VT52 Terminal Core for MiSTer

by Fred Van Eijk

Overview

This core implements a VT52-compatible terminal emulator for the MiSTer FPGA platform. It provides a terminal interface with keyboard input and UART communication capabilities.

Features

• 80x24 character display
• Basic VT52 control sequences
• Hardware scrolling buffer (25 rows total)
• PS/2 keyboard interface
• UART communication (115200 baud, 8N1)
• Configurable text color (White, Red, Green, Blue)
• Multiple aspect ratio options

Setup

UART Connection

The UART interface is available through the User port on the MiSTer:

• User port pin 1 (D-/TX): Connect to RX of your USB-to-Serial adapter
• User port pin 0 (D+/RX): Connect to TX of your USB-to-Serial adapter
• User port GND: Connect to GND of your USB-to-Serial adapter

Most USB-to-Serial adapters should work. Tested adapters include:

• FTDI FT232RL
• CH340
• CP2102

Note: Make sure your adapter supports 115200 baud and is configured for 8N1 operation.

OSD Menu Options

Access the On-Screen Display (OSD) menu using the Menu button on your MiSTer setup.

Available options:

1. Aspect Ratio

   • Original (4:3)
   • Full Screen
   • [Custom ratio 1]
   • [Custom ratio 2]

2. Text Color

   • White (default)
   • Red
   • Green
   • Blue

Reset Options

• Soft Reset: Available through OSD menu
• Hard Reset: Use the MiSTer reset button or OSD reset option

Terminal Commands

Control Characters

• Ctrl+H or Backspace: Move cursor left
• Tab: Move to next tab stop (every 8 columns)
• LF (Line Feed): Move cursor down one line
• CR (Carriage Return): Move cursor to start of current line

Implemented VT52 Escape Sequences

All escape sequences start with the ESC character (0x1B):

• ESC A: Cursor up
• ESC B: Cursor down
• ESC C: Cursor right
• ESC D: Cursor left
• ESC H: Cursor home (upper left corner)
• ESC I: Reverse line feed
• ESC J: Erase screen from cursor and home cursor
• ESC K: Erase to end of current line
• ESC Y row col: Direct cursor addressing
  • Row and column are sent as ASCII codes: space (0x20) + row/column number
  • Example: To move to row 5, column 10: ESC Y % * (% = 0x20 + 5, * = 0x20 + 10)

Display Characteristics

• Character Set: Standard ASCII (32-126)
• Cursor: Blinking block cursor
• Hardware scrolling using 25-row buffer (24 visible + 1 scroll buffer)

Test Server

A Python test server script (vt52-test-server.py) is included to verify terminal functionality. The server supports both serial and telnet connections.

Requirements

pip install pyserial

Usage

Serial mode (default):

python vt52-test-server.py --port COM9 --baudrate 115200

Telnet mode:

python vt52-test-server.py --mode telnet --port 2323

Test Suite Components

The server tests the following implemented features:

1. Control Characters

   • Backspace behavior
   • Line Feed/Carriage Return
   • Tab spacing (8-column stops)

2. Cursor Positioning

   • Absolute positioning using ESC Y sequences
   • Relative movement (up, down, left, right)
   • Pattern drawing demonstration

3. Erase Functions

   • Erase to end of line (ESC K)
   • Erase screen (ESC J)
   • Cursor positioning after erase

4. Scroll Behavior

   • Forward scrolling using hardware scroll buffer
   • Reverse scrolling with ESC I
   • Line wrapping at screen boundaries

5. Box Drawing Demo

   • Demonstrates cursor movement with visual pattern
   • Tests cursor positioning accuracy

Note: The test suite includes proper timing delays to account for hardware scrolling operations.

Guess the Animal Game

A sample game application is included (vt52-guess-the-animal-game.py) that demonstrates interactive terminal usage. The game implements a word-guessing game similar to Hangman, specifically themed around animals.

Game Features

• Visual ASCII art display of game progress
• 15 different animals to guess
• 6 attempts allowed per round
• Hardware-synchronized screen updates
• Proper handling of terminal scroll operations
• Support for both serial and telnet connections

Game Display Layout

     +---+
     |   |    Title and game status at top
     O   |    
    /|\  |    Word display in center
    / \  |    
         |    Input prompts at bottom
   =========

Usage

Serial mode (default):

python vt52-guess-the-animal-game.py --mode serial --port COM9 --baudrate 115200

Telnet mode:

python vt52-guess-the-animal-game.py --mode telnet --port 2323

Command Line Options

--mode     : Connection mode (serial or telnet)
--port     : Serial port or telnet port number
--baudrate : Serial baudrate (default: 115200)
--host     : Telnet server host (default: 0.0.0.0)

VT52 Features Used

• ESC H: Clear screen and home cursor
• ESC J: Erase screen
• ESC Y: Direct cursor addressing for UI elements
• Character output with proper timing
• Hardware scroll consideration

Technical Implementation

• Synchronous screen updates accounting for VT52 timing
• Proper handling of hardware scroll operations
• Clean connection handling for both serial and telnet modes
• Error detection and graceful disconnection handling
• Configurable timing delays for hardware operations

Connection Handling

• Supports multiple simultaneous telnet clients
• Automatic connection cleanup on client disconnect
• Proper serial port release on program exit
• Error handling for connection loss during gameplay

Requirements

pip install pyserial

The game serves as both a practical example of terminal usage and a test tool for various VT52 features including cursor positioning, screen clearing, and character I/O timing considerations.

