exxos blog - random goings on

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ijor
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Re: exxos blog - random goings on

Post by ijor »

exxos wrote: Mon May 22, 2023 11:57 am *should* yes. But something screws up with the CPU with the delay which can only be busgrant related as the CPU doesn't do much else. What I have done now is enter 32MHz on BGACK and its filled my 200MB partition up twice without tripping up once. Though I think BGACK is more complicated than one tends to think.

I guess BR comes too late, ends up tripping over to the next clock cycle, then everything is late after that.
Nothing wrong should happen if BR is delayed for a few cycles. If BR is delayed, then just the whole DMA transaction is delayed. GLUE (or GST in the STE) doesn't expect any specific timing between BR & BG. And as a matter of fact, the timing is normally not constant (the diagram you posted is a bit misleading in this regard). GLUE will wait for the CPU grating the bus as much as needed.

Conceivably, if DMA is delayed too much and the device pushes data too fast, there is the possibility that the DMA will fail by overrun. But this can only happen when reading a sector, not when writing. Because when writing, the computer sets the DMA pace, not the device.

However be careful with manipulating the DMA signals without synchronization with the other bus control signals. For example, the meaning of BG depends on AS (and also DTACK to a lesser extent).
Badwolf wrote: Mon May 22, 2023 11:30 amDuring DMA cycle the CPU is dormant ...
May be just for the record, but note this is not accurate. The CPU will continue processing as long as it doesn't need the bus. In theory, if the CPU happens to be processing an instruction that takes many internal cycles (such as DIV or MUL), it is perfectly possible that DMA will not produce any delay whatsoever. In practice this will rarely happen, if at all.
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Re: exxos blog - random goings on

Post by exxos »

ijor wrote: Mon May 22, 2023 1:32 pm Nothing wrong should happen if BR is delayed for a few cycles. If BR is delayed, then just the whole DMA transaction is delayed. GLUE (or GST in the STE) doesn't expect any specific timing between BR & BG. And as a matter of fact, the timing is normally not constant (the diagram you posted is a bit misleading in this regard). GLUE will wait for the CPU grating the bus as much as needed.
Yeah, I totally agree with you, but something gets screwed up nonetheless. Literally just 10ns delay on the CPU clock and the DMA starts failing. The system will run all day on 8Mhz or 32MHz without any issues otherwise. It has nothing to do with the actual DMA chip itself either as the "Good dma" actually works worse.

I think in one test I inverted the clock and IIRC @dad664npc reported it was more reliable, but the system was very slow.

I think if BR was delayed by a full clock or more, it probably would work, but a very small delay as just a few ns, something trips up.

It's so sensitive that even offloading the CPU to my booster board upsets the thing. 33pF on the 8Mhz clock seems to cause the same issues, only the blitter seems to screw up as it locks up just before it gets to desktop.

I've seen it before were swapping the CPU just from one brand to another can make or break DMA issues. So something is going on relating to the 8MHz clock timing to the CPU and DMA cycles.

There is some strange borderline timing somewhere but no idea what. But I just don't really have the time or equipment to invest in diagnosing all these bizarre issues all the time.
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Re: exxos blog - random goings on

Post by exxos »

My box of 30 pin simms just arrived :D Now I will be busy for the next foreseeable forever in testing all 400 of them :roll:

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JezC
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Re: exxos blog - random goings on

Post by JezC »

exxos wrote: Mon May 22, 2023 3:43 pm My box of 30 pin simms just arrived :D Now I will be busy for the next foreseeable forever in testing all 400 of them :roll:
I expect to see another post relating to a faulty STE in the very near future!

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Re: exxos blog - random goings on

Post by exxos »

JezC wrote: Mon May 22, 2023 5:25 pm I expect to see another post relating to a faulty STE in the very near future!
Yeah, you're probably right. I need to find my test machine which has the metal clip sockets I fitted from the last time I destroyed it testing simms :lol:
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Re: exxos blog - random goings on

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Had to connect my other STE up as the SIMM sockets are going iffy already. This one has my previous booster installed (solder in type) .

This one does not have the clock buffer but I tested it because it was unstable.. And...

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Re: exxos blog - random goings on

Post by exxos »

sporniket wrote: Tue May 23, 2023 4:48 pm That's quite an undershoot. The overshoot is high too.
Yeah it's on the external blitter machines, its really bad on those boards.
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Re: exxos blog - random goings on

Post by exxos »

BR on the external blitter boards is pretty bad as well.

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Re: exxos blog - random goings on

Post by exxos »

Thought I would do a timed test on how long it takes a diagnostic cartridge to test 4MB RAM in 8MHz and the STE boosters 32MHz mode. Rather a significant difference despite RAM itself not running any faster!

8MHz = 1:38
32MHz = 0:35

~280% speed at 32MHz 8-)


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Just because a lot of people agree on something, doesn't make it a fact. ~exxos ~
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