DFRobot FIT0127 LCD Character Display Emulation Specification
Overview
This document specifies how to emulate the DFRobot FIT0127 LCD Character Display module. FIT0127 is a 16x2 character LCD compatible with the HD44780 controller, commonly used in 6502 and microcontroller breadboard systems.
The goal is functional correctness for SBC emulators, firmware testing, and UI visualization rather than electrical signal simulation.
Module Summary
| Feature | Value |
|---|---|
| Display Type | Character LCD |
| Resolution | 16 columns x 2 rows |
| Controller | HD44780-compatible |
| Interface | 8-bit or 4-bit parallel |
| Character Matrix | 5x8 dots |
| Supply Voltage | 5V |
Pin Definitions
| Pin | Name | Function |
|---|---|---|
| 1 | GND | Ground |
| 2 | VCC | +5V |
| 3 | VO | Contrast control |
| 4 | RS | Register Select |
| 5 | R/W | Read / Write |
| 6 | E | Enable |
| 7-14 | D0-D7 | Data bus |
| 15 | A | Backlight + |
| 16 | K | Backlight - |
Logical Registers
| RS | Register |
|---|---|
| 0 | Instruction Register |
| 1 | Data Register |
Instruction Set (Subset)
| Command | Code | Description |
|---|---|---|
| Clear Display | 0x01 |
Clears DDRAM, cursor home |
| Return Home | 0x02 |
Cursor to home position |
| Entry Mode Set | 0x04-0x07 |
Cursor direction |
| Display Control | 0x08-0x0F |
Display, cursor, blink |
| Cursor/Shift | 0x10-0x1F |
Shift cursor/display |
| Function Set | 0x20-0x3F |
Data length, lines |
| Set CGRAM Addr | 0x40-0x7F |
Custom chars |
| Set DDRAM Addr | 0x80-0xFF |
Cursor position |
Internal Memory Model
DDRAM (Display Data RAM)
- Size: 80 bytes
- Line 1 base:
0x00 - Line 2 base:
0x40
Emulator mapping:
Row 0: DDRAM[0x00-0x0F]
Row 1: DDRAM[0x40-0x4F]
CGRAM (Character Generator RAM)
- Stores up to 8 custom characters
- 8 bytes per character
Data Write Cycle
A write occurs when:
RS = 1
R/W = 0
E: HIGH LOW
Emulator Behavior
- On falling edge of
E, latch data - Write data to DDRAM or CGRAM depending on address mode
- Auto-increment or decrement address based on entry mode
Instruction Write Cycle
A command write occurs when:
RS = 0
R/W = 0
E: HIGH LOW
Read Cycle (Optional)
Reads are uncommon in hobby systems.
RS = 0/1
R/W = 1
E: HIGH
Emulator may simplify:
- Ignore reads entirely
- Or return busy flag + address counter
Busy Flag Emulation
Real Hardware
- Busy flag = D7
- Commands take 37-1520 µs
Emulator Options
| Mode | Behavior |
|---|---|
| Simplified | Always ready |
| Timed | Busy for N cycles |
Recommended default: Always ready
Power-Up State
On reset:
- Display OFF
- Cursor OFF
- DDRAM cleared or undefined
- Address counter = 0
Emulator should:
- Clear DDRAM
- Set cursor to (0,0)
- Display enabled
Cursor and Display Model
State variables:
cursor_row
cursor_col
display_on
cursor_on
blink_on
Cursor moves automatically after writes based on entry mode.
4-bit vs 8-bit Interface
8-bit Mode
- Full byte transferred on D0-D7
4-bit Mode
- High nibble sent first
- Two enable pulses per byte
Emulator simplification:
- Accept full byte writes
- Ignore nibble timing
Rendering Model (Emulator UI)
Recommended approach:
- Maintain 16x2 character buffer
- Render ASCII subset
- Substitute unsupported glyphs
- Optionally render custom CGRAM chars
Emulator API Model
typedef struct {
uint8_t ddram[80];
uint8_t cgram[64];
uint8_t addr;
bool display_on;
bool cursor_on;
bool blink_on;
uint8_t entry_mode;
} FIT0127_LCD;
Common Wiring in 6502 Systems
VIA Port LCD D4-D7 (4-bit mode)
RS VIA bit
E VIA bit
R/W GND
Testing Checklist
- Clear display command
- Cursor positioning via DDRAM addresses
- Sequential character writes
- Line wrap behavior
- Custom character display
References
- HD44780U Datasheet (Hitachi)
- Ben Eater LCD Interface Notes
- Ben Eater's 6502 Computer
- Build a 6502 Computer
Notes
This spec intentionally prioritizes firmware-visible behavior over electrical accuracy, making it ideal for:
- SBC emulators
- ROM and monitor development
- Automated testing of LCD output
- Educational CPU projects