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MHF-002 (Mega Home FORTH)

ATmega2560 based SBC with VGA, PS/2 and SD cards - meant to run FORTH now and serve as graphic card later.

gilhadGilhad
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This 10 cm x 10 cm PCB will serve as Single Board Computer now, to develope software for display VGA/RCA based 40x25 character screen (and maybe later 320x200 graphics), managing PS/2 keyboard, SD card, USB Serial communication (with RTS/CTS flow controll) and maybe also other SPI and I2C periferials (like RTC). It will use external RAM and while developing it will also use FORTH as main system/language. Finally it will serve as graphic and I/O card for 8bit computer based on HD6309 processor. This is second PCB (so MHF-002), with more than 75 improvements over previous PCB (called MHF-001). PCBway.com generously sponsored manufacturing both those boards. Their work is flawless, easy to use, easy to solder/desolder and repeatedly resolder chips while testing different configuration and software options. Those improvements are result of actually using the MHF-001 board and adress discovered flaws in my design. Even with lot of reworks no trace nor pad was damaged during the tests

Current status:

MHF-001 proved, that VGA signal can be generated clean and stable. Due bad PORTs assignment was characters little distorted, this should be fixed on this new card. Also I failed to solder the external RAM perfectly, due footprints oriented for machine work, not hand soldering, so it sometime worked, sometime not and preesig it by hand improved the chace for right results. New version have much longer pads on footprints, which I can solder reliably.

More similar improvements was discovered over time, but basicaly I was able somehow confirm all wanted features. 

Software needs be rewritten for new port usage and for better functionality. It is just matter of time and effort.

FORTH helped very much, as it allows interactivelly program the atmega over keyboard or serial line, without recompiling and uploading everything from start for any and every action wanted.  Interactivity simply rules :)

Main idea for this card is to share 128kB RAM with the HD6309 computer, where it will be mapped to one or two 8kB windows and let the CPU fill commands and data in on its own pace, then map the RAM to MCU (ATmega) and read (and execute) everything on its own pace. This will enable high throuhtput at cost of little latency added and paralel work of both systems.

Generating of VGA signal needs like 90% of time on MCU and it runs in Interrupt routine, to provide needed regularity. The rest of time is used for everything else. CPU is not bothered with such details and runs all the time at full speed.

I do work on this project from begining of 2024 (so nearly two years) and it goes forward slowly, but surly. Now I estimate like another year is needed to explore this card and turn to building the CPU part.

  • 1 × Arduino Mega Pro Cheap way to get core parts - MCU with crystal, reset block, USB block with CH340G and crystal and connector, Power block for 5V and 3V3 and few LEDs
  • 1 × SD card reader with level converter needs fix MISO to use CS
  • 1 × CP2102 USB 2.0 to TTL UART module for alternate USB A connection and RTS/CTS flow controll
  • 1 × conectors VGA, RCA, PS/2, Barell jack 12V
  • 2 × CY7C109D 128kB RAM External + Shared RAMs

View all 14 components

  • RAM (External + Shared)

    Gilhad • 13 hours ago • 0 comments

    External RAM may be used to have visible 64kB RAM inside MCU (but those 56kB extra is 1 tick slower to access).

    Shared RAM may be shared with retrocomputer or used as just storage with slower time then RAM but faster then anything else.

    Both are 128kB chips CY7C109D, as I found them to be cheap, accessible and 5V friendly. The truth is, that their output is  lower then 3.4V, but Atmega should be able use it anyway. 74HC245 gate may need power little lower than +5V, so I added resistor there to slightly decrease it (or maybe Schottky diode). I will see, if it is really needed.

    Note that near the RAM datalines may be permutated any way, as well as adderss lines, because it does not matter, the permutation will negate when the data are readed back. So it may be used to have better physical traces there (as I did).

    Read more »

  • SD Card + SPI + ISCP

    Gilhad • 14 hours ago • 0 comments

    SD card formated with FAT system serve for storing and reading files. 

    Currently it uses SPI and Arduino library, but as I use timers, I update millis() just each screen refresh to make illusion of time for its timeouts.

