Falcon clock patch V4 series

[exxos]

Indeed that does seem a little odd. The original Atari F04 "hack" and only something like 3V logic high drive. The V4 should show the full 5V swing as it uses totally different driver chips. Also , I forget which pad, but it was either the central or the right pad on the V4, actually drive both FPU & SDMA clocks and should look pretty bad on the V4 itself because of signal reflections from both traces. Though it is what is at the "destination" which is important, which is directly on the FPU clock (next to the FPU itself) and the SDMA clock on the SDMA clock pin itself.

[exxos]

What I will say is that even though the phantom cleaned it up (SDMA) nicely there was still some 'artefacts' present in the signal (compared to the super clean output from the powerup2) and if I wanted to put a further speeder (DBFX or Nemesis etc.) then I would probably need a cleaner signal to SDMA as it would all be put under tighter tolerances and therefore I will probably need (in my case) the V4 patch.

Just a small note here in case of future readers etc.. While the V4 does give the best signals & fixes to date, I'm not suggesting people use it with any sort of bus accelerator like the Phantom etc for many reasons.

The problem is, Phantom etc is pushing bus timings so tightly that even something like 1ns difference on the clock can make or break the system. Originally I thought it was just a simple matter of a delay on the clock, hence using the non-LS buffers on the original Phantom clock patch, so I imitated that delay with variations, which was why myself and @Steve tried a delay board which did 10ns steps. However I did read somewhere that I think the original designer of the Phantom, changed the brand of buffer chips at some point and it caused problems. That seems to be how tight the timings are. In which case it is more of a fluke if some hardware combination works or not. The V2/V3 mimicked the Phantom delay, I think people had some success with that with bus speeders, but again I think it is more of a fluke of hardware timings as to if it would actually work or not. Unfortunately when you push the bus speed to the limits, you're going to get tight timings and with enormous amount of tolerances in the hardware across all machines, it further complicates issues.

So because the timings on the bus accelerators is pretty machine dependent and indeed such variances on tolerances, there is just no way to build a clock patch which is going to work in all machines with bus speeders. You are fighting minute tolerances in all the timings on all chips. Other than trial and error, your never going to get a "plug and play" solution to this problem.

The Falcon itself may not always boost to 25mhz (or whatever) reliably to start with. So developing a clock patch to correct such problems is basically impossible because it simply cannot be done. So in terms of the Phantom, people should probably use the Phantom clock patch exactly how it's creator intended it to be, and use the exact brands of chips as was used originally. Though even doing that, tolerances on the buffer chip itself or tolerances on the Falcon can still break such a system. I'm sure people know that overclocking is a bit of a roulette gamble anyway.

So in light of all these problems, I pretty much abandoned the bus accelerator aspect altogether. I would just rather keep out of it all, because I don't want to get drawn into all the chaos it would endeavour when people are trying to overclocked their machines and things don't work as expected.

Saying that some falcons with no clock patch whatsoever can run speeders/accelerators and the JOY demo absolutely fine, each Falcon must be considered unique and what works for one Falcon might not work for another. Another example is the capacitor in the video circuit that is the wrong way round, voltages need to be measured and there are variances in the circuit on the board that aren't in the schematics, mine is correctly orientated from factory but vast majority aren't again need to measure and check your own bird.

Yep. I don't think anyone has really looked into such specific details before. I think my original Falcon does not have a clock patch if I remember rightly. I definitely used it with a hard drive and don't recall having any issues as such. But mostly I was using STOS where I could easily blame buggy code for crashes, but its possible there is simply more to that now more is known these days.

Though it is still pretty weird why some falcons would seem to work without a clock patch at all. I'm not saying they are 100% stable. The odd crash every several hours can easily go unnoticed for example. How the resistors are wired to the single clock is just inherently bad start with. It is possible some machines may work fine on hot or cold days but not the other for example. Same with bus speeders.

Maybe there was some very early falcons which did work reasonably well, maybe Atari changed the supplier of chips or did something layout wise on the motherboard, even a totally separate production run could be enough upset things. But obviously Atari recognised the issues, hence doing the modification to pretty much every Falcon since. We could simply put it down to a fluke that some machines just inherently work while the majority don't. Just moving a trace couple of millimetres can be enough to cause total chaos, I have witnessed that first-hand.

