True 16MHz Mode

[exxos]

As shown in the new thread https://www.exxosforum.co.uk/forum/viewt ... =73&t=1916

This is how I have wired mine up.. I'm not saying its ideal, its just what works..

In the case if Ickys start, most of the resistors are 68R on my board (or no resistor at all, IE linked). So 33R may or may not work until tested.

Of course I am reusing the 33R's on the remake board as well.. so these may get removed in favour for 100R on the mod board at a later date for example. I just didn't want to make to many changes at once to my setup. and just documenting whats currently working on my board..

[ijor]

Yeah, Troed did a table of all this somewhere in his tread. Of course it needs the bandwidth to RAM to output the data fast enough, but also needs a shifter capable of using the bandwidth.
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So we basically go from 320x200x16 to 640x200x16 which is double bandwidth needed. So 640x200x16 would need double bandwidth again. At these points the shifter pixel clock is still running at stock speeds.
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Long long long term idea.. if we had a 32bit system and a shifter to match, we could transfer the data double speed again ( in effect a 64MHz system). In that case 640x400x256 would be possible.. but its only a dream of getting that far with the project.

There may be other 16bit options of just running RAM/MMU at 100MHz with the shifter, and using a 50MHz CPU.. but would need a lot of fast RAM to keep up as well. But I think going to 32bit after 50MHz is probably a better direction to go in.. but I think it will be unlikely the project will ever get that far.

For a board running on FPGA chipset the sky is virtually the limit. Running at x4 bandwidth shouldn't be a problem at all and wouldn't even need a fancy implementation. Just using low cost 45-50ns SRAM should probably be enough.

Going to x8 bandwidth will probably be more complicated but not too much. Using 32-bit RAM could be a good solution for SHIFTER, but the CPU won't benefit from the extra bandwidth. MMU (on FPGA) can easily run at 64MHZ, and 10ns SRAM is still affordable. Fast frequencies might require good impedance matching, and in general good board level analog characteristics.

[exxos]

I was just trying out my strip board version which had been working fine, and I get this screen on power up every time now..

Pressing reset generally does not do anything, but sometimes it goes to a white screen with bombs. Powering off and on again and the machine works fine then.. Odd..

In any case, I have started with the proper mod PCB now...

IMG_4273.JPG

[exxos]

I have started building my board, only to realise I am missing the 08 chip :cry: :cry:

What I will do is at least try and do some testing with the clock part today..

1.jpg

[exxos]

Basically installed, still having the odd reset issues :shrug:

IMG_4278.JPG

IMG_4277.JPG

Pressing the reset button does not do anything (just end up with the same screen again). But if I leave it powered down for about 20 seconds, and power backup again, seems to work fine. Though I should be getting the new reset circuit from Icky soon, so it will be interesting to see if this solves the problem.

Seems stable and looks like 200% scores as expected ( as said before I am missing the AND gate chip :( so this is why my screen is odd below, aka 1280x200 :lol: )

IMG_4276.JPG

[Icky]

Here is my header / jumper cable version of this mod

Firstly I placed the 47pF cap, exxos noticed needed adding, from GND to the 16MHz line underneath the board so its out the way.

IMG_3978.jpg

All the clock line 33R's have been replaced with headers. I then crafted jumper cables with the 33R resistors in the cables using heatshrink over the header plug with the resistor and then the wire.This way I can plug and play around with different options. I still think the brown patch cable on the left is too long. Will shorten it later. The CPU extension board is in place and working with the mod.

IMG_3979 (1).jpg

[PhilC]

Those wires need tydying. Setting off my OCD here.

[Icky]

Those wires need tydying. Setting off my OCD here.

When I was building the wires up I was thinking this is too far for OCD and then I noticed I was lining the resistors up in the same orientation into the header sockets. Thats when I had to stop for a while I think it got the better of me.

[PhilC]

:lol: not just me then

[Icky]

:lol: not just me then

Wait till you see the monstrosity coming in my extension board posts tonight :twisted:

https://www.exxosforum.co.uk/forum/viewt ... 9&start=10