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add reference files for new skill legacy-circuit-mockups
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jhauga
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skills/legacy-circuit-mockups/references/emulator-lcd.md
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skills/legacy-circuit-mockups/references/emulator-lcd.md
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# DFRobot FIT0127 LCD Character Display Emulation Specification
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## Overview
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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.
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The goal is **functional correctness** for SBC emulators, firmware testing, and UI visualization rather than electrical signal simulation.
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---
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## Module Summary
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| Feature | Value |
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| ---------------- | ----------------------- |
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| Display Type | Character LCD |
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| Resolution | 16 columns x 2 rows |
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| Controller | HD44780-compatible |
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| Interface | 8-bit or 4-bit parallel |
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| Character Matrix | 5x8 dots |
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| Supply Voltage | 5V |
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---
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## Pin Definitions
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| Pin | Name | Function |
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| ---- | ----- | ---------------- |
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| 1 | GND | Ground |
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| 2 | VCC | +5V |
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| 3 | VO | Contrast control |
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| 4 | RS | Register Select |
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| 5 | R/W | Read / Write |
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| 6 | E | Enable |
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| 7-14 | D0-D7 | Data bus |
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| 15 | A | Backlight + |
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| 16 | K | Backlight - |
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---
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## Logical Registers
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| RS | Register |
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| -- | -------------------- |
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| 0 | Instruction Register |
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| 1 | Data Register |
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---
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## Instruction Set (Subset)
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| Command | Code | Description |
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| --------------- | ----------- | ------------------------- |
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| Clear Display | `0x01` | Clears DDRAM, cursor home |
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| Return Home | `0x02` | Cursor to home position |
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| Entry Mode Set | `0x04-0x07` | Cursor direction |
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| Display Control | `0x08-0x0F` | Display, cursor, blink |
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| Cursor/Shift | `0x10-0x1F` | Shift cursor/display |
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| Function Set | `0x20-0x3F` | Data length, lines |
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| Set CGRAM Addr | `0x40-0x7F` | Custom chars |
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| Set DDRAM Addr | `0x80-0xFF` | Cursor position |
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---
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## Internal Memory Model
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### DDRAM (Display Data RAM)
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* Size: 80 bytes
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* Line 1 base: `0x00`
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* Line 2 base: `0x40`
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Emulator mapping:
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```text
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Row 0: DDRAM[0x00-0x0F]
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Row 1: DDRAM[0x40-0x4F]
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```
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### CGRAM (Character Generator RAM)
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* Stores up to 8 custom characters
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* 8 bytes per character
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---
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## Data Write Cycle
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A write occurs when:
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```
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RS = 1
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R/W = 0
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E: HIGH LOW
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```
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### Emulator Behavior
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* On falling edge of `E`, latch data
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* Write data to DDRAM or CGRAM depending on address mode
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* Auto-increment or decrement address based on entry mode
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---
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## Instruction Write Cycle
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A command write occurs when:
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```
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RS = 0
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R/W = 0
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E: HIGH LOW
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```
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---
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## Read Cycle (Optional)
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Reads are uncommon in hobby systems.
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```
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RS = 0/1
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R/W = 1
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E: HIGH
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```
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Emulator may simplify:
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* Ignore reads entirely
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* Or return busy flag + address counter
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---
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## Busy Flag Emulation
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### Real Hardware
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* Busy flag = D7
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* Commands take 37-1520 µs
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### Emulator Options
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| Mode | Behavior |
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| ---------- | ----------------- |
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| Simplified | Always ready |
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| Timed | Busy for N cycles |
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Recommended default: **Always ready**
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---
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## Power-Up State
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On reset:
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* Display OFF
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* Cursor OFF
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* DDRAM cleared or undefined
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* Address counter = 0
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Emulator should:
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* Clear DDRAM
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* Set cursor to (0,0)
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* Display enabled
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---
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## Cursor and Display Model
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State variables:
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```text
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cursor_row
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cursor_col
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display_on
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cursor_on
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blink_on
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```
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Cursor moves automatically after writes based on entry mode.
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---
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## 4-bit vs 8-bit Interface
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### 8-bit Mode
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* Full byte transferred on D0-D7
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### 4-bit Mode
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* High nibble sent first
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* Two enable pulses per byte
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Emulator simplification:
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* Accept full byte writes
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* Ignore nibble timing
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---
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## Rendering Model (Emulator UI)
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Recommended approach:
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* Maintain 16x2 character buffer
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* Render ASCII subset
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* Substitute unsupported glyphs
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* Optionally render custom CGRAM chars
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---
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## Emulator API Model
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```c
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typedef struct {
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uint8_t ddram[80];
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uint8_t cgram[64];
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uint8_t addr;
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bool display_on;
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bool cursor_on;
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bool blink_on;
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uint8_t entry_mode;
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} FIT0127_LCD;
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```
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---
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## Common Wiring in 6502 Systems
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```
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VIA Port LCD D4-D7 (4-bit mode)
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RS VIA bit
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E VIA bit
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R/W GND
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```
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---
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## Testing Checklist
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* Clear display command
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* Cursor positioning via DDRAM addresses
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* Sequential character writes
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* Line wrap behavior
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* Custom character display
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---
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## References
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* [HD44780U Datasheet (Hitachi)](https://academy.cba.mit.edu/classes/output_devices/44780.pdf)
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* [Ben Eater LCD Interface Notes](https://hackaday.io/project/174128-db6502/log/181838-adventures-with-hd44780-lcd-controller)
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* [Ben Eater's 6502 Computer](https://github.com/tedkotz/be6502)
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* [Build a 6502 Computer](https://eater.net/6502)
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---
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## Notes
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This spec intentionally prioritizes **firmware-visible behavior** over electrical accuracy, making it ideal for:
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* SBC emulators
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* ROM and monitor development
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* Automated testing of LCD output
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* Educational CPU projects
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