TL;DR;

The V4 solves all currently known clock issues to date, but it should only be used on a stock falcon. People who want to experiment with bus speeders are "on their own".

[leonk]

@exxos while I have the system open, I can do other tests with your V4 clock patch. Can you suggest other points you'd like me to check?

I know this might not be perfect because I don't have an FPU. Around the SDMA was one suggestion. Did you want me to test before/after the added SMD resistor? Other places? Let me know.

I do recall I did a quick test after the resistor, the curve looked perfectly clean, but the voltage was lower. I forget what it was, but I'm guessing closer to 4 Vpp rather than 5 Vpp as I can see in other locations.

[exxos]

@exxos while I have the system open, I can do other tests with your V4 clock patch. Can you suggest other points you'd like me to check?

I would check the SDMA clock on the clock pin itself...

I know this might not be perfect because I don't have an FPU.

No FPU shouldn't affect the results.

I do recall I did a quick test after the resistor, the curve looked perfectly clean, but the voltage was lower. I forget what it was, but I'm guessing closer to 4 Vpp rather than 5 Vpp as I can see in other locations.

If you see less than 5V on the SDMA clock, then I would check both sides of the output resistors on the V4 board ( I think they were 20R ) .

Should be more like this.

sdma68r.png

BUT,

One thought here, because your signals look more rounded than they should, are you using your scope & probes in X1 mode ? I can only assume this is the reason you are seeing the results you are.

[leonk]

Probes are now at 100X. Single shot, on the SDMA clock pin (what the resistor feeds into) As you can see, it's not the full 5Vpp but closer to 4.5Vpp. Will this be an issue?

IMG_7634.JPG

[exxos]

Probes are now at 100X. Single shot, on the SDMA clock pin (what the resistor feeds into) As you can see, it's not the full 5Vpp but closer to 4.5Vpp. Will this be an issue?

4.5V is still like 1V more than the original clock patch, so you should be fine.

There is still looks like there is too much capacitance for some reason.

You could just trying removing the 47pF from the SDMA line (might be easier to remove the GND from the AJAX) and see if the signal looks better. Without the capacitor you would normally start seeing a lot of overshoot, but it seems like that value capacitor might be too high for your machine for some reason. Assuming that is the reason.

[leonk]

Here's what I see at the R221 resistor on the V4 PCB.

After the resistor (on its way to the Falcon PCB's SDMA):

IMG_7635.JPG

Before the SMD resistor on the V4 PCB:

IMG_7636.JPG

I think it's the SMD resistor on the V4 PCB that lowers the levels down, but the modification next to the SDMA seem to do a great job at cleaning up the noise and smoothing out the curve. I think my best course of action at this point is to leave it alone. :)

[leonk]

There is still looks like there is too much capacitance for some reason.

You could just trying removing the 47pF from the SDMA line (might be easier to remove the GND from the AJAX) and see if the signal looks better. Without the capacitor you would normally start seeing a lot of overshoot, but it seems like that value capacitor might be too high for your machine for some reason. Assuming that is the reason.

Could this be due to the added FPU capacitor under the Falcon PCB but no FPU installed?

[exxos]

Here's what I see at the R221 resistor on the V4 PCB.

Looks normal on the V4 PCB because it is having to deal with the reflections.

I think it's the SMD resistor on the V4 PCB that lowers the levels down, but the modification next to the SDMA seem to do a great job at cleaning up the noise and smoothing out the curve.

The resistor does lower the voltage a little yes, but it is a necessary evil to deal with the reflections coming back. It is basically a tuned circuit. It just seemed that there seems to be too much capacitance causing too much of a curve on the signal.

I think my best course of action at this point is to leave it alone. :)

If your Falcon is operating fine then I would not worry about it.

[Swissfalcon]

It's a very interesting thread. May I ask what would be the best clock patch to use with a untouched Falcon (newer modified, nothing soldered/patched) that has the Atari first attempt to patch (C208 removed on the other side of PCB and R221short-circuited) and a CT60e? Thanks for your help and for what you do for the community!