Created and tested 2 V9958 designs. One using the Sony CXA1645 chip to generate RGB, Composite and S-Video signals. Another that only outputs RGBs signal.
The various designs for video boards that i found online, use DRAM memory chips to get 128k or 192k . I didnt want to use those for a few reasons, so I went with a design that uses an SRAM chip - the 512k chip (ASC6C4008). This is similiar to many of the TMS9918 modern implementations.
The benifits of going with SRAM are:
- Simplier board design, that can fit on the 8x10 cm boards
- Much easier to source the ram chip
I wasnt really sure if the timings would all work --- I looked at the datasheets and it all seemed to be in the realm of possibility - The paper by Tom LeMense was very handy in helping me understand some of the things that would need to be consider.
I mapped the 3 CAS lines from the V9958 to the top 3 Address lines of the SRAM, allowing the video to access a full 192k range - the other memory address are simply ignored. I havnt fully tested all RAM access operations - only been running it in TMS9918 compatibility mode....
A few things to note:
- I dont own an RGB Montior - only an old LCD screen with some cheap ebay converters.
- My RGB to HDMI converter - says it should handle the 15khz signal - but .... I cant seem to get that to work. When i look at the signals coming out of the board on the oscilloscope - it looks ok - not that I would really know - as I am such a noob at this.
- The composite and S-Video conversion works (although a bit noisy).
- I guess I will need to get the OSSC converter - cost a little bit - but i am not sure how else to test the RGB output.
RomWBW works fine with the board, using its TMS driver - the V9958 is backwards compatible to the TMS9918
I had a few design faults with my boards -- manually 'adjusted' the boards - but i will get new ones made soon.
I havn't tested PAL mode, nor tried writing software to utilise the extra features of the V9958 chip - such as its higher resolution.
The designs for the boards 'borrow' from many other projects, in particular the Rosco 68K video board, for understanding how to create a simple RGBs output. The Omega MSX for a working implementation using the CXA1645P chip. This guy. And the numerous google searches and various forum postings.
1. Get new boards made that fix minor layout issues - also try and improve signal quality
2. Write some software to use the higher graphics modes.
3. Update RomWBW tms driver to detect v9958 chip and flip to 80 column text.
4. Get an OSSC to test out RGBs video.