STE Blitter bus arbitration glitches?

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1st1
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STE Blitter bus arbitration glitches?

Post by 1st1 »

Hello here in the forum are several interesting discussions and observations about the behaviour of STE blitter.

Maybe there is something new. User R^2 of the german forum reports that when switching from CPU to Blitter operation that there are sometimes/often glitches/spikes on the /BG signal which prevents the switching on the 1st attempt. It happens even more often when that STE has a CPU acceleration board. Only after 2nd attempt of the switching process it is successfull and Blitter takes over. That means, everytime there is such a spike appears, there is a delay until Blitter begins to work, what means a loss of performance!

R^2 has also worked out a workarround for this behaviour, adding a ceramic 47pf capacitor at the /BG signal and ground as close as possible to the Blitter (U101 separate Blitter, U400 GSTMCU with integrated Blitter).

Near U400 you find a contact pass through with /BG, connected with pin 11 of the 68000, where you can solder the capacitor, and you can get ground nearby at C404.

With U101 blitter you find /BG at R111 solder point and for ground you can use C106.

According to R^2 this little modification accelerates the bus arbitration reliable. On Mega STE no need for this modification.

Source with pictures: https://forum.atari-home.de/index.php/topic,17216
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Re: STE Blitter bus arbitration glitches?

Post by TheNameOfTheGame »

Very interesting. Thanks for letting us know!
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Re: STE Blitter bus arbitration glitches?

Post by ijor »

1st1 wrote: Sun Sep 18, 2022 6:40 pm Maybe there is something new. User R^2 of the german forum reports that when switching from CPU to Blitter operation that there are sometimes/often glitches/spikes on the /BG signal which prevents the switching on the 1st attempt. It happens even more often when that STE has a CPU acceleration board. Only after 2nd attempt of the switching process it is successfull and Blitter takes over. That means, everytime there is such a spike appears, there is a delay until Blitter begins to work, what means a loss of performance!
Do you know if this was tested on multiple computers without an accelerator? Otherwise it is likely a specific issue of his setup, may be specific to the booster he is using. There are several demos that require cycle exact Blitter timing. Some require an exact total cycles spent by Blitter, some also require blitting to occur at precise cycles. If the mentioned behavior where Blitter timing is delayed or altered would be common, then we would likely have seen this already.
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Re: STE Blitter bus arbitration glitches?

Post by czietz »

If you have a specific demo in mind and can point me to it, I'll ask the original poster (R^2) to test it.
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Re: STE Blitter bus arbitration glitches?

Post by npomarede »

czietz wrote: Mon Sep 19, 2022 7:40 pm If you have a specific demo in mind and can point me to it, I'll ask the original poster (R^2) to test it.
Here you go :)
- RGBeast by Agression : fullscreen plasma using the blitter https://www.pouet.net/prod.php?which=29355
- We Were @ by Oxygene : many parts are using blitter during fullscreen https://www.pouet.net/prod.php?which=66702
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Re: STE Blitter bus arbitration glitches?

Post by metalages »

Blitter plasma and / or colors changes

https://youtu.be/riXJ6q0sk9g?t=24
https://youtu.be/jTW-htWG84A?t=77
https://youtu.be/jTW-htWG84A?t=113
https://youtu.be/iNbVcFThTxY?t=263

The first one (in Relapse demo) has one specificity : in order to compensate the mega STe 4 addtionnal cycles at launch, instead on changing the generated code according to machine type cookie STe / mega STe, I have used another way by testing blitter lag and generate code accordingly.
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Re: STE Blitter bus arbitration glitches?

Post by 12MHz »

Hi guys,

perhaps you know me via the german forum "Atari-Home". My name is Robert and at this forum my nick name is R^2. Here at Atari-Forum they call me "12MHz".

When transferring the report from Atari-Home to Atari-Forum, comprehension difficulties have unfortunately crept in during the translation. Sorry for this situation.

The Blitter doesn't every time rearbitrate the bus. This issue could be possible with a normal 1040-STE but additionally some items should be also happened. It is to be seen more strongly e.g. if the power supply isn't correct or great changes of Bits during blitting screenparts are changing her level (high/low).

This issue is forced to remark, if an accelator board is installed into the 1040-STE. It isn't depending on board revision. The workaround to stop this issue, the capacitor at the line /BG is - in my mind - a good solution. Atari change during producing the 1040-STE the pull up resistor to lower value at the /BG line. This is also a hint, that Atari remark some problems during changing the bus master from 68000 to blitter. But nevertheless. It's up to you, to make this workaround. If you haven't any trouble with your blitter inside your 1040-STE it isn't necessary to do this.

I try to give you some more details what's could happened. I've gladlessly (happily <- don't know to translate) make a logic analysator print of this issue.

download/file.php?mode=view&id=48463

In the first picture you see three time phases (parts). At the end of this article you'll find also three detail views of this three time parts.
First part the bus arbitration between 68000 and blitter works well.
Part two show some retries to get bus master of blitter. This part works also well but with retries at start.
Part three show the final break down of system.
download/file.php?mode=view&id=48464
download/file.php?mode=view&id=48465
download/file.php?mode=view&id=48466

I hope, this give you more detail informations, what I've tried to say.
Sorry for misunderstood.

Best regards
Robert
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Re: STE Blitter bus arbitration glitches?

Post by 12MHz »

npomarede wrote: Mon Sep 19, 2022 8:22 pm
czietz wrote: Mon Sep 19, 2022 7:40 pm If you have a specific demo in mind and can point me to it, I'll ask the original poster (R^2) to test it.
Here you go :)
- RGBeast by Agression : fullscreen plasma using the blitter https://www.pouet.net/prod.php?which=29355
- We Were @ by Oxygene : many parts are using blitter during fullscreen https://www.pouet.net/prod.php?which=66702
I've tested both demos. Here the result:
Both demos works in the mode "68000 with 8 MHz" well.
All others modes doesn't works.

In case of RGB-Ruler the program need explicit a 68000. It doesn't works on a 68020. Also in case RGB-Ruler the timing between Blitter and 68000er need exact the timing of a 68000 with 8 MHz. In the mode "68000 with 16 MHz" the demo runs, but without a screen. You could only listen the music.

In case of "we were" also only the mode "68000 with 8 MHz" works well.

Best regards
Robert
Best regards
Robert
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