Command Line Options

--mode     : Connection mode (serial or telnet)
--port     : Serial port or telnet port number
--baudrate : Serial baudrate (default: 115200)
--host     : Telnet server host (default: 0.0.0.0)

LED Status Indicator

The LED indicates various system states:

• Cursor blink
• UART errors (overrun, framing, parity)
• Keyboard activity
• PS/2 errors
• Scroll operation in progress

Known Limitations

1. No graphics character set support
2. No alternate keypad mode
3. Fixed character set (ASCII 32-126)
4. Fixed UART parameters
5. Hardware scrolling operations require timing considerations

Error Handling

The terminal provides visual feedback through the LED for:

• UART overrun errors
• UART framing errors
• UART parity errors
• PS/2 keyboard frame errors
• Scroll buffer busy states

Technical Notes

• Core Clock: 25MHz
• Video output compatible with standard VGA/HDMI displays
• Full hardware implementation with no CPU requirements
• 25-row buffer implementation (24 visible + 1 scroll)
• Synchronous scroll operations with busy/done signaling [Previous sections remain unchanged until Technical Notes]

Technical Architecture

Core Components

1. Terminal Control Modules

• VT52_terminal: Top-level module integrating all components
• Command Handler: Processes VT52 commands and screen operations
• 8251 like UART: Serial communication at 115200 baud, 8N1

2. Display Modules

• Character Buffer: 25-row display buffer with hardware scrolling

  • 24 visible rows + 1 scroll buffer row
  • Synchronous read/write operations
  • Hardware-accelerated scroll operations

• Character ROM: Font storage and rendering

  • 4K x 8 ROM using Terminus Font 8x16
  • Latin-1 character subset
  • Synchronous font data access

• Video Generator: Display timing and rendering

  • VGA/HDMI signal generation
  • Character and cursor rendering
  • Blanking signal control

3. Input Processing Modules

• Input Multiplexer: Input stream arbitration

  • Prioritizes keyboard input over UART
  • Handshaked data transfer
  • Source tracking (keyboard/UART)

• Keyboard Controller: PS/2 keyboard interface

  • Full PS/2 protocol implementation
  • Keycode to ASCII conversion
  • Modifier key handling (Shift, Control, Meta)
  • Error detection and reporting
  • Special key sequence processing

• Keymap ROM: Key translation tables

  • 2KB ROM for keycode mapping
  • Multiple keyboard planes (normal, shifted, caps)
  • Extended keycode support

4. Cursor Management

• Cursor: Position and state tracking

  • X/Y coordinate management
  • Synchronous position updates
  • Integration with command handler

• Cursor Blinker: Visual cursor control

  • ~1Hz blink rate using 6-bit counter
  • vblank-synchronized timing
  • Reset on cursor movement

5. Support Modules

• Simple Register: Generic storage element
  • Parameterized width
  • Synchronous operation
  • Reset support

Data Flow

Input Stage:
PS/2 Keyboard → Keyboard Controller →\
                                     Input Multiplexer → Command Handler
UART RX ───────→ UART Controller ───/           ↑            ↓
                        ↑                        |      Character Buffer
                        |                        |            ↓
Keyboard Data ←── Command Handler ←── UART TX ───┘     Character ROM
                                                           ↓
                                                    Video Generator
                                                           ↓
                                                    VGA/HDMI Output

This diagram shows:

• Both keyboard and UART inputs being arbitrated by the Input Multiplexer
• Bidirectional UART communication (keyboard data is echoed to UART TX)
• Character ROM providing font data to the Video Generator
• Character Buffer storing the screen contents
• Command Handler coordinating all operations
• Video Generator producing the final display output

Key Features

1. Synchronous Design

• Single 25MHz clock domain
• Clean handshaking between modules
• Proper reset propagation

2. Input Processing

• PS/2 protocol with error detection
• Priority-based input arbitration
• Special key sequence handling
• Full modifier key support

3. Display System

• Hardware scroll buffer
• Efficient character rendering
• Smooth cursor management
• VGA/HDMI output generation

4. Error Management

• PS/2 frame error detection
• UART error detection
• Input overflow prevention
• LED status indication

[Remaining sections continue as before...]

Debugging

If you encounter issues:

1. Check the LED status for error conditions and scroll operations
2. Verify UART connections and settings
3. Ensure PS/2 keyboard is properly connected
4. Allow sufficient time for scroll operations to complete
5. Try resetting the core through the OSD menu

Future Enhancements

Planned features:

1. Graphics character set support
2. Alternate keypad mode
3. Configurable UART parameters
4. Extended character set support
5. Save/restore terminal state