 0	0x00	0x80
Line 1, Column 1	0x01	0x81
Line 1, Column 15	0x0F	0x8F
Line 2, Column 0	0x40	0xC0
Line 2, Column 1	0x41	0xC1
Line 2, Column 15	0x4F	0xCF

Formula for cursor positioning:

Command = 0x80 + column for line 1 (row 0)
Command = 0xC0 + column for line 2 (row 1)

Timing Requirements

The HD44780 requires specific timing for reliable operation:

• Enable pulse width (PW_EH): Minimum 450 ns (typically use 1-2 μs for safety)
• Enable cycle time (t_cycE): Minimum 1 μs (1 MHz max frequency)
• Setup time (t_AS, t_DSW): RS and data must be stable 60 ns before E rises
• Hold time (t_AH, t_DHW): RS and data must remain stable 20 ns after E falls
• Command execution time:
  • Normal commands: ~40 μs (use 1-2 ms for safety)
  • Clear display (0x01): ~1.6 ms (use 2-3 ms)
  • Return home (0x02): ~1.6 ms (use 2-3 ms)

At 50 MHz system clock:

• 1 μs = 50 clock cycles
• 1 ms = 50,000 clock cycles

We implement microsecond delays using the SysTick timer to meet these timing requirements precisely.

LCD Driver Implementation Strategy

A well-structured LCD driver separates concerns into distinct layers:

Low-Level Functions

• LCD_EnablePulse(): Generates enable pulse (E high → delay → E low)
• LCD_SendNibble(): Sends 4 bits on D7-D4 with enable pulse
• delay_us(), delay_ms(): Precise timing using SysTick

Mid-Level Functions

• LCD_Command(): Sends 8-bit command (RS=0) as two nibbles
• LCD_Data(): Sends 8-bit character (RS=1) as two nibbles

High-Level API

• LCD_Init(): Complete initialization sequence
• LCD_Clear(): Clears display
• LCD_SetCursor(row, col): Positions cursor
• LCD_Print(string): Displays string at current cursor position