10MHz and Beyond!

Using, learning, programming and modding the Gigatron and anything related.
Forum rules
Be nice.
Post Reply
Posts: 16
Joined: 17 May 2018, 07:17

10MHz and Beyond!

Post by monsonite » 18 May 2018, 11:49

Hi All,

I received my Gigatron a few weeks ago, and after a successful build - apart for 1 bad soldered joint, which later failed, having a working TTL computer with colour graphics was a rewarding end to an evening of soldering.

I decided to experiment with different crystals, and an 8MHz was fitted - which remarkably produced a video image that my monitor could still synchronise to.

I found a 10MHz crystal to hand and fitted this. The Gigatron continued to run the Blinkenlights on power-up, just using the set of 74HCTxx ICs supplied with the kit - alhough with H-sync now being produced at 50kHz, there was no longer any chance of my monitor synchronising to the video output.

I had chosen to fit sockets throughout the design, so that 74Fxx series of ICs could be fitted. These I sourced on ebay, from various small suppliers, for about £0.50 each in small volume. I also bought some 10nS SRAM, some 45nS OTP ROM, and some cheap adaptor pcbs - so that a SRAM with a SMT package could be fitted into the 28 pin DIL socket on the Gigatron.

The last of the ICs arrived yesterday, so I now have a Gigatron made almost entirely with 74F seies ICs, faster SRAM and the opportunity of faster ROM.

With this arrangement it might be possible to overclock to perhaps 16MHz or 20MHz.

I have offered Marcel a set of 74F ICs - in exchange for programming my 45nS ROM with the V1 code.

I have sufficient IC sets for about 3 more fast ALU, SRAM and ROM. - If anyone is interested in experimenting with faster hardware - please get in touch.



Posts: 25
Joined: 14 May 2018, 09:04

Re: 10MHz and Beyond!

Post by Cwiiis » 19 May 2018, 18:43

Neat! So at 8Mhz, do you get a corresponding increased refresh rate of ~76Hz?

Posts: 16
Joined: 17 May 2018, 07:17

Re: 10MHz and Beyond!

Post by monsonite » 08 Mar 2019, 20:30

Hi All,

After a year of inactivity - I have caught the Gigatron bug again.

In May 2018 I was able to tweak the clock frequency up to 10.00MHz and still have the Blinkenlights run - just 160% of their normal speed.

Tonight, I have reduced the value of C1 and C2 in the crystal oscillator circuit from 47pF to 22pF, and removed C3 - so that now I can run the oscillator with higher frequency crystals and still achieve a clean clock signal.

I'm a bit limited with the choice of crystals I have at hand, but I can confirm that Blinkenlights still runs with an 11.0592MHz crystal, but it definitely does not run with a 12.00MHz crystal. :(

I think that I have reached the access time of the ROM - the cycle time is now 90.42nS (down from 160nS) and the poor old 100nS 27C1024-10 EPROM is struggling to get beyond that. 12MHz (83.33nS) is just too much for the old chap.

There is a solution to greater speed - and that is to replace the 100nS part with a 45nS part, and that might get me into the 20MHz region. If I am particularly lucky with an EPROM very conservatively rated at 45nS, I might even reach 25MHz - four times the standard clock frequency. This could open up some interesting possibilities of increased video resolution.

Whilst the standard Gigatron will run at 10.00MHz with the usual 74HCT devices, I have opted to replace most of the parts on the board with the faster 74F series. This includes the ICs used in the ALU, the PC, the MAU and the instruction and data registers and instruction decoder. The RAM has also been replaced with a 10nS part.

In order to widen the scope of this development, I have sent Marcel a set of "upgrade components" so that we can both see just how far the design can be pushed.

Above 20-25MHz it's anyone's guess. ST Microelectronics used to have the M27C1024 available with a 35nS access time which could get closer to 30MHz or so, but I guess that we will start to see the effects of board layout, track lengths and lack of adequate ground plane before we reach those lofty heights

https://www.jameco.com/Jameco/Products/ ... 3641ST.pdf

Replacing the EPROM with a 10nS 64Kx16 RAM could potentially achieve an even higher clock speed - but at the expense of power consumption. It would probably be necessary to redesign the board using smaller SMT ICs and reduce the track lengths.

In the meantime I shall be aiming for 25MHz with the faster EPROM.

Have a good weekend


User avatar
Posts: 178
Joined: 13 May 2018, 08:26

Re: 10MHz and Beyond!

Post by marcelk » 11 Mar 2019, 10:32

The 74F series was introduced in 1978 or 1979. I would have committed a crime for getting my hands on a 25 MHz computer back in 1988.

At higher power and frequencies, we may need to control the emissions and crosstalk a bit better. There are three options that are independent from one another:
  1. Add solid inner ground and Vcc planes
  2. Do a more thorough job on the existing outside ground pours
  3. Go to a smaller PCB with SMD components
Option 1 is what Dave J. Jones (EEVblog) has done, while simultaneously removing the outside power and grounds
Option 2 just requires changing the fill settings in KiCad, plus some minor local rearrangements (see below).
Option 3 takes a lot of effort, depending on how fussy you are about the layout. The amazing C74-6502 has done this.

Ground pour settings.png
Ground pour settings.png (249.98 KiB) Viewed 76 times

I've mixed feelings on how it looks. While it's cool, it also makes it look more like an Amiga instead of an Apple 1. Still I've ordered a couple of proto boards that combine option 1+2 and I'll send you one when they come in. I've reduced the "spoke width" on ground and power pins for this combo mod, for easier soldering. I will likely receive those boards after VCF PNW.

Post Reply