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A simple 8-bit home computer style application for the TI Tiva-C Launchpad
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Monotron.

A simple 1980's home computer style application for the Tiva-C Launchpad

Introduction

Monotron is powered by a Texas Instruments TM4C123 microcontroller, containing an ARM Cortex-M4 core and a number of peripherals. This processor was chosen because it is available on an inexpensive dev-kit - the Tiva-C Launchpad - and I happened to have some lying around. The challenge I set myself was, how much can you squeeze out of this tiny CPU? And can you do it all in pure-Rust?

YouTube screenshot of a video of Monotron See Monotron in Action!

Features

  • 800x600 8-colour VGA output
  • 32 KiB SRAM (24 KiB usable for applications)
  • 256 KiB Flash ROM
  • Choice of low-memory 48x36 text display mode or full 384x288 bitmap graphics mode
  • 8-bit mono audio output with 3 channel, 4 waveform synthesiser
  • USB Serial I/O
  • Atari 9-pin joystick interface
  • Simple command-line interface
  • I2C expansion interface
  • Loadable apps
  • Battery backed real-time clock*
  • 25-pin IBM PC style parallel printer port*
  • MIDI In, Out and Through*
  • PS/2 Keyboard and Mouse*
  • RS-232 Serial port*
  • Watch this space!

* Requires additional hardware, included on the Monotron PCB

Video

Monotron generates an 800x600 VGA video signal at 60 Hz using three SPI peripherals and a timer to generate the horizontal sync signals. It can do this because the VGA signal has a pixel clock of 40 MHz and the Tiva-C Launchpad's TM4C123 CPU runs at 80 MHz. We save on the number of pixels we have to push through the SPIs by running at half-resolution horizontally (giving 400x600), which also halves the pixel clock to 20 MHz. I did try 40 MHz mode and it didn't work.

Monotron has two 'modes' it can display on this VGA output.

Text Mode

Text Mode has a 48 character by 36 line display. Each character cell is 8 pixels wide and 16 pixels high and can take any character from the 8-bit MS- DOS Code Page 850 character set, and can have any foreground and background colour from the supported set:

  • White
  • Red
  • Yellow
  • Green
  • Cyan
  • Blue
  • Magenta
  • Black

The text buffer takes up 48 x 36 x 2 = 3,456 bytes of SRAM.

The built-in font is taken from FreeBSD. There's also a second font which implements Teletext block graphics (or 'sixels').

Any line of text can be displayed in "double height" mode, showing either the top-half or bottom-half.

Finally, the display is framed with an 8 pixel border at the sides and a 12 pixel border at the top and bottom, to make everything fit neatly and to help with any minor overscan issues if you use an actual CRT monitor.

Graphics Mode

Graphics Mode can be enabled and disabled at run-time. It's not enabled by default because bitmap graphics take up a lot of RAM!

You can attach a 1-bit-per-pixel graphics buffer that is some multiple of 384 pixels (i.e. 384 bits or 48 bytes) long. This buffer is displayed using line- doubling (i.e. every line is shown twice) so you can go up to 384x288 resolution maximum, which will fill the screen. Each bit from the bitmap is coloured according to the text cell (see above) it sits on top of, much like a ZX Spectrum. A full-screen bitmap therefore uses the 3,456 bytes of SRAM from text-mode plus an additional 384x288 / 8 = 13,824 bytes of SRAM.

Compiling

You will need to build using Rust Nightly, as we need various experimental features for Embedded development that are not yet available in Stable.

$ rustup toolchain install nightly
$ git clone https://github.com/thejpster/monotron.git
$ cd monotron
$ rustup override set nightly
$ rustup target add thumbv7em-none-eabihf
$ cargo build --release

To program the board, you can use lm4flash:

$ cargo build --release
$ arm-none-eabi-objcopy -O binary ./target/thumbv7em-none-eabihf/release/monotron ./target/thumbv7em-none-eabihf/release/monotron.bin
$ lm4flash ./target/thumbv7em-none-eabihf/release/monotron.bin

Or you can debug in GDB (which will automatically load the program first):

$ openocd
<switch to a different terminal>
$ cargo run --release

OpenOCD should read our openocd.cfg file, which directs it to use the correct configuration. You may need to run sudo openocd if your user doesn't have permission to open the USB device.

