Gigatron Clone Ideas

Using, learning, programming and modding the Gigatron and anything related.
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alastair
Posts: 34
Joined: 10 Oct 2019, 14:28

Gigatron Clone Ideas

Post by alastair »

I've been thinking about doing a Gigatron clone for some time and finally sketched out an initial plan. This would be binary compatible with the existing code base, but with some of the newer ideas integrated on a new PCB. These include:
* Pluggy Reloaded
* 6-bit Audio
* High Resolution as an optional output

The other changes would be replacing the diode ROMs with era-appropriate PALs (16L8) and adding a second 32k RAM. This would reduce the TTL chip count to 22 with a PAL for the Mode Decoder and a pair of (identical) PALs for the ALU (4 bits each). The skinny 28-pin RAMs chips are a lot faster and there would be no problem running at 12.5MHz as an option.

I'm still buried in other projects but thought I would float this out there. I don't want to deviate too far, but anything else worth considering?
at67
Posts: 479
Joined: 14 May 2018, 08:29

Re: Gigatron Clone Ideas

Post by at67 »

Video Repeater : viewtopic.php?t=303
bmwtcu
Posts: 111
Joined: 01 Nov 2018, 12:02

Re: Gigatron Clone Ideas

Post by bmwtcu »

viewtopic.php?p=2948#p2948

25MHz oscillator with jumper selectable /1,/2 and /4 clock options using 74LS74.

If you're doing the PALs/GALs already, does it make sense to integrate the SPI/RAM Expander vs Pluggy Reloaded?
https://github.com/lb3361/gigatron-lb/t ... sion-retro
Not sure if that forces you to upgrade to 128kB SRAM or if it would work fine with 64kB.
alastair
Posts: 34
Joined: 10 Oct 2019, 14:28

Re: Gigatron Clone Ideas

Post by alastair »

at67 wrote: 03 May 2022, 21:42 Video Repeater : viewtopic.php?t=303
I was debating the video repeater but might not go that far.
bmwtcu wrote: 03 May 2022, 23:15 25MHz oscillator with jumper selectable /1,/2 and /4 clock options using 74LS74.
This is probably how I will go. I need a symmetrical clock from a flip-flop with a Q and ~Q output for the high res mode. I would need to start at 25MHz to get a 12.5MHz clock. I wouldn't get to 25MHz, so just the /2 and /4 clock options.
at67
Posts: 479
Joined: 14 May 2018, 08:29

Re: Gigatron Clone Ideas

Post by at67 »

With the high res mode, rather than coming up with some ad hoc set of resistor values providing a murky colour palette; how about using a few extra gates to implement a 4bit LUT and fetch the colours either from the current 6bit palette, (much lower gate count), or from an 8bit palette, (either from Gigatron RAM or extra logic). Hi-res mode is completely undefined in the current gigatron eco-system, so having to allocate a few bytes of RAM for palette's would be just a simple step in shoe-horning hi-res mode into the system.

Not only would this allow for much more vibrant and pure colours, (high res text will look better with high contrast pure colours), but also opens up the magnificent world of ye ole palette animation.

P.S. Remember Boing! on the Amiga? Most of the hard work done in that demo was through palette animation, i.e. there was no blitting whatsoever.
alastair
Posts: 34
Joined: 10 Oct 2019, 14:28

Re: Gigatron Clone Ideas

Post by alastair »

I was leaning towards a 74HC244 ganged to this with a 3PDT switch to select between the two resistor DACs. I could also look at using some 74HC670 register files as a RAMDAC and have a way to load the palette colors during blanking.

Anther option with a RAMDAC is doubling up again, so 4 pixels per cycle. This would be CGA-style 4 colors and would only work if you had a programable palette. This would allow the 6.25MHz version to output graphics at the native VGA resolution.
alastair
Posts: 34
Joined: 10 Oct 2019, 14:28

Re: Gigatron Clone Ideas

Post by alastair »

alastair wrote: 04 May 2022, 13:12 I was leaning towards ... this
Scratch that. There's only 6 bits available because I still need the H & V sync bits. That would be 3 bits each per pixel and the basic 1 bit RGB colors. These can be maxed out (>1V) to fully saturate the colors and get the "classic" 8 color palette.
bmwtcu
Posts: 111
Joined: 01 Nov 2018, 12:02

Re: Gigatron Clone Ideas

Post by bmwtcu »

Is 5V x16 flash/EEPROM a viable option? I know more folks with PROM programmers than UV erasers these days:P
alastair
Posts: 34
Joined: 10 Oct 2019, 14:28

Re: Gigatron Clone Ideas

Post by alastair »

at67 wrote: 03 May 2022, 21:42 Video Repeater : viewtopic.php?t=303
I think I'm going to go with the video repeater, at least layout the components on the board as an option. The FIFOs are kind of pricey but not crazy and they are still in production.

The ALU won't fit in a pair of 16L8 PALs. It probably wouldn't fit in a pair of 22V10 PALs and this is assuming ripple carry (there is similar ALU done this way). If it was possible then a pair of 15ns 22V10s would have a total propagation delay of around 90ns (with the ripple carry) and over clocking would be out of the question. I plan to divide and conquer with a pair of 16L8s coupled to the existing 74HCT283s. I also need a way to zero out the accumulator, so another one or two chips there.
veekoo
Posts: 78
Joined: 07 Jun 2021, 07:07

Re: Gigatron Clone Ideas

Post by veekoo »

What if someone designed new larger board where all these upgrades are added to circuitry of PCB. You can take the ICs from old board and add them to new plus all the new feature ICs. Larger RAM and ROM offcourse and maybe SSD-chip. SD-card interface as default.
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