MegaHomeFORTH: A C repository from githubgilhad

Many thanks to PCBway, which sponsored this project by manufacturing the PCB for free. The code for this PCB is W828834AS5P3 and I created it as free project here https://www.pcbway.com/project/shareproject/W828834AS5P3_MegaHomeFORTH_MHF_001_cheap_computer_with_VGA_PS_2_SD_card_ba_2abd40e2.html

MHF-001 results:

The PCBway PCB was excelent. Communications with PCBway was easy and straightforward, PCB was made exactly to my specification and it was delivered in good time too. Packaging was standart and the PCB was perfect. It was easy to solder, desolder, resolder parts as the testing went and the PCB was not damaged even with some parts (like USB connector) was resoldered many times.

The PCB was perfect, there is nothing what could be improved on the manufacturing. Bad design decisions are problem on my side, PCBway could not discover or prevent them :)

I will fully recomend PCBway as PCB manufacturer.

Also their sponsorship encouraged me to speed progress on this project and to write about it, which is really important part. I thank PCBway for their encouragement.

I did run into lot of problems in this project, many of them was on my part, some not (none on PCBway part)

I found error in compiler (it is fixed now and was really obscure https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119989 )
CH341G is not fully supported in kernel. There is an unofficial fix, but I may add another option too. And I need way to disable this chip alltogether, to be able use Serial + DTR + line flow over connector (and not fight signalling from chip)
I underestimated how difficult is to reliably handsolder MCU and memories - I will make better footprints for it, and for USB too
I had no testing program for the HW
bad understanding of some technicalities (like that RTS/CTS are crossed between MCU and CH341G in similar way as RX/TX)
bad planning for port usage
- VGA output must be on lower port, which allows OUT instruction - STS is one tick slower and it matter
- CTS(in) should be on some Pin Change Interrupt pin
74HC165 is level driven so manipulating it "manually" leads to doubled first bit value, I will use 74KC166, which is edge driven, to prevent this problem
USB connector is better to be massive (like USB A) and easily repaired (as it is physically manipulated often) - and rather be on small module for repairability
I need even better testability - more pinheaders, add GND (and maybe +5V) to each connector etc...
see https://github.com/githubgilhad/MegaHomeFORTH/tree/master/docs/MHF-001#errata-and-improvements for more details

All in all - I will make another PCB. This one is OK, but I have much better design ideas now.

MegaHomeFORTH

Mega Home Computer with FORTH on ATmega2560 - step to graphic card for HD6309 - VGA, RCA, PS/2 - see Larger Picture

This project is more about the HW part of solution (but with enought SW to be fully tested and expanded (will be build on the way)).

Look at MHF-001 for actual progress just now

Index

MegaHomeFORTH

Project Goals

Depending on how much components will be soldered to the PCB, it could offer many things (at cost of using some pins):

full breakout of ATmega2560 - like Arduino Mega, but all pins accessible - simply, plain, 86 I/O pins blink_all
USB serial connection - 2 pins
memory extended to 64 kB RAM - at cost of 16+4 pins
another 128 kB RAM (somehow) accessible - another 24+3 pins
- and shared over bus with HD6309/6502/other 8 bit computer - +5pins?
VGA output (~40x25 characters text screen or 320x200 B/W graphic) - 8+3 pins
- 4+4 bits forefround+background colors for full text lines (or single graphic lines) - 8 pins
RCA ("composite") B/W output ( can do this OR VGA, not both at the same time, but may switch it SW way ) - 3 pins
PS/2 input - 2 or 8+1 pins (much smoother operation)
SD card reader (may interfere with video interrupts?) - 4pins