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Amiga Editorials / Re: Dominik Diamond writes about the Amiga for the Guardian
« Last post by BozzerBigD on April 28, 2024, 11:03:16 AM »Great article! Thank you for sharing!
The only reason this sloppy E clock timing works at all is because the 8520 CIA is not a Motorola 6800 peripheral chip. If it was, this 14 MHz accelerator design would fail. Commodore designed these custom chips for the Amiga and using the E clock to drive them somewhat simplified the Amiga's I/O hardware design.That's the impression that I got reading the 68000 datasheet and noting that the Amiga has no actual M6800 peripherals.
But just because you can sometimes get away with sloppy timing does not means it's often good idea to do so.Definitely.
The fundamental timing problem is still not corrected because @ 28 MHz the CPU's E clock cycle will end in one 7 MHz clock. A more reliable way to handle this problem is to disable the CPU E clock cycle by disconnecting VPA and connecting a pull-up resistor. You then add logic to generate VMA when the E clock is low and keep it latched when the E clock is high.That's a point. Currently /VMA is unconnected on the PCB (and per the schematic), I guess there's an internal pullup...? What's the consequence of the short E pulse?
Here are some example boolean logic equations:I did not find the pound sign (#) in the list of standard boolean operators. What did you mean?
/VMA = /E * /VPA # /VMA * E
Next you need to generate ETERM to terminate the cycle:I'm not familiar with the .D and .T notation. What did you mean? ETERM and EDTACK symbols are connected to what?
/ETERM.D = ETERM * E * /AS * /VMA
/EDTACK.D = EDTACK * /AS * /ETERM
/DTACK.D = DTACK * /AS * /EDTACK
/DTACK.T = DTACK * /AS * /EDTACK
NOTE: ETERM and EDTACK are registered on the rising edge of the 7 MHz clock (but you can experiment with the falling edge). DTACK is registered on the 14 MHz clock. Also, DTACK is a Tri-State signal.
As far as creating the proper 60/40 duty cycle, that's probably not as important as having the E clock at the proper frequency and having the rising edge synchronized with the 7 MHz clock.I found a dedicated divider part that will generate the proper frequency E pulse in under 1ns. That will definitely get it working, at least on par with the unmodified 14MHz accelerator. I like the fact that it keeps with the minimalist spirit of the original project in OP. But latching with the rising edge of the 7MHz clock is definitely something that I can do. I also found a decade counter that would let me generate it from the 7MHz reference.
The main reason to keep the correct duty cycle is performance concerns.Yeah, I had a hunch about that, are you able to quantify what is affected by the pulse width?
BTW, some 68020+ systems can be modified for improved performance. Here is the link:Nice project, useful toward "Amiga-like computer from scratch" projects that don't use any original Amiga components (which will be my final goal, eventually). Thanks for providing the code. I wouldn't mind learning that type of PLD syntax/notation. I'm more fluent with VHDL and Verilog.
https://forum.amiga.org/index.php?topic=74945.msg850556
Actually yes it is a newer panasonic drive. This is an Escom Amiga made in France. I've suspected it could actually be the floppy drive drawing too much current on motor start. Its not a shielding problem, its the 5V rail dipping and the monitor loses sync, I can see it on my scope. It happens even with all shielding installed. I've tried different power supplies so its not the supply. I've tried a different video cable. This is a bare bones mainboard no accelerator.
Hi,
Simple answer is “Yes”, have had this even on a Rev 1D1 mainboard. The answer maybe that the top RF shielding is needed. Or the drive motor has become too noisy on the drive.
Is the drive by chance a Panasonic one?
From memory D215A. Is there a component there?
Fix is normally, ceramic cap across collector and emitter pads.
@mykrowyre, I've seen this to varying degrees and it has varied for me by screen and connection. Sorry if I've missed it on previous posts but how do you connect e.g. RF modulator, composite, RGB, and to what type of screen?