To exit GDB, you may need to press Ctrl-C multiple times, as it seems it can get a bit stuck.

Connecting

You have two options when running a Monotron.

  1. You can take a bare Tiva-C Launchpad and wire up various connectors using the pin-outs in the following sections.
  2. You can skip all that make yourself a Monotron PCB!

VGA

Your VGA connector requires five wires:

  • Pin 1: Red - connect to PF1 via a 330 Ohm resistor.
  • Pin 2: Green - connect to PB7 via a 330 Ohm resistor.
  • Pin 3: Blue - connect to PD3 via a 330 Ohm resistor.
  • Pin 5: Ground - connect to GND
  • Pin 6: Red Return - connect to GND
  • Pin 7: Green Return - connect to GND
  • Pin 8: Blue Return - connect to GND
  • Pin 13: H-Sync - connect to PB4
  • Pin 14: V-Sync - connect to PB5

I'm using this arrangement using random resistors I found on my desk, and it works for me (although the picture is a bit dim, as it actually produces about 0.6V peak rather than 0.7V):

-----+
     |     +------+ 330 Ohm        Co-ax in the VGA cable
PB7 o+-----|      |------------(o)==================)+
     |     +------+                                  |
-----+                                               |
                                                    +-+
                                                    | |
                                                    | | 75 Ohm
                                                    | | (in Monitor)
                                                    +-+
                                                     |
                                                     o
                                                    GND

The 330 Ohm resistor forms a resistive divider with the 75 Ohm resistor in the monitor. This is needed to drop the 3.3V output down to 0.7V. Some monitors are more tolerant of over voltage than others. The higher the resistor you use, the less current you pulling out of the GPIO pin (we're just over 8mA currently, which is a bit high) but the lower the voltage the monitor will see and the dimmer your picture will be. Conversely if you lower the resistor, more current will flow but it'll be a brigher picture. I'd save your chip from damage and just wind the brightness control up!

Obviously only one channel is shown above - wire up the blue and red channels in exactly the same fashion. Finally, don't forget to keep your wires short! You will have noise if you try and send a 20 MHz signal down 10cm of unshielded wire.

In a perfect world, your board would offer a 75 ohm source impendance matching the monitor's 75 ohm impedance, to reduce reflections, but at this resolution it doesn't seem to matter. If you want do do that, you'll need to make a resistive divider to drop the 3.3V to 1.4V, and then feed that through a high-bandwidth (>20 MHz) unity-gain amplifier, with a 75 ohm resistor on the output. The pair of 75 ohm resistors will then drop the 1.4V to 0.7V in the monitor.

UART

Monotron primarily uses UART0, which is converted to USB Serial by the on-board companion chip on the Tiva-C Launchpad. Connect with your favourite Serial terminal at 115,200bps, then send UTF-8 characters and they'll get converted to virtual keyboard input, allowing you to drive the Monotron.

There's also a second UART (UART1), which has RTS/CTS hardware handshaking lines connected (but not DSR, DTR or RI). On the Monotron PCB these are brought out to pin header J8. By fitting six jumpers on this header, the on-board MAX3232 level shifter is activated, driving RS-232 signals on the DE9M connector, J12. This connector is wired as Data Terminal Equipment or DTE (as opposed to Data Communications Equipment or DCE). This means that Monotron transmits data on DE9 pin 3, and it's designed to connect to Serial AT modems or other peripherals. Other computers (like an old IBM PC) and even USB to RS-232 adaptors are most likely wired DTE and so to connect one to the Monotron you'll most likely need a null-modem cable (one that swaps pins 3 + 4 and pins 7 + 8).