    SPI have its LED and pinheader placed near SD card and may be separated by cutting solder jumpers (default connected). MISO/MOSI may serve also for PS/2 serial input when its (default open) solderjumpers are solrdered.

    ISCP connector is also there with pin 1 marked by white rectangle under SD reader. 

    Read more »

  • Serial + USB

    Gilhad • a day ago • 0 comments

    Common

    All marks are from MCU signal side of view. (RX is input, TX output, CTS is input, RTS is output, DTR is input).

    Default Serial uses USART0 (RX0/TX0 - PE0/PE1 pins) and RTS/CTS (PB5/PB4 pins) and DTR connected to Reset.

    There are 3 possible sources for Serial Communication

    • Serial pinhead - always conected to MCU, need all signals to be attached
    • CH340G with micro USB from Arduino Mega Pro - need to solder connect
    • CP2102 module with USB A - need to solder connect

    There can be two modes of communication

    • full with RTS/CTS flow controll - recomended - protect agains buffer overflow, when PC transmits more data (like a file, or Copy-Paste larger block)
    • simply RX/TX like on Arduino, buffer may overflow, cannot paste longer lines
    Read more »

  • PS/2

    Gilhad • a day ago • 0 comments

    PS/2 have 2 lines - Clock and Data. See https://wiki.osdev.org/PS/2_Keyboard

    Clock runs at 10-17 kHz, 11 pulses (normal High, active LOW), data came as 

    • 1 start bit (always 0)
    • 8 data bits (least significant bit first)
    • 1 parity bit (odd parity)
    • 1 stop bit (always 1)

    The clock/data can be connected directly to PB2/PB3 MOSI/MISO via solder jumpers (default OPEN, solder to USE) and decoded as serial communication, but I will use two shift registers and R-C to detect, if we are Inside the packet or not, so I could read the data as one byte on port PF, when I do not use it as VGA data output. 

    Inside signal (connected via closed solder jumper, cut if not wanted) is HIGH when clock is changing, LOW, when clock stops and PSDat is valid. 

    PS-OE is active LOW and let the captured PS data go to port PF

    Read more »

  • Composite output

    Gilhad • 2 days ago • 0 comments

    The composite output is currently done via USART running on half system clock.

    One line is 64 uS long.

    Sync pulse should be 0 V, black should be 0.3 V and white should be 1 V.

    Read more »

  • VGA

    Gilhad • 11/07/2025 at 12:32 • 0 comments

    VGA need Vertical Sync (at start of each screen), Horizontal Sync (at start of each line) and Signal (inside the line, it is the pixels).

    Vertical Sync is generated by timer on pin PE4 (every 16.64 mS ~ 60 Hz)

    Horizontal Sync is generated by timer on pin PB6 (every 32 uS)

    VGA signal is generated in interrupt on timer overflow (every 32 uS = each 512  clock ticks)

    Read more »

  • How it started

    Gilhad • 11/05/2025 at 12:46 • 0 comments

    Many years ago I bought  Omen Kilo (SBC with HD6309) and start to play with it. It worked, but did not anything visible (does it even run?), just communicated over Serial. Which was not so much satisfying.

    Read more »

View all 7 project logs

  • 1
    Building MHF-002 step by step

    I first time just took parts from Arduino Mega Pro and solder them to the PCB at right places, then I tested I it works and it worked. 

    Then I added one subsystem at time and tested that. This way it was not too much complicated and I wrote SW for each test extra. Also it makes more familiarity with the whole system, as it is build by logical steps.

    Problem is, that smaller parts are difficult to solder, when larger parts are already there.

  • 2
    Building it easier way

    Decide, which all subsystems are to be used, then solder the MCU first, then the RAMs, then the SMD caps and resistors and diodes, then ICs, then normal caps, then connectors, modules and pinheaders.

  • 3
    Submodules links

    Each submodule have its own description in logs. There is also schema, and top and bottom view where the parts mentioned are shown in green. Still is recomended to use KiCad and the full project files as main reference, but this should help with orientation.

View all 3 instructions

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