Bonus points to the first person to write a BBS program for Monotron that lets you dial up on a 56k modem.

Extra bonus points to the first person to write an xmodem file transfer program so you can transfer files from SD card over RS-232 to an xmodem utility on your old MS-DOS 3.3 IBM PC.

Note: The Joystick connector looks the same as the RS232 connector - don't mix them up!

On the Monotron PCB, a third UART (UART3) is routed through various opto-isolators to the MIDI In and MIDI Out ports. The MIDI Through port just repeats everything received on the MIDI In port.

Finally, also on the Monotron PCB, a fourth UART (UART7) connects to a 5V AtMega48 which is used as an I/O expander. This microcontroller drives two PS/2 ports (one for keyboard, one for the mouse) as well as a full IBM PC-style 25-pin parallel printer port.

Launchpad Pin Tiva-C Pin External Pin Function (from Monotron's point of view)
N/A PA0 N/A USB Serial Rx
N/A PA1 N/A USB Serial Tx
J4.8 PD6 J12 pin 3 RS-232 Tx
J4.9 PD7 J12 pin 2 RS-232 Rx
J4.10 PF4 J12 pin 7 RS-232 RTS
J3.2 GND J12 pin 8 RS-232 CTS
J4.6 PC6 J11 pin 4/5 MIDI In
J4.7 PC7 J9 pin 4/5 MIDI Out
N/A N/A J10 pin 4/5 MIDI Through

Audio

Monotron can generate 8-bit audio output using PWM on pin PE4. I use the monotron- synth which has a three-channel wavetable synthesiser which can bleep and bloop with square waves, sine waves, sawtooth waves and generate white noise.

You'll need to run the pin through a low-pass filter to remove the noise, and connect it to an amplifier as the GPIO pin won't really supply much current.

Launchpad Pin Tiva-C Pin TRS Function
J1.5 PE4 Ring/Tip Audio Left
J1.6 PE5 Tip Audio Right
J3.2 GND Sleeve Common

Currently audio only comes out as Mono on PE4 (Left). If this is a problem for your amplifier, setting jumper J1 on the PCB to 1-2 allows you to route Audio Left to Sleeve as well as Tip. Setting J1 to 2-3 enables stereo support, should that ever get developed.

Joystick

There are five active-low inputs corresponding to Up, Down, Left, Right and Fire. You can connect these inputs to a standard Atari or Commodore 9-pin Joystick as follows:

Launchpad Pin Tiva-C Pin Joystick Pin Function
J3.8 PE2 1 Up
J3.9 PE3 2 Down
J4.8 PD6 3 Left
J4.9 PD7 4 Right
J4.10 PF4 6 Fire
J3.2 GND 8 Ground

A Sega Master System (tm) controller should work, except you'll only be able to use Button 1 not Button 2. A Sega Mega Drive (tm) controller probably won't work because it has more buttons than there are pins, and so uses a 5V powered multiplexer to select between the two banks of buttons. 15-pin Analog PC joysticks won't work either.

SD Card

You can load programs and data from an SD card, from the first Primary MBR (standard old-style MS-DOS) partition, formatted as either FAT16 or FAT32. Use a standard SD Card SPI breakout adaptor, connected up as follows:

Launchpad Pin Tiva-C Pin Function
J2.10 PA2 Clock
J2.9 PA3 Chip Select
J2.8 PA4 MISO/Data Out
J1.8 PA5 MOSI/Data In

SD cards operate at 3.3v so no level shifters are required, but many cards will require a 47k (well, between 10k and 100k) pull-up to 3.3v on all four data pins. If you don't use these pull-ups, the pins float while the Monotron is booting, and these fluctuations may upset the SD card, giving you random timeouts and other spurious effects. Some cards tolerate this better than others, so YMMV. If in doubt, just add the pull-ups (they're missing on rev 0.7.0 of the PCB, but will be on 0.8.0+).

On the console you use the mount command to scan the disk, then dir to show the root directory contents. You can also dload, ddump and dpage commands to load, hex-dump and print files to the screen. A program loaded with dload can subsequently be executed with the run command.

I2C

The Monotron PCB has an I2C expansion header connected to I2C1. The PCB can also optionally be fitted with an Microchip MCP7940N battery-backed real time clock chip. Driver support for this chip is TBC.

The PCB pulls-up the I2C bus to 5V through 4.7k pull-ups. On these particular pins the TM4C123 happens to be 5V tolerant and so it simply runs the I2C pins in open-drain mode without a level shifter.

Launchpad Pin Tiva-C Pin External Pin Function
J2.1 GND J6 pin 1 Ground
J3.1 5V J6 pin 2 5V Power
J1.10 PA7 J6 pin 3 SDA
J1.9 PA6 J6 pin 4 SCL

PS/2 Keyboard

The initial PS/2 keyboard support sort of worked, but it wasn't reliable I so took it out again. The fundamental problem was that it's really hard to sample a 10 to 15 kHz incoming synchronous signal without using SPI slave peripheral (they're all in use) and while bit-bashing timing-critical video signals at 20 MHz. Either you drop random key events (leading to missing characters or stuck keys), or the video wobbles every time a key is pressed (like a ZX81). Neither was ideal.

On the Monotron PCB I work around this issue by adding an Atmel AtMega48 microcontroller as an I/O expander, connected to UART 7. Look in the avr_kb folder for more information.

Note: There's a bug in the 0.7.0 PCB where both PS/2 connector pinouts are mirror image, compared to how they should be. It happened because although the connector drawings said "Bottom View" quite clearly, I didn't pay enough attention and assumed they were viewed from above! To use the PS/2 connectors, they must be fitted to the underside of the PCB (i.e. opposite to all the other connectors). This will be fixed in the next PCB revision.

MIDI

The Monotron PCB has three 5-pin DIN MIDI ports: In, Out and Through. These are connected to UART 3.

Parallel Printer Port

The 25-pin Parallel Printer Port is connected to the Monotron PCB's AtMega48 I/O controller. You send commands to the AtMega to get it to drive the printer port. Currently only SPP (classic mono-directional support as found on the original IBM PC) is planned for support. Support for the fancier EPP and ECP modes is TBD.

Monotron PCB

There's now a PCB! It features:

  • 2x20 pin headers, to mate with the TM4C123 Launchpad (other Launchpads like the TM4C129 and MSP430 have not been tested and almost certainly won't work)
  • 5V power jack (2.1mm barrel jack, centre +ve)
  • VGA port (DE15HD female)
  • Atari/Commodore joystick port (DE9 male)
  • Micro-SD Card slot
  • 3.5mm stereo line out
  • RS-232 port (DE9 male)
  • 2.54mm (100 mil) jumpers to disconnect the RS232 driver and allow you to connect a 3.3v FTDI type TTL serial cable instead.
  • MIDI In, Out and Through ports, with opto-isolation (5-pin DIN)
  • 4-pin 2.54mm pitch (100 mil) I2C expansion header (GND, 5V, SDA, SCL)
  • Microchip MCP7940N I2C Real-Time Clock with battery back-up
  • AtMega48 microcontroller, controlling:
    • IBM PC style parallel printer port (DB25 female)
    • PS/2 Keyboard port (6-pin mini-DIN)
    • PS/2 Mouse port (6-pin mini-DIN)

See the pcb folder for Kicad files, Gerbers and PDF Schematics. If you want to buy a PCB (with or without a kit of parts) send a message to @therealjpster on Twitter, or via my Github e-mail. Rev 0.7.0 has some rough edges, but I'm working on it.

Running

When running, a simple command driven interface is presented. Commands can be entered over serial, or using a PS/2 keyboard. Commands are split on whitespace and then interpreted based on the left-most word. Enter the command 'help' to see a list of commands. Some commands place you in to a sub-menu - use 'exit' to return to the previous menu.

Loading apps

Applications can be compiled and loaded into RAM for exection. They must be linked to run from address 0x2000_2000 and take less than 24 KiB for all code and data combined. See the table below:

Address Length Description
0x0000_0000 256 KiB OS Code
0x2000_0000 8 KiB OS Data
0x2000_2000 24 KiB Application

The first four bytes of the image must be the address of the start function, with prototype fn start(const struct callbacks_t* callbacks) -> int32. Apart from that, applications are free to apportion the remaining 24,572 bytes as they see fit.

Note: The application does not need to provide a stack region - the Monotron ROM will handle that using the system stack.

The callback structure supplied to the application's entry function is defined in api.rs, but in C looks like:

struct callbacks_t {
    int32_t (*putchar)(void* p_context, char ch);
    int32_t (*puts)(void* p_context, const char*);
    int32_t (*readc)(void* p_context);
    void (*wfvbi)(void* p_context);
    int32_t (*kbhit)(void* p_context);
    void (*move_cursor)(void* p_context, unsigned char row, unsigned char col);
    int32_t (*play)(void* p_context, uint32_t frequency, uint8_t channel, uint8_t waveform, uint8_t volume);
    void (*change_font)(void* p_context, uint32_t mode, const void* p_font);
    uint8_t (*get_joystick)(void* p_context);
    void (*set_cursor_visible)(void* p_context, uint8_t visible);
};

The C functions exported to the apps are:

  • puts - print an 8-bit string (certain escape sequences are understood). Note that unlike the C routine of the same name, this function does not append a newline automatically. It is more like fputs(s, stdout).
  • putchar - print an 8-bit character
  • readc - blocking wait for keyboard/serial input
  • wfvbi - Wait For next Vertical Blanking Interval
  • kbhit - return 1 if a key has been pressed (and so readc won't block), else return 0
  • move_cursor - move the cursor to change where the next print goes
  • play - play a note on one of the synthesizer channels
  • change_font - changes the font used on screen to the normal CodePage 850, the Teletext font, or a custom font supplied by the application.
  • get_joystick - returns the current state of the joystick input. Bits 0-4 correspond to Fire, Right, Left, Down and Up respectively.
  • set_cursor_visible - Pass 0 to disable the _ cursor, or non-zero to enable it.

You can use the upload Python script in this repo to upload binary images into RAM, or you can use the dload to load them from SD card.

See monotron-apps for example apps which will run from Monotron's RAM, along with a wrapper which makes using the callbacks as simple as using a normal C library.

Unreleased changes (will be 0.10.0)

  • Fixed video interrupt jitter by entering WFI before drawing pixels.
  • Updated VGA framebuffer callback API

Changelog

  • Version 0.9.0 - Added I2C commands and support for the AtMega keyboard controller (WIP) on the Monotron PCB.
  • Version 0.8.0 - Added cursor support to ABI. Added basic SD Card support (read-only).
  • Version 0.7.0 - Move application RAM to 0x2000_2000. Added cursor support. Moved callback pointer.
  • Version 0.6.3 - Fixed Joystick support.
  • Version 0.6.2 - Add Joystick support.
  • Version 0.6.1 - Add Teletext font and support for font-switching in apps.
  • Version 0.6.0 - Added sound and support for apps running from RAM. Removed PS/2 keyboard support.
  • Version 0.5.0 - Added 1bpp graphics mode.
  • Version 0.4.0 - Added PS/2 keyboard support.
  • Version 0.3.0 - Backspace works.
  • Version 0.2.0 - Switch to a text buffer to save RAM. Basic animations work.
  • Version 0.1.0 - First release. VGA output works but menu is full of dummy commands and there's no keyboard input.

License

Licensed under either of

at your option.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.

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